Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1506.00299
New scalar constraint operator for loop quantum gravity
Mehdi Assanioussi, Jerzy Lewandowski, Ilkka Mäkinen
(Submitted on 31 May 2015)
We present a concrete and explicit construction of a new scalar constraint operator for loop quantum gravity. The operator is defined on the recently introduced space of partially diffeomorphism invariant states, and this space is preserved by the action of the operator. To define the Euclidean part of the scalar constraint operator, we propose a specific regularization based on the idea of so-called "special" loops. The Lorentzian part of the quantum scalar constraint is merely the curvature operator that has been introduced in an earlier work. Due to the properties of the special loops assignment, the adjoint operator of the non-symmetric constraint operator is densely defined on the partially diffeomorphism invariant Hilbert space. This fact opens up the possibility of defining a symmetric scalar constraint operator as a suitable combination of the original operator and its adjoint. We also show that the algebra of the scalar constraint operators is anomaly free, and describe the structure of the kernel of these operators on a general level.
14 pages.

http://arxiv.org/abs/1506.00183
The Polymer Bouncer
A. Martin-Ruiz, A. Frank, L. F. Urrutia
(Submitted on 31 May 2015)
Polymer Quantization (PQ) is a background independent quantization scheme that is deployed in Loop Quantum Gravity. This framework leads to a new short-distance (discretized) structure characterized by a fundamental length. In this paper we use PQ to analyze the problem of a particle bouncing on a perfectly reflecting surface under the influence of Earth's gravitational field, what we have called "The Polymer Bouncer". In this scenario, deviations from the usual quantum effects are induced by the spatial discreteness, but not by a new short-range gravitational interaction. We solve the polymer Schrödinger equation in an analytical fashion, and we evaluate numerically the corresponding energy levels. We find that the polymer energy spectrum exhibits a negative shift compared to the obtained for the quantum bouncer. The comparison of our results with those obtained in the GRANIT experiment leads to an upper bound for the fundamental length scale, namely λ≪0.6A∘. We find polymer corrections to the probability of transitions between levels, induced by small vibrations, together with the probability of spontaneous emission in the quadrupole approximation.
22 pages, 1 figure (a matrix of 9 individual graphs), 1 table.

http://arxiv.org/abs/1506.00398
Quantum from principles
Giulio Chiribella, Giacomo Mauro D'Ariano, Paolo Perinotti
(Submitted on 1 Jun 2015)
Quantum theory was discovered in an adventurous way, under the urge to solve puzzles-like the spectrum of the blackbody radiation-that haunted the physics community at the beginning of the 20th century. It soon became clear, though, that quantum theory was not just a theory of specific physical systems, but rather a new language of universal applicability. Can this language be reconstructed from first principles? Can we arrive at it from logical reasoning, instead of ad hoc guesswork? A positive answer was provided in Refs. [1, 2], where we put forward six principles that identify quantum theory uniquely in a broad class of theories. We first defined a class of "theories of information", constructed as extensions of probability theory in which events can be connected into networks. In this framework, we formulated the six principles as rules governing the control and the accessibility of information. Directly from these rules, we reconstructed a number of quantum information features, and eventually, the whole Hilbert space framework. In short, our principles characterize quantum theory as the theory of information that allows for maximal control of randomness.
50 pages. Contribution to the book "Quantum Theory: Informational Foundations and Foils", Springer Verlag, in press

 

[marcus]

http://arxiv.org/abs/1506.00927
The strange equation of quantum gravity
Carlo Rovelli
(Submitted on 2 Jun 2015)
Disavowed by one its fathers, ill defined, never empirically tested, the Wheeler-DeWitt equation has nevertheless had a powerful influence on fundamental physics. A well deserved one.
7 pages. Appeared in the Classical and Quantum Gravity Focus issue: Milestones of general relativity.

http://arxiv.org/abs/1506.01018
Hawking Radiation Energy and Entropy from a Bianchi-Smerlak Semiclassical Black Hole
Shohreh Abdolrahimi, Don N. Page
(Submitted on 2 Jun 2015)
Eugenio Bianchi and Matteo Smerlak have found a relationship between the Hawking radiation energy and von Neumann entropy in a conformal field emitted by a semiclassical two-dimensional black hole. We compare this relationship with what might be expected for unitary evolution of a quantum black hole in four and higher dimensions. If one neglects the expected increase in the radiation entropy over the decrease in the black hole Bekenstein-Hawking A/4 entropy that arises from the scattering of the radiation by the barrier near the black hole, the relation works very well, except near the peak of the radiation von Neumann entropy and near the final evaporation. These discrepancies are calculated and discussed as tiny differences between a semiclassical treatment and a quantum gravity treatment.
17 pages.

http://arxiv.org/abs/1506.00946
Low Energy Theorems of Quantum Gravity from Effective Field Theory
John F. Donoghue, Barry R. Holstein
(Submitted on 2 Jun 2015)
In this survey, we review some of the low energy quantum predictions of General Relativity which are independent of details of the yet unknown high-energy completion of the gravitational interaction. Such predictions can be extracted using the techniques of effective field theory.
35 pages. Invited Topical Review for J. Phys. G.

challenging:
http://arxiv.org/abs/1506.01354
Marginal evidence for cosmic acceleration from Type Ia supernovae
Jeppe Trøst Nielsen, Alberto Guffanti, Subir Sarkar
(Submitted on 3 Jun 2015)
The `standard' model of cosmology is founded on the basis that the expansion rate of the universe is accelerating at present --- as was inferred originally from the Hubble diagram of Type Ia supernovae. There exists now a much bigger database of supernovae so we can perform rigorous statistical tests to check whether these `standardisable candles' indeed indicate cosmic acceleration. Taking account of the empirical procedure by which corrections are made to their absolute magnitudes to allow for the varying shape of the light curve and extinction by dust, we find, rather surprisingly, that the data are still quite consistent with a constant rate of expansion.
5 pages, 6 figures

possible side interest:
http://arxiv.org/abs/1506.01247
Arrow of time in dissipationless cosmology
Varun Sahni, Yuri Shtanov, Aleksey Toporensky
(Submitted on 3 Jun 2015)
It is generally believed that a cosmological arrow of time must be associated with entropy production. Indeed, in his seminal work on cyclic cosmology, Tolman introduced a viscous fluid in order to make successive expansion/contraction cycles larger than previous ones, thereby generating an arrow of time. However, as we demonstrate in this letter, the production of entropy is not the only means by which a cosmological arrow of time may emerge. Remarkably, systems which are dissipationless may nevertheless demonstrate a preferred direction of time provided they possesses attractors. An example is provided by a homogeneous scalar-field driven cyclic cosmology where the presence of cosmological hysteresis causes an arrow of time to emerge in a system which is formally dissipationless.
14 pages, 6 figures

http://arxiv.org/abs/1506.01213
Indirect retrieval of information and the emergence of facts in quantum mechanics
Miguel Ballesteros, Martin Fraas, Jürg Fröhlich, Baptiste Schubnel
(Submitted on 3 Jun 2015)
Long sequences of successive direct (projective) measurements or observations of a few "uninteresting" physical quantities of a quantum system may reveal indirect, but precise and unambiguous information on the values of some very "interesting" observables of the system. In this paper, the mathematics underlying this claim is developed; i.e., we attempt to contribute to a mathematical theory of indirect and, in particular, non-demolition measurements in quantum mechanics. Our attempt leads us to make novel uses of classical notions and results of probability theory, such as the "algebra of functions measurable at infinity", the Central Limit Theorem, results concerning relative entropy and its role in the theory of large deviations, etc.
29 pages

http://arxiv.org/abs/1506.01369
Measuring the lensing potential with galaxy clustering
Francesco Montanari, Ruth Durrer
(Submitted on 3 Jun 2015)
We investigate how the lensing potential can be constrained with future galaxy surveys using their number counts. Such a measurement is an independent test of the standard LCDM framework and can be used to discern modified gravity models. We perform a Fisher matrix forecast based on galaxy angular power spectra, assuming specifications consistent with future photometric Euclid-like surveys, for which we provide a computation and fitting formula of magnification bias, and spectroscopic SKA-like surveys. The analysis suggests that the amplitude of the lensing potential can be constrained at the same level as other standard LCDM cosmological parameters.
24 pages, 11 figures

http://arxiv.org/abs/1506.00725
3D gravity with dust: classical and quantum theory
Viqar Husain, Jonathan Ziprick
(Submitted on 2 Jun 2015)
We study the Einstein gravity and dust system in three spacetime dimensions as an example of a non-perturbative quantum gravity model with local degrees of freedom. We derive the Hamiltonian theory in the dust time gauge and show that it has a rich class of exact solutions. These include the Bañados-Teitelboim-Zanelli black hole, static solutions with naked singularities and traveling wave solutions with dynamical horizons. We give a complete quantization of the wave sector of the theory, including a definition of a self-adjoint spacetime metric operator. This operator is used to demonstrate the quantization of deficit angle and the fluctuation of dynamical horizons.
14 pages

http://arxiv.org/abs/1506.00675
The Quantum Hilbert Hotel
Václav Potocek, Filippo M. Miatto, Mohammad Mirhosseini, Omar S. Magaña-Loaiza, Andreas C. Liapis, Daniel K. L. Oi, Robert W. Boyd, John Jeffers
(Submitted on 1 Jun 2015)
In 1924 David Hilbert conceived a paradoxical tale involving a hotel with an infinite number of rooms to illustrate some aspects of the mathematical notion of "infinity". In continuous-variable quantum mechanics we routinely make use of infinite state spaces: here we show that such a theoretical apparatus can accommodate an analog of Hilbert's hotel paradox. We devise a protocol that, mimicking what happens to the guests of the hotel, maps the amplitudes of an infinite eigenbasis to twice their original quantum number in a coherent and deterministic manner, producing infinitely many unoccupied levels in the process. We demonstrate the feasibility of the protocol by experimentally realising it on the orbital angular momentum of a paraxial field. This new non-Gaussian operation may be exploited for example for enhancing the sensitivity of N00N states, for increasing the capacity of a channel or for multiplexing multiple channels into a single one.
4 figures 5 pages
==excerpt==
V.P., F.M.M., D.K.L.O. and J.J. developed the theory. F.M.M., M.M., O.S.M.L., A.C.L. and R.W.B. conceived the experiment. M.M. O.S.M.L. and A.C.L. carried out the experiment. F.M.M. performed the data analysis. All authors contributed to writing the paper.
==endquote==
http://arxiv.org/abs/1506.01337
Violations of the Born rule in cool state-dependent horizons
Donald Marolf, Joseph Polchinski
(Submitted on 3 Jun 2015)
The black hole information problem has motivated many proposals for new physics. One idea, known as state-dependence, is that quantum mechanics must be generalized to describe the physics of black holes, and that fixed linear operators do not provide the fundamental description of experiences for infalling observers. Instead, such experiences are to be described by operators with an extra dependence on the global quantum state. We show that any implementation of this idea strong enough to remove firewalls from generic states requires massive violations of the Born rule. We also demonstrate a sense in which such violations are visible to infalling observers involved in preparing the initial state of the black hole. We emphasize the generality of our results; no details of any specific proposal for state-dependence are required.
20 pages, 1 figure

 

[atyy]

http://arxiv.org/abs/1506.01623
Area Law from Loop Quantum Gravity
Alioscia Hamma, Ling-Yan Hung, Antonino Marciano, Mingyi Zhang
(Submitted on 4 Jun 2015)
We explore the constraints following from requiring the Area Law in the entanglement entropy in the context of loop quantum gravity. We find a unique solution to the single link wave-function in the large j limit, believed to be appropriate in the semi-classical limit. We then generalize our considerations to multi-link coherent states, and find that the area law is preserved very generically using our single link wave-function as a building block. Finally, we develop the framework that generates families of multi-link states that preserve the area law while avoiding macroscopic entanglement, the space-time analogue of "Schroedinger cat". We note that these states, defined on a given set of graphs, are the ground states of some local Hamiltonian that can be constructed explicitly. This can potentially shed light on the construction of the appropriate Hamiltonian constraints in the LQG framework.

 

[marcus]

http://arxiv.org/abs/1506.03053
Encoding Curved Tetrahedra in Face Holonomies: a Phase Space of Shapes from Group-Valued Moment Maps
Hal M. Haggard, Muxin Han, Aldo Riello
(Submitted on 9 Jun 2015)
We present a generalization of Minkowski's classic theorem on the reconstruction of tetrahedra from algebraic data to homogeneously curved spaces. Euclidean notions such as the normal vector to a face are replaced by Levi-Civita holonomies around each of the tetrahedron's faces. This allows the reconstruction of both spherical and hyperbolic tetrahedra within a unified framework. A new type of hyperbolic simplex is introduced in order for all the sectors encoded in the algebraic data to be covered. Generalizing the phase space of shapes associated to flat tetrahedra leads to group valued moment maps and quasi-Poisson spaces. These discrete geometries provide a natural arena for considering the quantization of gravity including a cosmological constant. A concrete realization of this is provided by the relation with the spin-network states of loop quantum gravity. This work therefore provides a bottom-up justification for the emergence of deformed gauge symmetries and quantum groups in 3+1 dimensional covariant loop quantum gravity in the presence of a cosmological constant.
38 pages and 9 figures

http://arxiv.org/abs/1506.03320
Generalised conservation laws in non-local field theories
Alexander Kegeles, Daniele Oriti
(Submitted on 10 Jun 2015)
We propose a geometrical treatment of symmetries in non-local field theories, where the non-locality is due to a lack of identification of field arguments in the action. We show that the existence of a symmetry of the action leads to a generalised conservation law, in which the usual conserved current acquires an additional non-local correction term, obtaining a generalisation of the standard Noether theorem. We illustrate the general formalism by discussing the specific physical example of complex scalar field theory of the type describing the hydrodynamic approximation of Bose-Einstein condensates. We expect our analysis and results to be of particular interest for the group field theory formulation of quantum gravity.
24 pages

http://arxiv.org/abs/1506.02648
Complex Quantum Network Manifolds in Dimension d>2 are Scale-Free
Ginestra Bianconi, Christoph Rahmede
(Submitted on 8 Jun 2015)
In quantum gravity, several approaches have been proposed until now for the quantum description of discrete geometries. These theoretical frameworks include loop quantum gravity, causal dynamical triangulations, causal sets, quantum graphity, and energetic spin networks. Most of these approaches describe discrete spaces as homogeneous network manifolds. Here we define Complex Quantum Network Manifolds (CQNM) describing the evolution of quantum network states, and constructed from growing simplicial complexes of dimension d. We show that in d=2 CQNM are homogeneous networks while for d>2 they are scale-free i.e. they are characterized by large inhomogeneities of degrees like most complex networks. From the self-organized evolution of CQNM quantum statistics emerge spontaneously. Here we define the generalized degrees associated with the δ-faces of the d-dimensional CQNMs, and we show that the statistics of these generalized degrees can either follow Fermi-Dirac, Boltzmann or Bose-Einstein distributions depending on the dimension of the δ-faces.
29 pages, 4 figures

http://arxiv.org/abs/1506.02882
Quantum gravity with torsion and non-metricity
Carlo Pagani, Roberto Percacci
(Submitted on 9 Jun 2015)
We study the renormalization of theories of gravity with an arbitrary (torsionful and non-metric) connection. The class of actions we consider is of the Palatini type, including the most general terms with up to two derivatives of the metric, but no derivatives of the connection. It contains 19 independent parameters. We calculate the one loop beta functions of these parameters and find their fixed points. The Holst subspace is discussed in some detail and found not to be stable under renormalization. Some possible implications for ultraviolet and infrared gravity are discussed.
18 pages

http://arxiv.org/abs/1506.02965
Avoidance of singularities in asymptotically safe Quantum Einstein Gravity
Georgios Kofinas, Vasilios Zarikas
(Submitted on 9 Jun 2015)
New general spherically symmetric solutions have been derived with a cosmological "constant" Λ as a source. This Λ field is not constant but it satisfies the properties of the asymptotically safe gravity at the ultraviolet fixed point. The importance of these solutions comes from the fact that they describe the near to the centre region of black hole spacetimes as this is modified by the Renormalization Group scaling behaviour of the fields. The consistent set of field equations which respect the Bianchi identities is derived and solved. One of the solutions (with conventional sign of temporal-radial metric components) is timelike geodesically complete, and although there is still a curvature divergent origin, this is never approachable by an infalling massive particle which is reflected at a finite distance due to the repulsive origin. Another family of solutions (of both signatures) range from a finite radius outwards, they cannot be extended to the centre of spherical symmetry, and the curvature invariants are finite at the minimum radius.
15 pages

http://arxiv.org/abs/1506.02938
Quantum mechanics and the principle of maximal variety
Lee Smolin
(Submitted on 9 Jun 2015)
Quantum mechanics is derived from the principle that the universe contain as much variety as possible, in the sense of maximizing the distinctiveness of each subsystem.
The quantum state of a microscopic system is defined to correspond to an ensemble of subsystems of the universe with identical constituents and similar preparations and environments. A new kind of interaction is posited amongst such similar subsystems which acts to increase their distinctiveness, by extremizing the variety. In the limit of large numbers of similar subsystems this interaction is shown to give rise to Bohm's quantum potential. As a result the probability distribution for the ensemble is governed by the Schroedinger equation.
The measurement problem is naturally and simply solved. Microscopic systems appear statistical because they are members of large ensembles of similar systems which interact non-locally. Macroscopic systems are unique, and are not members of any ensembles of similar systems. Consequently their collective coordinates may evolve deterministically.
This proposal could be tested by constructing quantum devices from entangled states of a modest number of quits which, by its combinatorial complexity, can be expected to have no natural copies.
24 pages. For a talk based on this paper, see this http URL

briefly noted:
http://arxiv.org/abs/1506.03733
A naturalist account of the limited, and hence reasonable, effectiveness of mathematics in physics
Lee Smolin
(Submitted on 11 Jun 2015)
The aim of this essay is to propose a conception of mathematics that is fully consonant with naturalism. By that I mean the hypothesis that everything that exists is part of the natural world, which makes up a unitary whole.
10 pages. Awarded third place in the 2015 FQXi essay contest

http://arxiv.org/abs/1506.03788
Cognitive science and the connection between physics and mathematics
Anshu Gupta Mujumdar, Tejinder P. Singh
(Submitted on 11 Jun 2015)
...
16 pages. This essay received the Special Prize for Creative Thinking in the 2015 Essay Contest "Trick or Truth: the Mysterious Connection Between Physics and Mathematics" conducted by the Foundational Questions Institute, USA [this http URL]

http://arxiv.org/abs/1506.02669
A Quantum Focussing Conjecture
Raphael Bousso, Zachary Fisher, Stefan Leichenauer, and Aron C. Wall
(Submitted on 8 Jun 2015)
We propose a universal inequality that unifies the Bousso bound with the classical focussing theorem. Given a surface σ that need not lie on a horizon, we define a finite generalized entropy S_gen as the area of σ in Planck units, plus the von Neumann entropy of its exterior. Given a null congruence N orthogonal to σ, the rate of change of S_gen per unit area defines a quantum expansion. We conjecture that the quantum expansion cannot increase along N. This extends the notion of universal focussing to cases where quantum matter may violate the null energy condition. ...
45 pages, 6 figures

 

[marcus]

http://arxiv.org/abs/1506.04749
Coupled intertwiner dynamics - a toy model for coupling matter to spin foam models
Sebastian Steinhaus
The universal coupling of matter and gravity is one of the most important features of general relativity. In quantum gravity, in particular spin foams, matter couplings have been defined in the past, yet the mutual dynamics, in particular if matter and gravity are strongly coupled, are hardly explored, which is related to the definition of both matter and gravitational degrees of freedom on the discretisation. However extracting this mutual dynamics is crucial in testing the viability of the spin foam approach and also establishing connections to other discrete approaches such as lattice gauge theories.
Therefore, we introduce a simple 2D toy model for Yang--Mills coupled to spin foams, namely an Ising model coupled to so--called intertwiner models defined for SU(2)_k. The two systems are coupled by choosing the Ising coupling constant to depend on spin labels of the background, as these are interpreted as the edge lengths of the discretisation. We coarse grain this toy model via tensor network renormalization and uncover an interesting dynamics: the Ising phase transition temperature turns out to be sensitive to the background configurations and conversely, the Ising model can induce phase transitions in the background. Moreover, we observe a strong coupling of both systems if close to both phase transitions.
31 + 6 pages, 8 figures, 7 tables

http://arxiv.org/abs/1506.04984
Is there a quantum gravity effect on the cosmic microwave background power spectrum?
Donato Bini, Giampiero Esposito
(Submitted on 16 Jun 2015)
An assessment is made of recent attempts to evaluate how quantum gravity may affect the anisotropy spectrum of the cosmic microwave background. A perturbative scheme for the solution of the Wheeler-DeWitt equation has been found to allow for enhancement of power at large scales, whereas the alternative predicts a suppression of power at large scales. Both effects are corrections which, although conceptually interesting, turn out to be too small to be detected. Another scheme relies upon a Born-Oppenheimer analysis: by using a perturbative approach to the nonlinear ordinary differential equation obeyed by the two-point function for scalar fluctuations, a new family of power spectra have been obtained and studied by the authors.
4 pages, prepared for the AT1 Session of MG14 in Rome

http://arxiv.org/abs/1506.03814
Momentum density of spacetime and the gravitational dynamics
T. Padmanabhan
(Submitted on 11 Jun 2015)
I introduce a covariant four-vector G^a[v], which can be interpreted as the momentum density attributed to the spacetime geometry by an observer with velocity v^a, and describe its properties: (a) Demanding that the total momentum of matter plus geometry is conserved for all observers, leads to the gravitational field equations. Thus, how matter curves spacetime is entirely determined by this principle of momentum conservation. (b) The G^a[v] can be related to the gravitational Lagrangian in a manner similar to the usual definition of Hamiltonian in, say, classical mechanics. (c) Geodesic observers in a spacetime will find that the conserved total momentum vanishes on-shell. (d) The on-shell, conserved, total energy in a region of space, as measured by the comoving observers, will be equal to the total heat energy of the boundary surface. (e) The off-shell gravitational energy in a region will be the sum of the ADM energy in the bulk plus the thermal energy of the boundary. These results suggest that G^a[v] can be a useful physical quantity to probe the gravitational theories.
6 pages

http://arxiv.org/abs/1506.03829
A note on cutting spin networks and the area spectrum in loop quantum gravity
Yu Asato
(Submitted on 11 Jun 2015)
In this paper, I show that if a spin network is cut by a surface separating space-time into two regions, the sum of spins of edges crossing the surface must be an integer. This gives a restriction on the area spectrum of such surfaces, including black hole horizons, in loop quantum gravity.
5 pages, 1 figure

possible general interest:
http://arxiv.org/abs/1506.03975
The Hawking cascade from a black hole is extremely sparse
Finnian Gray (Victoria University of Wellington), Sebastian Schuster (Victoria University of Wellington), Alexander Van-Brunt (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 12 Jun 2015)
The Hawking flux from a black hole, (at least as seen from large distances), is extremely sparse and thin, with the average time between emission of successive Hawking quanta being hundreds of times larger than the natural timescale set by the energies of the emitted quanta. Some aspects of this result have been known for over 30 years, but have been largely forgotten, possibly because authors focussed mainly on the late-time high-temperature regime. We shall instead focus on the early-stage low-temperature regime, and shall both quantify and significantly extend these observations in a number of different ways.
First we shall identify several natural dimensionless figures of merit, and thereby compare the mean time between emission of successive Hawking quanta to several quite natural timescales that can be associated with the emitted quanta, demonstrating that ratios of 300 or more are typical for emission of photons or gravitons from a Schwarzschild black hole. Furthermore these ratios are independent of the mass of the black hole as it slowly evolves. The situation for fermion emission (massless neutrinos) is actually worse. Second, we shall then show that the situation for Reissner-Nordstrom, Kerr, Kerr-Newman and "dirty" black holes is even worse. Third, we consider the effects of particle rest mass. Overall, the Hawking quanta are seen to be dribbling out of the black hole one at a time, in an extremely slow cascade of 3-body decays. This implies that the Hawking flux is subject to "shot noise". Observationally, the Planck spectrum of the Hawking flux can only be determined by collecting and integrating data over a very long time. We conclude by connecting these points back to various kinematic aspects of the Hawking evaporation process.
15 pages

 

[marcus]

http://arxiv.org/abs/1506.05526
Ultraviolet Fixed Points in Conformal Gravity and General Quadratic Theories
Nobuyoshi Ohta, Roberto Percacci
(Submitted on 18 Jun 2015)
We study the beta functions for four-dimensional conformal gravity using two different parametrizations of metric fluctuation, linear split and exponential parametrization. We find that after imposing the traceless conditions, the beta functions are the same in four dimensions though the dependence on the dimensions are quite different. This indicates the universality of these results. We also examine the beta functions in general quadratic theory with the Einstein and cosmological terms for exponential parametrization, and find that it leads to results for beta functions of dimensionful couplings different from linear split, though the fact that there exists nontrivial fixed point remains the same and the fixed points also remain the same.
19 pages.

http://inspirehep.net/record/1376835?ln=en
Thermodynamics of spherically symmetric spacetimes in loop quantum gravity
http://inspirehep.net/author/profile/M%C3%A4kel%C3%A4%2C%20Jarmo?recid=1376835&ln=en (Vaasa Poly.)
Abstract (APS)
The choice of the area operator in loop quantum gravity is by no means unique. In addition to the area operator commonly used in loop quantum gravity there is also an area operator introduced by Krasnov in 1998, which gives uniformly spaced area spectra for the horizons of spacetime. Using Krasnov’s area operator we consider the thermodynamics of spherically symmetric spacetimes equipped with horizons in loop quantum gravity. Among other things, our approach implies, in a pretty simple manner, that every horizon of spacetime emits thermal radiation and possesses entropy which, in the natural units, is one-quarter of its area. When applied to the de Sitter spacetime loop quantum gravity provides an explanation both to the presence and the smallness of the cosmological constant.
18 pages Phys.Rev. D91 (2015) 124050

http://inspirehep.net/record/1376832?ln=en
Warm Chaplygin inflation in loop quantum cosmology in light of Planck data
http://inspirehep.net/author/profile/Setare%2C%20M.%E2%80%89R.?recid=1376832&ln=en (Kurdistan U.) , http://inspirehep.net/author/profile/Kamali%2C%20V.?recid=1376832&ln=en (Bou Ali Sina U.)
Abstract (APS)
The warm Chaplygin inflationary universe model in the context of the effective field theory of loop quantum cosmology (LQC) is studied. In the slow-roll phase of inflation primordial perturbation spectrums, tensor-to-scalar ratio and spectral indices of our model are presented. Using chaotic potential, we present the main characteristics of this model in great detail. The parameters of the model are restricted by recent observational results i.e. WMAP9 and Planck data.
8 pages. Phys.Rev. D91 (2015) 123517

http://arxiv.org/abs/1506.05531
A mirror moving in quantum vacuum of a massive scalar field
Qingdi Wang, William G. Unruh
(Submitted on 18 Jun 2015)
We present a mirror model moving in the quantum vacuum of a massive scalar field and study its motion under infinitely fluctuating quantum vacuum stress. The model is similar to the one in \cite{PhysRevD.89.085009}, but this time there is no divergent effective mass to weaken the effect of divergent vacuum energy density. We show that this kind of weakening is not necessary. The vacuum friction and strong anti-correlation property of the quantum vacuum are enough to confine the mirror's position fluctuations.
5 pages, 1 figure

http://arxiv.org/abs/1506.05597
Arbitrary Dimensional Loop Quantum Cosmology
Xiangdong Zhang
(Submitted on 18 Jun 2015)
Loop quantum cosmology(LQC) is the symmetric model of loop quantum gravity. In this paper, we generalize the structure of loop quantum cosmology to the theories with arbitrary spacetime dimensions. The isotropic and homogenous cosmological model in n+1 dimensions is quantized by the loop quantization method. Interestingly, we find that the underlying quantum theories are divided into two qualitatively different sectors according to spacetime dimensions. The effective Hamiltonian and modified dynamical equations of n+1 dimensional LQC are obtained. Moreover, our results indicate that the classical big bang singularity is resolved in arbitrary spacetime dimensions by a quantum bounce. We also briefly discuss the similarities and differences between the n+1 dimensional model and the 3+1 dimensional one.
14 pages.

http://arxiv.org/abs/1506.05739
Fix Immirzi parameter by quasinormal modes in four and higher spacetime dimensions
Xiangdong Zhang
(Submitted on 18 Jun 2015)
One parameter quantization ambiguity is existed in Loop quantum gravity which is called the Immirzi parameter. In this paper, we fix this free paremater by considering the quasinormal mode spectrum of black holes in four and higher spacetime dimensions. As a consequence, our result consistents with Bekenstein-Hawking entropy of a black hole. Moreover, we also give a possible quantum gravity explanation of the universal ln(3) behavior of the quasinormal mode spectrum.
6 pages

 

[marcus]

http://arxiv.org/abs/1506.08393
Renormalization of a tensorial field theory on the homogeneous space SU(2)/U(1)
Vincent Lahoche, Daniele Oriti
(Submitted on 28 Jun 2015)
We study the renormalization of a general field theory on the 2-sphere with tensorial interaction and gauge invariance under the diagonal action of SU(2). We derive the power counting for arbitrary dimension d. For the case d=4, we prove perturbative renormalizability to all orders via multi-scale analysis, study both the renormalised and effective perturbation series, and establish the asymptotic freedom of the model. We also outline a general power counting for the homogeneous space SO(D)/SO(D-1), of direct interest for quantum gravity models in general dimensions, and point out the obstructions to the direct generalisation of our results to these cases.
48 pages, 17 figures

http://arxiv.org/abs/1506.08383
4D covariance of holographic quantization of Einstein gravity
I. Y. Park
(Submitted on 28 Jun 2015)
It has been observed in [Park:2014tia] that the physical states of 4D Einstein gravity holographically reduce and can be described by a 3D language. We examine the 4D covariance issue in this work. While doing so, we solve the puzzle encountered in [Park:2014noa] in which the one-loop counterterms for the action expanded around a given background could not be expressed in terms of covariant quantities expanded around the same background (a flat background presently and in [Park:2014noa]. At intermediate steps, a renormalization program can be carried out covariantly to any loop order; it is only at the final stage that one should consider the physical external states. With the physical external states, the 1PI effective action reduces to 3D and renormalizability is restored just as in the entirely-3D approach. We revisit the one-loop two-point renormalization, and in particular outline one-loop renormalization of the Newton's constant.
25 pages, 5 figures

http://arxiv.org/abs/1506.07759
3d Lorentzian loop quantum gravity and the spinor approach
Florian Girelli, Giuseppe Sellaroli
(Submitted on 25 Jun 2015)
We consider the generalization of the "spinor approach" to the Lorentzian case, in the context of 3d loop quantum gravity with cosmological constant Λ=0. The key technical tool that allows this generalization is the recoupling theory between unitary infinite-dimensional representations and non-unitary finite-dimensional ones, obtained in the process of generalizing the Wigner-Eckart theorem to SU(1,1). We use SU(1,1) tensor operators to build observables and a solvable quantum Hamiltonian constraint, analogue of the one introduced by V. Bonzom and his collaborators in the Euclidean case (with both Λ=0 and Λ≠0). We show that the Lorentzian Ponzano-Regge amplitude is solution of the quantum Hamiltonian constraint by recovering the Biedenharn-Elliott relation (generalized to the case where unitary and non-unitary SU(1,1) representations are coupled to each other). Our formalism is sufficiently general that both the Lorentzian and the Euclidean case can be recovered (with Λ=0).
28 pages, 3 figures

http://arxiv.org/abs/1506.07484
Quantum Reduced Loop Gravity
Emanuele Alesci, Francesco Cianfrani
(Submitted on 24 Jun 2015)
Quantum Reduced Loop Gravity provides a promising framework for a consistent characterization of the early Universe dynamics. Inspired by BKL conjecture, a flat Universe is described as a collection of Bianchi I homogeneous patches. The resulting quantum dynamics is described by the scalar constraint operator, whose matrix elements can be analytically computed. The effective semiclassical dynamics is discussed, and the differences with Loop Quantum Cosmology are emphasized.
6 pages, proceedings of Frontiers of Fundamental Physics 14 - FFP14, 15-18 July 2014, Marseille, accepted for publication in Proceedings of Science

http://arxiv.org/abs/1506.07835
Quantum Reduced Loop Gravity: a realistic Universe
Emanuele Alesci, Francesco Cianfrani
(Submitted on 25 Jun 2015)
We describe the quantum flat universe in QRLG in terms of states based at cuboidal graphs with six-valent nodes. We investigate the action of the scalar constraint operator at each node and we construct proper semiclassical states. This allows us to discuss the semiclassical effective dynamics of the quantum universe, which resembles that of LQC. In particular, the regulator is identified with the third root of the inverse number of nodes within each homogeneous patch, while inverse-volume corrections are enhanced.
15 pages

http://arxiv.org/abs/1506.07016
Local Quantum Gravity
Nicolai Christiansen, Benjamin Knorr, Jan Meibohm, Jan M. Pawlowski, Manuel Reichert
(Submitted on 23 Jun 2015)
We investigate the ultraviolet behaviour of quantum gravity within a functional renormalisation group approach. The present setup includes the full ghost and graviton propagators and, for the first time, the dynamical graviton three-point function. The latter gives access to the coupling of dynamical gravitons and makes the system minimally self-consistent. The resulting phase diagram confirms the asymptotic safety scenario in quantum gravity with a non-trivial UV fixed point. A well-defined Wilsonian block spinning requires locality of the flow in momentum space. This property is discussed in the context of functional renormalisation group flows. We show that momentum locality of graviton correlation functions is non-trivially linked to diffeomorphism invariance, and is realized in the present setup.
5 pages, 3 figures

http://arxiv.org/abs/1506.06839
Causal Dynamical Triangulations in the Spincube Model of Quantum Gravity
Marko Vojinovic
(Submitted on 23 Jun 2015)
We study the implications of the simplicity constraint in the spincube model of quantum gravity. Relating the edge-lengths to integer triangle areas, the simplicity constraint imposes a very strong restrictions between them, ultimately leading to a requirement that all 4-simplices in the triangulation must be almost mutually identical. As a surprising and unexpected consequence of this property, one can obtain the CDT state sum as a special case of the spincube state sum. This relationship brings new insight into the long-standing problem of the relationship between the spinfoam approach and the CDT approach to quantum gravity. In particular, it turns out that the spincube model contains properties of both approaches, providing a single unifying framework for their analysis and comparison. In addition, the spincube state sum also contains some other special cases, very similar but not equivalent to the CDT state sum.
12 pages

possible interest:
http://arxiv.org/abs/1506.07576
Towards a Grand Unified Theory of Mathematics and Physics
Peter Woit
(Submitted on 24 Jun 2015)
Wigner's "unreasonable effectiveness of mathematics" in physics can be understood as a reflection of a deep and unexpected unity between the fundamental structures of mathematics and of physics. Some of the history of evidence for this is reviewed, emphasizing developments since Wigner's time and still poorly understood analogies between number theory and quantum field theory.
9 pages. Submitted to a 2015 FQXi essay contest

http://arxiv.org/abs/1506.07306
Fine Tuning May Not Be Enough
S. P. Miao (National Cheng Kung University, Taiwan), R. P. Woodard (University of Florida, USA)
(Submitted on 24 Jun 2015)
We argue that the fine tuning problems of scalar-driven inflation may be worse than is commonly believed. The reason is that reheating requires the inflaton to be coupled to other matter fields whose vacuum fluctuations alter the inflaton potential. The usual response has been that even more fine-tuning of the classical potential V(φ) can repair any damage done in this way. We point out that the effective potential in de Sitter background actually depends in a complicated way upon the dimensionless combination of φ/H. We also show that the factors of H which occur in de Sitter do not even correspond to local functionals of the metric for general geometries, nor are they Planck-suppressed.
15 pages,

http://arxiv.org/abs/1506.06152
Asymptotics with a positive cosmological constant: II. Linear fields on de Sitter space-time
Abhay Ashtekar, Béatrice Bonga, Aruna Kesavan
(Submitted on 19 Jun 2015)
Linearized gravitational waves in de Sitter space-time are analyzed in detail to obtain guidance for constructing the theory of gravitational radiation in presence of a positive cosmological constant in full, nonlinear general relativity. Specifically:
i) In the exact theory, the intrinsic geometry of $\mathcal{I}$ is often assumed to be conformally flat in order to reduce the asymptotic symmetry group from Diff($\mathcal{I}$) to the de Sitter group. Our results show explicitly that this condition is physically unreasonable;
ii) We obtain expressions of energy-momentum and angular momentum fluxes carried by gravitational waves in terms of fields defined at $\mathcal{I}^+$;
iii) We argue that, although energy of linearized gravitational waves can be arbitrarily negative in general, gravitational waves emitted by physically reasonable sources carry positive energy; and, finally
iv) We demonstrate that the flux formulas reduce to the familiar ones in Minkowski space-time in spite of the fact that the limit Λ→0 is discontinuous (since, in particular, $\mathcal{I}$ changes its space-like character to null in the limit).
21 pages, 4 figures

briefly noted:
http://arxiv.org/abs/1506.06749
http://arxiv.org/abs/1506.07001

 

[atyy]

http://arxiv.org/abs/1506.07795
Scaling Exponents for Lattice Quantum Gravity in Four Dimensions
Herbert W. Hamber
(Submitted on 25 Jun 2015)
In this work nonperturbative aspects of quantum gravity are investigated using the lattice formulation, and some new results are presented for critical exponents, amplitudes and invariant correlation functions. Values for the universal scaling dimensions are compared with other nonperturbative approaches to gravity in four dimensions, and specifically to the conjectured value for the universal critical exponent ν=1/3. It is found that the lattice results are generally consistent with gravitational anti-screening, which would imply a slow increase in the strength of the gravitational coupling with distance, and here detailed estimates for exponents and amplitudes characterizing this slow rise are presented. Furthermore, it is shown that in the lattice approach (as for gauge theories) the quantum theory is highly constrained, and eventually by virtue of scaling depends on a rather small set of physical parameters. Arguments are given in support of the statement that the fundamental reference scale for the growth of the gravitational coupling G with distance is represented by the observed scaled cosmological constant λ, which in gravity acts as an effective nonperturbative infrared cutoff. In the vacuum condensate picture a fundamental relationship emerges between the scale characterizing the running of G at large distances, the macroscopic scale for the curvature as described by the observed cosmological constant, and the behavior of invariant gravitational correlation functions at large distances. Overall, the lattice results suggest that the infrared slow growth of G with distance should become observable only on very large distance scales, comparable to λ. It is hoped that future high precision satellite experiments will possibly come within reach of this small quantum correction, as suggested by a vacuum condensate picture of quantum gravity.

 

[marcus]

http://arxiv.org/abs/1506.08571
A new realization of quantum geometry
Benjamin Bahr, Bianca Dittrich, Marc Geiller
(Submitted on 29 Jun 2015)
We construct in this article a new realization of quantum geometry, which is obtained by quantizing the recently-introduced flux formulation of loop quantum gravity. In this framework, the vacuum is peaked on flat connections, and states are built upon it by creating local curvature excitations. The inner product induces a discrete topology on the gauge group, which turns out to be an essential ingredient for the construction of a continuum limit Hilbert space. This leads to a representation of the full holonomy-flux algebra of loop quantum gravity which is unitarily-inequivalent to the one based on the Ashtekar-Isham-Lewandowski vacuum. It therefore provides a new notion of quantum geometry. We discuss how the spectra of geometric operators, including holonomy and area operators, are affected by this new quantization. In particular, we find that the area operator is bounded, and that there are two different ways in which the Barbero-Immirzi parameter can be taken into account. The methods introduced in this work open up new possibilities for investigating further realizations of quantum geometry based on different vacua.
72 pages, 6 figures

http://arxiv.org/abs/1506.08579
Quantum reduced loop gravity: extension to scalar field
Jakub Bilski, Emanuele Alesci, Francesco Cianfrani
(Submitted on 29 Jun 2015)
The quantization of the Hamiltonian for a scalar field is performed in the framework of Quantum Reduced Loop Gravity. We outline how the regularization can be performed by using the analogous tools adopted in full Loop Quantum Gravity and the matrix elements of the resulting operator between basis states are analytic coefficients. These achievements open the way for a consistent analysis of the Quantum Gravity corrections to the classical dynamics of gravity in the presence of a scalar field in a cosmological setting.
14 pages

http://arxiv.org/abs/1506.08273
Bounce Loop Quantum Cosmology Corrected Gauss-Bonnet Gravity
J. Haro, A.N. Makarenko, A.N. Myagky, S. D. Odintsov, V.K. Oikonomou
(Submitted on 27 Jun 2015)
We develop a Gauss-Bonnet extension of Loop Quantum Cosmology, by introducing holonomy corrections in modified F(G) theories of gravity. Within the context of our formalism, we provide a perturbative expansion in the critical density, a parameter characteristic of Loop Quantum Gravity theories, and we result in having leading order corrections to the classical F(G) theories of gravity. After extensively discussing the formalism, we present a reconstruction method that makes possible to find the Loop Quantum Cosmology corrected F(G) theory that can realize various cosmological scenarios. Specifically, we studied exponential and power-law bouncing cosmologies, emphasizing on the behavior near the bouncing point and in some cases, the behavior for all the values of the cosmic time is obtained. We exemplify our theoretical constructions by using bouncing cosmologies, and we investigate which Loop Quantum Cosmology corrected Gauss-Bonnet modified gravities can successfully realize such cosmologies
24 pages.

http://arxiv.org/abs/1506.08775
Dimensional reduction in causal set gravity
S. Carlip
(Submitted on 29 Jun 2015)
Results from a number of different approaches to quantum gravity suggest that the effective dimension of spacetime may drop to d=2 at small scales. I show that two different dimensional estimators in causal set theory display the same behavior, and argue that a third, the spectral dimension, may exhibit a related phenomenon of "asymptotic silence."
6+1 pages

http://arxiv.org/abs/1506.08794
No fermion doubling in quantum geometry
Rodolfo Gambini, Jorge Pullin
(Submitted on 29 Jun 2015)
In loop quantum gravity the discrete nature of quantum geometry acts as a natural regulator for matter theories. Studies of quantum field theory in quantum space-times in spherical symmetry in the canonical approach have shown that the main effect of the quantum geometry is to discretize the equations of matter fields. This raises the possibility that in the case of fermion fields one could confront the usual fermion doubling problem that arises in lattice gauge theories. We suggest, again based on recent results on spherical symmetry, that since the background space-times will generically involve superpositions of states associated with different discretizations the phenomenon may not arise. This opens a possibility of incorporating chiral fermions in the framework of loop quantum gravity.
2 page

http://arxiv.org/abs/1506.08015
Loop quantization of the Schwarzschild interior revisited
Alejandro Corichi, Parampreet Singh
(Submitted on 26 Jun 2015)
The loop quantization of the Schwarzschild interior region, as described by a homogenous anisotropic Kantowski-Sachs model, is re-examined. As several studies of different --inequivalent-- loop quantizations have shown, to date there exists no fully satisfactory quantum theory for this model. This fact poses challenges to the validity of some scenarios to address the black hole information problem. Here we put forward a novel viewpoint to construct the quantum theory that builds from some of the models available in the literature. The final picture is a quantum theory that is both independent of any auxiliary structure and possesses a correct low curvature limit. It represents a subtle but non-trivial modification of the original prescription given by Ashtekar and Bojowald. It is shown that the quantum gravitational constraint is well defined past the singularity and that its effective dynamics possesses a bounce into an expanding regime. The classical singularity is avoided, and a semiclassical spacetime satisfying vacuum Einstein's equations is recovered on the "other side" of the bounce. We argue that such metric represents the interior region of a white-hole spacetime, but for which the corresponding "white-hole mass" differs from the original black hole mass. Furthermore, we find that the value of the white-hole mass is proportional to the third power of the starting black hole mass. We discuss possible implications of this phenomena.
12 pages, 3 figures

http://arxiv.org/abs/1506.08067
Braids as a representation space of SU(5)
Daniel Cartin
(Submitted on 23 Jun 2015)
The Standard Model of particle physics provides very accurate predictions of phenomena occurring at the sub-atomic level, but the reason for the choice of symmetry group and the large number of particles considered elementary, is still unknown. Along the lines of previous preon models positing a substructure to explain these aspects, Bilson-Thompson showed how the first family of elementary particles is realized as the crossings of braids made of three strands, with charges resulting from twists of those strands with certain conditions; in this topological model, there are only two distinct neutrino states. Modeling the particles as braids implies these braids must be the representation space of a Lie algebra, giving the symmetries of the Standard Model. In this paper, this representation is made explicit, obtaining the raising operators associated with the Lie algebra of SU(5), one of the earliest grand unified theories. Because the braids form a group, the action of these operators are braids themselves, leading to their identification as gauge bosons. Possible choices for the other two families are also given. Although this realization of particles as braids is lacking a dynamical framework, it is very suggestive, especially when considered as a natural method of adding matter to loop quantum gravity.
9 pages, 7 figures

http://arxiv.org/abs/1506.08068
Quantum and classical areas of black hole thermodynamics
A. Ghosh, P. Mitra
(Submitted on 23 Jun 2015)
Most calculations of black hole entropy in loop quantum gravity indicate a term proportional to the area eigenvalue A with a correction involving the logarithm of A. This violates the additivity of the entropy. An entropy proportional to A, with a correction term involving the logarithm of the classical area k, which is consistent with the additivity of entropy, is derived in both U(1) and SU(2) formulations.
7 pages; to appear in Classical and Quantum Gravity

http://arxiv.org/abs/1506.08094
Thermodynamics of Spherically Symmetric Spacetimes in Loop Quantum Gravity
Jarmo Mäkelä
(Submitted on 18 Jun 2015)
The choice of the area operator in loop quantum gravity is by no means unique. In addition to the area operator commonly used in loop quantum gravity there is also an area operator introduced by Krasnov in 1998, which gives uniformly spaced area spectra for the horizons of spacetime. Using Krasnov's area operator we consider the thermodynamics of spherically symmetric spacetimes equipped with horizons in loop quantum gravity. Among other things, our approach implies, in a pretty simple manner, that every horizon of spacetime emits thermal radiation and possesses entropy which, in the natural units, is one-quarter of its area. When applied to the de Sitter spacetime loop quantum gravity provides an explanation both to the presence and the smallness of the cosmological constant.
26 pages. Published in Physical Review D. Comments welcome

applicable to LQG:
http://arxiv.org/abs/1506.08613
Stable coherent states
Antonia Zipfel, Thomas Thiemann
(Submitted on 29 Jun 2015)
We analyze the stability under time evolution of complexifier coherent states (CCS) in one-dimensional mechanical systems. A system of coherent states is called stable if it evolves into another coherent state. It turns out that a system can only poses stable CCS if the classical evolution of the variable for a given complexifier C depends only on z itself and not on its complex conjugate. This condition is very restrictive in general so that only few systems exist that obey this condition. However, it is possible to access a wider class of models that in principle may allow for stable coherent states associated to certain regions in the phase space by introducing action-angle coordinates.

not LQG related but possibly of broader interest:
http://arxiv.org/abs/1506.08073
Lie Group Cosmology
A. Garrett Lisi
(Submitted on 24 Jun 2015)
Our universe is a deforming Lie group.
42 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1506.09164
General Relativity in the radial gauge I. Reduced phase space and canonical structure
Norbert Bodendorfer, Jerzy Lewandowski, Jedrzej Świeżewski
(Submitted on 30 Jun 2015)
Firstly, we present a reformulation of the standard canonical approach to spherically symmetric systems in which the radial gauge is imposed. This is done via the gauge unfixing techniques, which serves as their exposition in the context of the radial gauge. Secondly, we apply the same techniques to the full theory, without assuming spherical symmetry, resulting in a reduced phase space description of General Relativity. The canonical structure of the theory is analysed.
In a companion paper a quantization of the reduced phase space is presented. The construction is well suited for the treatment of spherically symmetric situations and allows for a quantum definition thereof.
26 pages, 1 figure

[related to this LQG paper by the same authors:
http://arxiv.org/abs/1410.5609
A quantum reduction to spherical symmetry in loop quantum gravity
Norbert Bodendorfer, Jerzy Lewandowski, Jedrzej Świeżewski
(Submitted on 21 Oct 2014 (v1), last revised 27 May 2015 (this version, v2))
5 pages.]

possible wider interest:
http://arxiv.org/abs/1506.09143
Inflation, evidence and falsifiability
Giulia Gubitosi, Macarena Lagos, Joao Magueijo, Rupert Allison
(Submitted on 30 Jun 2015)
==================JULY PAPERS===============
http://arxiv.org/abs/1507.00226
Cyclic universe from Loop Quantum Gravity
Francesco Cianfrani, Jerzy Kowalski-Glikman, Giacomo Rosati
(Submitted on 1 Jul 2015)
We discuss how a cyclic model for the flat universe can be constructively derived from Loop Quantum Gravity. This model has a lower bounce, at small values of the scale factor, which shares many similarities with that of Loop Quantum Cosmology. We find that quantum gravity corrections can be also relevant at energy densities much smaller than the Planckian one and that they can induce an upper bounce at large values of the scale factor.
4 pages

http://arxiv.org/abs/1507.00329
Covariance in models of loop quantum gravity: Spherical symmetry
Martin Bojowald, Suddhasattwa Brahma, Juan D. Reyes
(Submitted on 1 Jul 2015)
Spherically symmetric models of loop quantum gravity have been studied recently by different methods that aim to deal with structure functions in the usual constraint algebra of gravitational systems. As noticed by Gambini and Pullin, a linear redefinition of the constraints (with phase-space dependent coefficients) can be used to eliminate structure functions, even Abelianizing the more-difficult part of the constraint algebra. The Abelianized constraints can then easily be quantized or modified by putative quantum effects. As pointed out here, however, the method does not automatically provide a covariant quantization, defined as an anomaly-free quantum theory with a classical limit in which the usual (off-shell) gauge structure of hypersurface deformations in space-time appears. The holonomy-modified vacuum theory based on Abelianization is covariant in this sense, but matter theories with local degrees of freedom are not. Detailed demonstrations of these statements show complete agreement with results of canonical effective methods applied earlier to the same systems (including signature change).
29 pages

http://arxiv.org/abs/1507.00330
Spectral Dimension from Causal Set Nonlocal Dynamics
Alessio Belenchia, Dionigi M.T. Benincasa, Antonino Marciano, Leonardo Modesto
(Submitted on 1 Jul 2015)
We investigate the spectral dimension obtained from non-local continuum d'Alembertians derived from causal sets. We find a universal dimensional reduction to 2 dimensions, in all dimensions. We conclude by discussing the validity and relevance of our results within the broader context of quantum field theories based on these nonlocal dynamics.
9 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1507.01229
Dynamics of the cosmological and Newton's constant
Lee Smolin
(Submitted on 5 Jul 2015)
A modification of general relativity is presented in which Newton's constant and the cosmological constant become a conjugate pair of dynamical variables.
18 pages, For a related talk see this http URL

http://arxiv.org/abs/1507.01149
Loop quantum gravity coupled to a scalar field
Jerzy Lewandowski, Hanno Sahlmann
(Submitted on 4 Jul 2015)
We reconsider the Rovelli-Smolin model of gravity coupled to the Klein-Gordon time field with an eye towards capturing the degrees of freedom of the scalar field lost in the framework in which time is deparametrized by the scalar field. Several new results for loop quantum gravity are obtained:
(i) a Hilbert space for the gravity-matter system and a non-standard representation of the scalar field thereon is constructed, (ii) a new operator for the scalar constraint of the coupled system is defined and investigated, (iii) methods for solving the constraint are developed.
Commutators of the new constraint do not vanish, but seem to reproduce a part of the Dirac algebra. This, however, poses problems for finding solutions. Hence the states we consider -- and perhaps the whole setup -- still needs some improvement.
As a side result we describe a representation of the gravitational degrees of freedom in which the flux is diagonal. This representation bears a strong resemblance to the BF vacuum of Dittrich and Geiller.
18 pages

http://arxiv.org/abs/1507.01153
Coherent State Operators in Loop Quantum Gravity
Emanuele Alesci, Andrea Dapor, Jerzy Lewandowski, Ilkka Makinen, Jan Sikorski
(Submitted on 4 Jul 2015)
We present a new method for constructing operators in loop quantum gravity. The construction is an application of the general idea of "coherent state quantization", which allows one to associate a unique quantum operator to every function on a classical phase space. Using the heat kernel coherent states of Hall and Thiemann, we show how to construct operators corresponding to functions depending on holonomies and fluxes associated to a fixed graph. We construct the coherent state versions of the fundamental holonomy and flux operators, as well as the basic geometric operators of area, angle and volume. Our calculations show that the corresponding canonical operators are recovered from the coherent state operators in the limit of large spins.
34 pages, 4 figures (plus many pretty diagrams)

http://arxiv.org/abs/1507.01232
Fermion Doubling in Loop Quantum Gravity
Jacob Barnett, Lee Smolin
(Submitted on 5 Jul 2015)
In this paper, we show that the Hamiltonian approach to loop quantum gravity has a fermion doubling problem. To obtain this result, we couple loop quantum gravity to a free massless scalar and a chiral fermion field, gauge fixing the many fingered time gauge invariance by interpreting the scalar field as a physical clock. We expand around a quantum gravity state based on a regular lattice and consider the limit where the bare cosmological constant is large but the fermonic excitations have energies low in Planck units. We then make the case for identifying the energy spectrum in this approximation with that of a model of lattice fermion theory which is known to double.
20 pages.

[However see:
http://arxiv.org/abs/1506.08794
No fermion doubling in quantum geometry
Rodolfo Gambini, Jorge Pullin
(Submitted on 29 Jun 2015)
In loop quantum gravity the discrete nature of quantum geometry acts as a natural regulator for matter theories. Studies of quantum field theory in quantum space-times in spherical symmetry in the canonical approach have shown that the main effect of the quantum geometry is to discretize the equations of matter fields. This raises the possibility that in the case of fermion fields one could confront the usual fermion doubling problem that arises in lattice gauge theories. We suggest, again based on recent results on spherical symmetry, that since the background space-times will generically involve superpositions of states associated with different discretizations the phenomenon may not arise. This opens a possibility of incorporating chiral fermions in the framework of loop quantum gravity.
2 page]

http://arxiv.org/abs/1507.00851
Ashtekar-Barbero holonomy on the hyperboloid: Immirzi parameter as a Cut-off for Quantum Gravity
Christoph Charles, Etera R. Livine
(Submitted on 3 Jul 2015)
In the context of the geometrical interpretation of the spin network states of Loop Quantum Gravity, we look at the holonomies of the Ashtekar-Barbero connection on loops embedded in space-like hyperboloids. We use this simple setting to illustrate two points. First, the Ashtekar-Barbero connection is not a space-time connection, its holonomies depend on the spacetime embedding of the canonical hypersurface. This fact is usually interpreted as an inconvenience, but we use it to extract the extrinsic curvature from the holonomy and separate it from the 3d intrinsic curvature. Second, we show the limitations of this reconstruction procedure, due to a periodicity of the holonomy in the Immirzi parameter, which underlines the role of a real Immirzi parameter as a cut-off for general relativity at the quantum level in contrast with its role of a mere coupling constant at the classical level.
8 pages

http://arxiv.org/abs/1507.00986
New Hamiltonian constraint operator for loop quantum gravity
Jinsong Yang, Yongge Ma
(Submitted on 3 Jul 2015)
A new symmetric Hamiltonian constraint operator is proposed for loop quantum gravity, which is well defined in the Hilbert space of diffeomorphism invariant states up to non-planar vertices. On one hand, it inherits the advantage of the original regularization method, so that its regulated version in the kinematical Hilbert space is diffeomorphism covariant and creates new vertices to the spin networks. On the other hand, it overcomes the problem in the original treatment, so that there is less ambiguity in its construction and its quantum algebra is anomaly-free in a suitable sense. The regularization procedure for the Hamiltonian constraint operator can also be applied to the symmetric model of loop quantum cosmology, which leads to a new quantum dynamics of the cosmological model.
5 pages

http://arxiv.org/abs/1507.01054
Black hole entropy and Lorentz-diffeomorphism Noether charge
Ted Jacobson, Arif Mohd
(Submitted on 4 Jul 2015)
We show that, in the first or second order orthonormal frame formalism, black hole entropy is the horizon Noether charge for a combination of diffeomorphism and local Lorentz symmetry involving the Lie derivative of the frame. The Noether charge for diffeomorphisms alone is unsuitable, since a regular frame cannot be invariant under the flow of the Killing field at the bifurcation surface. We apply this formalism to Lagrangians polynomial in wedge products of the frame field 1-form and curvature 2-form, including general relativity, Lovelock gravity, and "topological" terms in four dimensions.
8 pages.

http://arxiv.org/abs/1507.01567
Entanglement entropy of squeezed vacua on a lattice
Eugenio Bianchi, Lucas Hackl, Nelson Yokomizo
(Submitted on 6 Jul 2015)
We derive a formula for the entanglement entropy of squeezed states on a lattice in terms of the complex structure J. The analysis involves the identification of squeezed states with group-theoretical coherent states of the symplectic group and the relation between the coset Sp(2N,R)/Isot(J_0) and the space of complex structures. We present two applications of the new formula: (i) we derive the area law for the ground state of a scalar field on a generic lattice in the limit of small speed of sound, (ii) we compute the rate of growth of the entanglement entropy in the presence of an instability and show that it is bounded from above by the Kolmogorov-Sinai rate.
35 pages, 2 figures

briefly noted:
http://arxiv.org/abs/1507.01588
A reasonable thing that just might work
Daniel Rohrlich
(Submitted on 6 Jul 2015)
In 1964, John Bell proved that quantum mechanics is "unreasonable" (to use Einstein's term): there are nonlocal bipartite quantum correlations. But they are not the most nonlocal bipartite correlations consistent with relativistic causality ("no superluminal signalling"): also maximally nonlocal "superquantum" (or "PR-box") correlations are consistent with relativistic causality. I show that---unlike quantum correlations---these correlations do not have a classical limit consistent with relativistic causality. The generalization of this result to all stronger-than-quantum nonlocal correlations is a derivation of Tsirelson's bound---a theorem of quantum mechanics---from the three axioms of relativistic causality, nonlocality, and the existence of a classical limit. But is it reasonable to derive (a part of) quantum mechanics from the unreasonable axiom of nonlocality?! I consider replacing the nonlocality axiom with an equivalent axiom that even Bell and Einstein might have considered reasonable: an axiom of local retrocausality.
14 pages, 2 figures. To appear in Quantum Nonlocality and Reality: 50 Years of Bell's theorem, eds. S. Gao and M. Bell (Cambridge U. Press), 2015, in press
http://inspirehep.net/author/profile/D.Rohrlich.1

http://arxiv.org/abs/1507.01583
Taking the Universe's Temperature with PIXIE
J. Colin Hill, Nick Battaglia, Jens Chluba, Simone Ferraro, Emmanuel Schaan, David N. Spergel
(Submitted on 6 Jul 2015)
The cosmic microwave background (CMB) energy spectrum is a near-perfect blackbody. The standard model of cosmology predicts small spectral distortions to this form, but no such distortion of the sky-averaged CMB spectrum has yet been measured. We calculate the largest expected distortion, which arises from the inverse Compton scattering of CMB photons off hot, ionized electrons in the universe, known as the thermal Sunyaev-Zel'dovich (tSZ) effect...but will be detected at enormous significance (≳1000σ) by the proposed Primordial Inflation Explorer (PIXIE)... PIXIE will thus determine the global thermodynamic properties of ionized gas in the universe with unprecedented precision. These measurements will impose a fundamental "integral constraint" on models of galaxy formation and the injection of feedback energy over cosmic time.
5 pages, 2 figures, comments welcome

http://arxiv.org/abs/1507.00968
A flow equation for f(R) gravity and some of its exact solutions
Nobuyoshi Ohta, Roberto Percacci, Gian Paolo Vacca
(Submitted on 3 Jul 2015)
We write a Renormalization Group (RG) equation for the function f in a theory of gravity in the f(R) truncation...
5 pages.

http://arxiv.org/abs/1507.01281
Inflation and Dirac [Operator] in the Causal Set Approach to Discrete Quantum Gravity
Stan Gudder
(Submitted on 5 Jul 2015)
In this approach to discrete quantum gravity the basic structural element is a covariant causal set (c-causet). The geometry of a c-causet is described by a shell-sequence that determines the discrete gravity of a universe. In this growth model, universes evolve in discrete time by adding new vertices to their generating c-causet. We first describe an inflationary period that is common to all universes. After this very brief cycle, the model enters a multiverse period in which the system diverges in various ways forming paths of c-causets. At the beginning of the multiverse period, the structure of a four-dimensional discrete manifold emerges and quantum mechanics enters the picture. A natural Hilbert space is defined and a discrete, free Dirac operator is introduced. We determine the eigenvalues and eigenvectors of this operator. Finally, we propose values for coupling constants that determine multiverse probabilities. These probabilities predict the dominance of pulsating universes.
23 pages

http://arxiv.org/abs/1507.00727
A Cyclic Universe Approach to Fine Tuning
Stephon Alexander, Sam Cormack, Marcelo Gleiser

 

[marcus]

It turns out that there is to be a special issue of Modern Physics Letters A (July 2015) devoted to time variation of fundamental constants. The following is just an overview and introduction to the collection of articles, by the guest editor. However it and the other articles referred to could possibly be of wider interest:
http://arxiv.org/abs/1507.02229
Fundamental Constants in Physics and Their Time Variation
Joan Solà
(Submitted on 8 Jul 2015)
There is no doubt that the field of Fundamental Constants in Physics and Their Time Variation is one of the hottest subjects in modern theoretical and experimental physics, with potential implications in all fundamental areas of physics research, such as particle physics, gravitation, astrophysics and cosmology. In this Special Issue, the state-of-the-art in the field is presented in detail.
6 pages. Preface to the Special Issue on Fundamental Constants in Physics and Their Time Variation

[6] X. Calmet and M. Keller, Cosmological Evolution of Fundamental Constants: From Theory to Experiment, Mod. Phys. Lett. A30 (2015) 1540028 [13 pages].
http://arxiv.org/abs/1410.2765

[17] J. D. Barrow, J. Magueijo, Local Varying-Alpha Theories, Mod. Phys. Lett. A30 (2015) 1540029 [16 pages].
http://arxiv.org/abs/1412.3278

[18] B.F.L. Ward, Running of the cosmological constant and estimate of its value in quantum general relativity, Mod. Phys. Lett. A30 (2015) 1540030 [15 pages].
http://arxiv.org/abs/1412.7417

[19] S. Basilakos, Cosmic expansion and structure formation in running vacuum cosmologies, Mod. Phys. Lett. A30 (2015) 1540031 [17 pages].
http://arxiv.org/abs/1501.03749

[20] S. Capozziello, G. Lambiase, Propagation of quantum particles in BransDicke spacetime: The case of gamma ray bursts, Mod. Phys. Lett. A30 (2015) 1540032 [13 pages].
http://arxiv.org/abs/1501.05429

[21] S. D. Bass, Vacuum energy and the cosmological constant, Mod. Phys. Lett. A30 (2015) 1540033 [15 pages].
http://arxiv.org/abs/1503.05483
Vacuum energy and the cosmological constant
Steven D. Bass
(Submitted on 18 Mar 2015)
The accelerating expansion of the Universe points to a small positive value for the cosmological constant or vacuum energy density. We discuss recent ideas that the cosmological constant plus LHC results might hint at critical phenomena near the Planck scale.
Comments: 15 pages, Contributed paper to the Special Issue "Fundamental Constants in Physics and Their Time Variation" (Modern Physics Letters A, Guest Editor Joan Sol\`a)

[22] H. Fritzsch, J. Sola`, Fundamental constants and cosmic vacuum: The micro and macro connection, Mod. Phys. Lett. A30 (2015) 1540034 [16 pages].
http://arxiv.org/abs/1502.01411

 

[marcus]

http://arxiv.org/abs/1507.02573
Quantum gravity at the corner
Laurent Freidel, Alejandro Perez
(Submitted on 9 Jul 2015)
We investigate the quantum geometry of 2d surface S bounding the Cauchy slices of 4d gravitational system. We investigate in detail and for the first time the symplectic current that naturally arises boundary term in the first order formulation of general relativity in terms of the Ashtekar-Barbero connection. This current is proportional to the simplest quadratic form constructed out of the triad field, pulled back on S. We show that the would-be-gauge degrees of freedom---arising from SU(2) gauge transformations plus diffeomorphisms tangent to the boundary, are entirely described by the boundary 2-dimensional symplectic form and give rise to a representation at each point of S of SL(2,ℝ)×SU(2). Independently of the connection with gravity, this system is very simple and rich at the quantum level with possible connections with conformal field theory in 2d. A direct application of the quantum theory is modelling of the black horizons in quantum gravity.
16 pages, 3 figures.

http://arxiv.org/abs/1507.03875
The anamorphic universe
Anna Ijjas, Paul J. Steinhardt
(Submitted on 14 Jul 2015)
We introduce "anamorphic" cosmology, an approach for explaining the smoothness and flatness of the universe on large scales and the generation of a nearly scale-invariant spectrum of adiabatic density perturbations. The defining feature is a smoothing phase that acts like a contracting universe based on some Weyl frame-invariant criteria and an expanding universe based on other frame-invariant criteria. An advantage of the contracting aspects is that it is possible to avoid the multiverse and measure problems that arise in inflationary models. Unlike ekpyrotic models, anamorphic models can be constructed using only a single field and can generate a nearly scale-invariant spectrum of tensor perturbations. Anamorphic models also differ from pre-big bang and matter bounce models that do not explain the smoothness. We present some examples of cosmological models that incorporate an anamorphic smoothing phase.
Comments: 35 pages, 3 figures, 1 table

briefly mentioned:

http://arxiv.org/abs/1507.03592
How to Recover a Qubit That Has Fallen Into a Black Hole
Aidan Chatwin-Davies, Adam S. Jermyn, Sean M. Carroll
(Submitted on 13 Jul 2015)
We demonstrate an algorithm for the retrieval of a qubit, encoded in spin angular momentum, that has been dropped into a no-firewall unitary black hole. Retrieval is achieved analogously to quantum teleportation by collecting Hawking radiation and performing measurements on the black hole. Importantly, these methods only require the ability to perform measurements from outside the event horizon and to collect the Hawking radiation emitted after the state of interest is dropped into the black hole.
5 pages

http://arxiv.org/abs/1507.02287
The Typical-State Paradox: Diagnosing Horizons with Complexity
Leonard Susskind
18 pages, 7 figures

 

[marcus]

http://arxiv.org/abs/1507.04703
Loop quantum cosmology, non-Gaussianity, and CMB power asymmetry
Ivan Agullo
(Submitted on 16 Jul 2015)
We argue that the anomalous power asymmetry observed in the cosmic microwave background (CMB) may have originated in a cosmic bounce preceding inflation. In loop quantum cosmology (LQC) the big bang singularity is generically replaced by a bounce due to quantum gravitational effects. We compute the spectrum of inflationary non-Gaussianity and show that strong correlation between observable scales and modes with longer (super-horizon) wavelength arise as a consequence of the evolution of perturbations across the LQC bounce. These correlations are strongly scale dependent and induce a dipole-dominated modulation on large angular scales in the CMB, in agreement with observations.
7 pages, 3 figure

http://arxiv.org/abs/1507.04566
Locally Causal Dynamical Triangulations in Two Dimensions
Renate Loll, Ben Ruijl
(Submitted on 16 Jul 2015)
We analyze the universal properties of a new two-dimensional quantum gravity model defined in terms of Locally Causal Dynamical Triangulations (LCDT). Measuring the Hausdorff and spectral dimensions of the dynamical geometrical ensemble, we find numerical evidence that the continuum limit of the model lies in a new universality class of two-dimensional quantum gravity theories, inequivalent to both Euclidean and Causal Dynamical Triangulations.
34 pages, 21 figures.

http://arxiv.org/abs/1507.04810
Curvature and Quantum Mechanics on Covariant Causal Sets
Stanley Gudder
(Submitted on 17 Jul 2015)
This article begins by reviewing the causal set approach in discrete quantum gravity. In our version of this approach a special role is played by covariant causal sets which we call c-causets. The importance of c-causets is that they support the concepts of a natural distance function, geodesics and curvature in a discrete setting. We then discuss curvature in more detail. By considering c-causets with a maximum and minimum number of paths, we are able to find c-causets with large and small average curvature. We then briefly discuss our previous work on the inflationary period when the curvature was essentially zero. Quantum mechanics on c-causets is considered next. We first introduce a free wave equation for c-causets. We then show how the state of a particle with a specified mass (or energy) can be derived from the wave equation. It is demonstrated for small examples that quantum mechanics predicts that particles tend to move toward vertices with larger curvature.
19 pages

http://arxiv.org/abs/1507.04742
Conflation: a new type of accelerated expansion
Angelika Fertig, Jean-Luc Lehners, Enno Mallwitz
(Submitted on 16 Jul 2015)
In the framework of scaler-tensor theories of gravity, we construct a new kind of cosmological model that conflates inflation and ekpyrosis. During a phase of conflation, the universe undergoes accelerated expansion, but with crucial differences compared to ordinary inflation. In particular, the potential energy is negative, which is of interest for supergravity and string theory where both negative potentials and the required scalar-tensor couplings are rather natural. A distinguishing feature of the model is that it does not amplify adiabatic scalar and tensor fluctuations, and in particular does not lead to eternal inflation and the associated infinities. We also show how density fluctuations in accord with current observations may be generated by adding a second scalar field to the model. Conflation may be viewed as complementary to the recently proposed anamorphic universe of Ijjas and Steinhardt.
20 pages, 5 figures

http://arxiv.org/abs/1507.04348
How to (Path-) Integrate by Differentiating
Achim Kempf, David M. Jackson, Alejandro H. Morales
(Submitted on 15 Jul 2015)
Recently, it was found that a new set of simple techniques allow one to conveniently express ordinary integrals through differentiation. These techniques add to the general toolbox for integration and integral transforms such as the Fourier and Laplace transforms. The new methods also yield new perturbative expansions when the integrals cannot be solved analytically. Here, we add new results, for example, on expressing the Laplace transform and its inverse in terms of derivatives. The new methods can be used to express path integrals in terms of functional differentiation, and they also suggest new perturbative expansions in quantum field theory.
7 pages. Based on a presentation given by AK at the 7th International Workshop DICE2014 in Castiglioncello, Italy, September 15-19, 2014

 

[marcus]

http://arxiv.org/abs/1507.05424
Phenomenology of bouncing black holes in quantum gravity: a closer look
Aurelien Barrau, Boris Bolliet, Francesca Vidotto, Celine Weimer
(Submitted on 20 Jul 2015)
It was recently shown that black holes could be bouncing stars as a consequence of quantum gravity. We investigate the astrophysical signals implied by this hypothesis, focusing on primordial black holes. We consider different possible bounce times and study the integrated diffuse emission.
8 pages, 8 figures

a correction, I somehow missed these back in May:
http://arxiv.org/abs/1505.04400
Kinematical Foundations of Loop Quantum Cosmology
Christian Fleischhack
(Submitted on 17 May 2015)
First, we review the C^∗-algebraic foundations of loop quantization, in particular, the construction of quantum configuration spaces and the implementation of symmetries. Then, we apply these results to loop quantum gravity, focusing on the space of generalized connections and on measures thereon. Finally, we study the realm of homogeneous isotropic loop quantum cosmology: once viewed as the loop quantization of classical cosmology, once seen as the symmetric sector of loop quantum gravity. It will turn out that both theories differ, i.e., quantization and symmetry reduction do not commute. Moreover, we will present a uniqueness result for kinematical measures. These last two key results have originally been due to Hanusch; here, we give drastically simplified and direct proofs.
28 pages, LaTeX with Birkhäuser style

http://arxiv.org/abs/1505.04404
Kinematical Uniqueness of Loop Quantum Gravity
Christian Fleischhack
(Submitted on 17 May 2015)
We review uniqueness results for the kinematical part of loop quantum gravity. After sketching the general loop formalism, the holonomy-flux and the Weyl algebras are introduced. In both cases, then, diffeomorphism invariant representations are described.
17 pages, LaTeX with Birkhäuser style, version accepted for publication in 2006, Quantum Gravity: Mathematical Models and Experimental Bounds (Edited Volume, Birkhäuser, Basel, 2007), pp. 203-219

http://arxiv.org/abs/1507.05669
Renormalized spacetime is two-dimensional at the Planck scale
T. Padmanabhan, Sumanta Chakraborty, Dawood Kothawala
(Submitted on 16 Jul 2015)
Quantum field theory distinguishes between the bare variables -- which we introduce in the Lagrangian -- and the renormalized variables which incorporate the effects of interactions. This suggests that the renormalized, physical, metric tensor of spacetime (and all the geometrical quantities derived from it) will also be different from the bare, classical, metric tensor in terms of which the bare gravitational Lagrangian is expressed. We provide a physical ansatz to relate the renormalized metric tensor to the bare metric tensor such that the spacetime acquires a zero-point-length ℓ₀ of the order of the Planck length L_P. This prescription leads to several remarkable consequences. In particular, the Euclidean volume V_D(ℓ,ℓ₀) in a D-dimensional spacetime of a region of size ℓ scales as V_D(ℓ,ℓ₀)∝ℓ^D−2₀ℓ² when ℓ∼ℓ_0, while it reduces to the standard result V_D(ℓ,ℓ₀)∝ℓ^D at large scales (ℓ≫ℓ₀). The appropriately defined effective dimension, Deff, decreases continuously from D_eff=D (at ℓ≫ℓ₀) to D_eff=2 (at ℓ∼ℓ₀₎. This suggests that the physical spacetime becomes essentially 2-dimensional near Planck scale.
8 Pages

http://arxiv.org/abs/1507.05733
Is Gravity Quantum?
M. Bahrami, A. Bassi, S. McMillen, M. Paternostro, H. Ulbricht
(Submitted on 21 Jul 2015)
What gravitational field is generated by a massive quantum system in a spatial superposition? This is one of the most important questions in modern physics, and after decades of intensive theoretical and experimental research, we still do not know the answer. On the experimental side, the difficulty lies in the fact that gravity is weak and requires large masses to be detectable. But for large masses, it becomes increasingly difficult to generate spatial quantum superpositions, which live sufficiently long to be detected. A delicate balance between opposite quantum and gravitational demands is needed. Here we show that this can be achieved in an optomechanics scenario. We propose an experimental setup, which allows to decide whether the gravitational field generated by a quantum system in a spatial superposition is the superposition of the two alternatives, or not. We estimate the magnitude of the effect and show that it offers good perspectives for observability. Performing the experiment will mark a breakthrough in our understanding of the relationship between gravity and quantum theory.
5 pages; 4 figures

http://arxiv.org/abs/1507.05826
Interaction Vertex for Classical Spinning Particles
Trevor Rempel, Laurent Freidel
(Submitted on 21 Jul 2015)
We consider a model of the classical spinning particle in which the coadjoint orbits of the Poincare group are parametrized by two pairs of canonically conjugate four vectors, one representing the standard position and momentum variables and the other which encodes the spinning degrees of freedom. This "Dual Phase Space Model" is shown to be a consistent theory of both massive and massless particles and allows for coupling to background fields such as electromagnetism. The on-shell action is derived and shown to be a sum of two terms, one associated with motion in spacetime and the other with motion in "spin space." Interactions between spinning particles are studied and a necessary and sufficient condition for consistency of a three-point vertex is established.
26 pages, 2 figures

 

[marcus]

http://arxiv.org/abs/1507.08112
Running of the scalar spectral index in bouncing cosmologies
Jean-Luc Lehners, Edward Wilson-Ewing
(Submitted on 29 Jul 2015)
We calculate the running of the scalar index in the ekpyrotic and matter bounce cosmological scenarios, and find that it is typically negative for ekpyrotic models, while it is typically positive for realizations of the matter bounce where multiple fields are present. This can be compared to inflation, where the observationally preferred models typically predict a negative running. The magnitude of the running is expected to be between 10⁻⁴ and up to 10⁻², leading in some cases to interesting expectations for near-future observations.
6 pages

http://arxiv.org/abs/1507.07591
Discrete Hamiltonian for General Relativity
Jonathan Ziprick, Jack Gegenberg
(Submitted on 27 Jul 2015)
Beginning from canonical general relativity written in terms of Ashtekar variables, we derive a discrete phase space with a physical Hamiltonian for gravity. The key idea is to define the gravitational fields within a complex of three-dimensional cells such that the dynamics is completely described by discrete boundary variables, and the full theory is recovered in the continuum limit. Canonical quantization is attainable within the loop quantum gravity framework, and we believe this will lead to a promising candidate for quantum gravity.
6 pages

http://arxiv.org/abs/1507.08194
Quantum Gravity: A Brief History of Ideas and Some Prospects
Steven Carlip, Dah-Wei Chiou, Wei-Tou Ni, Richard Woodard
(Submitted on 29 Jul 2015)
We present a bird's-eye survey on the development of fundamental ideas of quantum gravity, placing emphasis on perturbative approaches, string theory, loop quantum gravity, and black hole thermodynamics. The early ideas at the dawn of quantum gravity as well as the possible observations of quantum gravitational effects in the foreseeable future are also briefly discussed.
17 pages + bibliography. Invited review article. A few parts based on arXiv:1410.1486 and arXiv:1412.4362. To appear in Int. J. Mod. Phys. D and in "One Hundred Years of General Relativity: From Genesis and Empirical Foundations to Gravitational Waves, Cosmology and Quantum Gravity," edited by Wei-Tou Ni (World Scientific, Singapore, 2015)

possible general interest:
http://arxiv.org/abs/1507.07921
Diffeomorphism-invariant observables and their nonlocal algebra
William Donnelly, Steven B. Giddings
(Submitted on 28 Jul 2015)
Gauge-invariant observables for quantum gravity are described, with explicit constructions given in linearized gravity analogous to and extending constructions first given by Dirac in quantum electrodynamics. These can be thought of as operators that create a particle, together with its inseparable gravitational field, and reduce to usual field operators of quantum field theory in the weak-gravity limit; they include both Wilson-line operators, and those creating a Coulombic field configuration. We also describe operators creating the field of a particle in motion; as in the electromagnetic case, these are expected to help address infrared problems. An important characteristic of the quantum theory of gravity is the algebra of its observables. We show that the commutators of the simple observables of this paper are nonlocal, with nonlocality becoming significant in strong field regions, as predicted previously on general grounds.
29 pages

http://arxiv.org/abs/1507.07956
CODATA Recommended Values of the Fundamental Physical Constants: 2014
Peter J. Mohr, David B. Newell, Barry N. Taylor
(Submitted on 21 Jul 2015)
This report gives the 2014 self-consistent set of values of the constants and conversion factors of physics and chemistry recommended by the Committee on Data for Science and Technology (CODATA). These values are based on a least-squares adjustment that takes into account all data available up to 31 December 2014. The recommended values may also be found on the World Wide Web at physics.nist.gov/constants.
Comments: 11 pages, 8 tables

http://arxiv.org/abs/1507.08102
Random walks across the sea: the origin of rogue waves?
Simon Birkholz, Carsten Brée, Ivan Veselić, Ayhan Demircan, Günter Steinmeyer

 

[marcus]

http://arxiv.org/abs/1507.08807
The entropy of isolated horizons in non-minimally coupling scalar field theory from BF theory
Jingbo Wang, Chao-Guang Huang
(Submitted on 31 Jul 2015)
In this paper, the entropy of isolated horizons in non-minimally coupling scalar field theory and in the scalar-tensor theory of gravitation is calculated by counting the degree of freedom of quantum states in loop quantum gravity. Instead of boundary Chern-Simons theory, the boundary BF theory is used. The advantages of the new approaches are that no spherical symmetry is needed, and that the final result matches exactly with the Wald entropy formula.
10 pages

possibly of general interest:
http://arxiv.org/abs/1508.01140
Disentangling the quantum world
Huw Price, Ken Wharton
(Submitted on 5 Aug 2015)
Correlations related to related to quantum entanglement have convinced many physicists that there must be some at-a-distance connection between separated events, at the quantum level. In the late 1940's, however, O. Costa de Beauregard proposed that such correlations can be explained without action at a distance, so long as the influence takes a zigzag path, via the intersecting past lightcones of the events in question. Costa de Beauregard's proposal is related to what has come to be called the retrocausal loophole in Bell's Theorem, but -- like that loophole -- it receives little attention, and remains poorly understood. Here we propose a new way to explain and motivate the idea. We exploit some simple symmetries to show how Costa de Beauregard's zigzag needs to work, to explain the correlations at the core of Bell's Theorem. As a bonus, the explanation shows how entanglement might be a much simpler matter than the orthodox view assumes -- not a puzzling feature of quantum reality itself, but an entirely unpuzzling feature of our knowledge of reality, once zigzags are in play.
14 pages, 3 figures

http://arxiv.org/abs/1508.00939
Comments on Microcausality, Chaos, and Gravitational Observables
Donald Marolf
(Submitted on 4 Aug 2015)
Observables in gravitational systems must be non-local so as to be invariant under diffeomorphism gauge transformations. But at the classical level some such observables can nevertheless satisfy an exact form of microcausality. This property is conjectured to remain true at all orders in the semiclassical expansion, though with limitations at finite ℏ or ℓ_Planck. We also discuss related issues concerning observables in black hole spacetimes and comment on the senses in which they do and do not experience the form of chaos identified by Shenker and Stanford. In particular, in contrast to the situation in a reflecting cavity, this chaos does not afflict observables naturally associated with Hawking radiation for evaporating black holes.
16 pages, 1 figure

http://arxiv.org/abs/1507.08665
Cosmological Axion and neutrino mass constraints from Planck 2015 temperature and polarization data
Eleonora Di Valentino, Elena Giusarma, Massimiliano Lattanzi, Olga Mena, Alessandro Melchiorri, Joseph Silk
(Submitted on 30 Jul 2015)
Axions currently provide the most compelling solution to the strong CP problem. These particles may be copiously produced in the early universe, including via thermal processes. Therefore, relic axions constitute a hot dark matter component and their masses are strongly degenerate with those of the three active neutrinos, as they leave identical signatures in the different cosmological observables. In addition, thermal axions, while still relativistic states, also contribute to the relativistic degrees of freedom, parameterised via Neff. We present the cosmological bounds on the relic axion and neutrino masses, exploiting the full Planck mission data, which include polarization measurements. In the mixed hot dark matter scenario explored here, we find the tightest and more robust constraint to date on the sum of the three active neutrino masses, ∑m_ν<0.136 eV at 95% CL, obtained in the well-known linear perturbation regime. The Planck Sunyaev-Zeldovich cluster number count data further tightens this bound, providing a 95% CL upper limit of ∑m_ν<0.126 eV in this very same mixed hot dark matter model, a value which is very close to the expectations in the inverted hierarchical neutrino mass scenario. Using this same combination of data sets we find the most stringent bound to date on the thermal axion mass, m_a<0.529eV at 95% CL.

http://arxiv.org/abs/1508.00567
New constraints on the structure and dynamics of black hole jets
William J. Potter, Garret Cotter
(Submitted on 3 Aug 2015)
20 pages, 7 figures, MNRAS accepted.

 

[John86]

http://arxiv.org/abs/1507.08576
Non-local beables
Lee Smolin
(Submitted on 30 Jul 2015)
I discuss the idea that the beables underlying quantum physics are non-local and relational, and give an example of a dynamical theory of such beables based on a matrix model, which is the bosonic sector of the BFSS model. Given that the same model has been proposed as a description of M theory, this shows that quantum mechanics may be emergent from a theory of gravity from which space is also emergent.

 

[marcus]

http://arxiv.org/abs/1508.01416
Spin Foams Without Spins
Jeff Hnybida
(Submitted on 6 Aug 2015)
We formulate the spin foam representation of discrete SU(2) gauge theory as a product of vertex amplitudes each of which is the spin network generating function of the boundary graph dual to the vertex. Thus the sums over spins have been carried out. We focus on the character expansion of Yang-Mills theory which is an approximate heat kernel regularization of BF theory. The boundary data of each n-valent node is an element of the Grassmannian Gr(2,n) which carries a coherent representation of U(n) and a geometrical interpretation as a framed polyhedron of fixed total area. Ultimately, sums over spins are traded for contour integrals over simple poles and recoupling theory is avoided using generating functions.
21 pages, 2 figures

http://arxiv.org/abs/1508.01277
Restoration of four-dimensional diffeomorphism covariance in canonical general relativity: An intrinsic Hamilton-Jacobi approach
Donald Salisbury, Jürgen Renn, Kurt Sundermeyer
(Submitted on 6 Aug 2015)
21 pages.

http://arxiv.org/abs/1508.01855
Functional Renormalisation Group analysis of a Tensorial Group Field Theory on ℝ³
Joseph Ben Geloun, Riccardo Martini, Daniele Oriti
(Submitted on 8 Aug 2015)
7 pages, 2 Figures

 

[marcus]

http://arxiv.org/abs/1508.01947
Chaos, Dirac observables and constraint quantization
Bianca Dittrich, Philipp A. Hoehn, Tim A. Koslowski, Mike I. Nelson
(Submitted on 8 Aug 2015)
There is good evidence that full general relativity is non-integrable or even chaotic. We point out the severe repercussions: differentiable Dirac observables and a reduced phase space do not exist in non-integrable constrained systems and are thus unlikely to occur in a generic general relativistic context. Instead, gauge invariant quantities generally become discontinuous, thus not admitting Poisson-algebraic structures and posing serious challenges to a quantization. Non-integrability also renders the paradigm of relational dynamics cumbersome, thereby straining common interpretations of the dynamics. We illustrate these conceptual and technical challenges with simple toy models. In particular, we exhibit reparametrization invariant models which fail to be integrable and, as a consequence, can either not be quantized with standard methods or lead to sick quantum theories without a semiclassical limit. These troubles are qualitatively distinct from semiclassical subtleties in unconstrained quantum chaos and can be directly traced back to the scarcity of Dirac observables. As a possible resolution, we propose to change the method of quantization by refining the configuration space topology until the generalized observables become continuous in the new topology and can acquire a quantum representation. This leads to the polymer quantization method underlying loop quantum cosmology and gravity. Remarkably, the polymer quantum theory circumvents the problems of the quantization with smooth topology, indicating that non-integrability and chaos, while a challenge, may not be a fundamental obstruction for quantum gravity.
48 pages, 9 figures, lots of discussion

http://arxiv.org/abs/1508.03036
Constraint Lie algebra and local physical Hamiltonian for a generic 2D dilatonic model
Alejandro Corichi, Asieh Karami, Saeed Rastgoo, Tatjana Vukašinac
(Submitted on 12 Aug 2015)
We consider a class of two dimensional dilatonic models, and revisit them from the perspective of a new set of "polar type" variables. These are motivated by recently defined variables within the spherically symmetric sector of 4D general relativity. We show that for a large class of models, with and without matter, one can perform a series of canonical transformations in such a way that the Poisson algebra of the constraints becomes a Lie algebra. Furthermore, we construct Dirac observables and a reduced Hamiltonian that accounts for the time evolution of the system. Thus, with our formulation, the systems under consideration are amenable to be quantized with loop quantization methods.
23 pages, 3 appendices

 

[marcus]

http://arxiv.org/abs/1508.04141
The evolution of cosmological perturbations and the production of non-Gaussianities through a nonsingular bounce: indications for a no-go theorem in single field matter bounce cosmologies
Jerome Quintin, Zeinab Sherkatghanad, Yi-Fu Cai, Robert H. Brandenberger
(Submitted on 17 Aug 2015)
Assuming that curvature perturbations and gravitational waves originally arise from vacuum fluctuations in a matter-dominated phase of contraction, we study the dynamics of the cosmological perturbations evolving through a nonsingular bouncing phase described by a generic single scalar field Lagrangian minimally coupled to Einstein gravity. ... By studying the general form of the bispectrum we show that the non-Gaussianity parameter f_NL (which is of order unity before the bounce phase) is enhanced during the bounce phase if the curvature fluctuations grow. Hence, in such nonsingular bounce models with matter given by a single scalar field, there appears to be a tension between obtaining a small enough tensor-to-scalar ratio and not obtaining a value of f_NL in excess of the current upper bounds. This conclusion may be considered as a "no-go" theorem for single field matter bounce cosmologies starting with vacuum initial conditions for the fluctuations.
31 pages, 2 figures.
==quote from conclusions==
We note that this “no-go” theorem might be circumvented by dropping certain assumptions imposed above... As another example, if the initial Bunch-Davies vacuum is non-canonical (e.g., in the ΛCDM bounce [6], the initial quantum vacuum has cs ≪ 1), the initial ratio of the tensor modes to the scalar modes can be suppressed, in which case there is no need for the curvature perturbations to be enhanced during the bounce. Our analysis also does not immediately apply to nonsingular bouncing models in which the violation of the Null Energy Condition is obtained by changes in the gravitational action (e.g., in Loop Quantum Cosmology [65, 66] or Horava-Lifshitz gravity [13]). It would be interesting to analyze the conditions under which the bispectrum constraints can be made consistent with the observed bound on the tensor-to-scalar ratio in such models.
==endquote==

http://arxiv.org/abs/1508.04576
Dark energy as a fixed point of the Einstein Yang-Mills Higgs Equations
Massimiliano Rinaldi
(Submitted on 19 Aug 2015)
We study the Einstein Yang-Mills Higgs equations in the SO(3) representation on a isotropic and homogeneous flat Universe, in the presence of radiation and matter fluids. We map the equations of motion into a closed dynamical system of first-order differential equations and we find the equilibrium points. We show that there is only one stable fixed point that corresponds to an accelerated expanding Universe in the future. In the past, instead, there is an unstable fixed point that implies a stiff-matter domination. In between, we find three other unstable fixed points, corresponding, in chronological order, to radiation domination, to matter domination, and, finally, to a transition from decelerated expansion to accelerated expansion. We solve the system numerically and we confirm that there are smooth trajectories that correctly describe the evolution of the Universe, from a remote past dominated by radiation to a remote future dominated by dark energy, passing through a matter-dominated phase.
16 pages, 3 figures.
Some earlier papers by Rinaldi:
http://arxiv.org/abs/1309.7332
http://arxiv.org/abs/1404.0532
http://arxiv.org/abs/1507.05886

briefly noted:
http://arxiv.org/abs/1508.04773
Accretion of dark matter by stars
Richard Brito, Vitor Cardoso, Hirotada Okawa
(Submitted on 19 Aug 2015)
5 pages, accepted by PRL
[My comment: this paper contains a remarkable and possibly useful prediction of an observable effect.]

http://arxiv.org/abs/1508.04797
Black hole binary inspiral: Analysis of the plunge
Richard H. Price, Sourabh Nampalliwar, Gaurav Khanna
(Submitted on 19 Aug 2015)
28 pages, 24 figures

 

[marcus]

Proceedings of 2014 conference on Noncommutative Geometry and Quantum Gravity
http://iopscience.iop.org/1742-6596/634/1/012001/
J. Phys.: Conf. Ser.634 012001 doi:10.1088/1742-6596/634/1/012001

IMHO unusually interesting contribution from Fabien Besnard
Journal of Physics: Conference Series 634 (2015) 012009 doi:10.1088/1742-6596/634/1/012009
http://iopscience.iop.org/1742-6596/634/1/012009/pdf/1742-6596_634_1_012009.pdf
Two roads to noncommutative causality
Fabien Besnard
Abstract. We review the physical motivations and the mathematical results obtained so far in the isocone-based approach to noncommutative causality. We also give a briefer account of the alternative framework of Franco and Eckstein which is based on Lorentzian spectral triples. We compare the two theories on the simple example of the product geometry of the Minkowski plane by the finite noncommutative space with algebra M2(C).

For background, I'll list the recent Besnard papers on arxiv:
1. arXiv:1508.01917 [pdf, ps, other]
Two roads to noncommutative causality
Fabien Besnard
Comments: The title is a tribute to a well-known book about the search for quantum gravity

2. arXiv:1504.03890 [pdf, ps, other]
The Standard Model as an extension of the noncommutative algebra of forms
Christian Brouder (IMPMC), Nadir Bizi (IMPMC), Fabien Besnard
Comments: The new version includes the Standard Model with a Lorentzian signature

3. arXiv:1411.0878 [pdf, ps, other]
The disappearance of causality at small scale in almost-commutative manifolds
Nadir Bizi, Fabien Besnard
Comments: 28 pages, 4 figures 1 typo fixed. 2 references added

4. arXiv:1104.4551 [pdf, ps, other]
Time of Philosophers, Time of Physicists, Time of Mathematicians
Fabien Besnard
Comments: 24 pages, 10 figures

and also list a couple by Franco and Eckstein:

4. arxiv:1409.1480 [pdf, other]
Noncommutative geometry, Lorentzian structures and causality
Nicolas Franco, Michał Eckstein

6. arXiv:1310.8225 [pdf, other]
Exploring the Causal Structures of Almost Commutative Geometries
Nicolas Franco, Michał Eckstein
Journal-ref: SIGMA 10 (2014), 010, 23 pages
===========================
http://arxiv.org/abs/1508.05543
Relational Quantum Cosmology
Francesca Vidotto
(Submitted on 22 Aug 2015)
The application of quantum theory to cosmology raises a number of conceptual questions, such as the role of the quantum-mechanical notion of "observer" or the absence of a time variable in the Wheeler-DeWitt equation. I point out that a relational formulation of quantum mechanics, and more in general the observation that evolution is always relational, provides a coherent solution to this tangle of problems.
20 pages, 4 figures. Contribution to the forthcoming book on Philosophy of Cosmology edited by K. Chamcham, J. Barrow, J. Silk and S. Saunders for Cambridge University Press

http://arxiv.org/abs/1508.05578
Scalar field as a time variable during gravitational evolution
Anna Nakonieczna, Jerzy Lewandowski
(Submitted on 23 Aug 2015)
Using a scalar field as an intrinsic 'clock' while investigating the dynamics of gravitational systems has been successfully pursued in various researches on the border between classical and quantum gravity. The objective of our research was to check explicitly whether the scalar field can serve as a time variable during dynamical evolution of the matter-geometry system, especially in regions of high curvature, which are essential from the perspective of quantum gravity. For this purpose, we analyzed a gravitational collapse of a self-interacting scalar field within the framework of general relativity. The obtained results indicated that the hypersurfaces of constant scalar field are spacelike in dynamical regions nearby the singularities formed during the investigated process. The scalar field values change monotonically in the areas, in which the constancy hypersurfaces are spacelike.
11 pages, 6 figures

http://arxiv.org/abs/1508.05953
Locality and entanglement in bandlimited quantum field theory
Jason Pye, William Donnelly, Achim Kempf
(Submitted on 24 Aug 2015)
We consider a model for a Planck scale ultraviolet cutoff which is based on Shannon sampling. Shannon sampling originated in information theory, where it expresses the equivalence of continuous and discrete representations of information. When applied to quantum field theory, Shannon sampling expresses a hard ultraviolet cutoff in the form of a bandlimitation. This introduces nonlocality at the cutoff scale in a way that is more subtle than a simple discretization of space: quantum fields can then be represented as either living on continuous space or, entirely equivalently, as living on anyone lattice whose average spacing is sufficiently small. We explicitly calculate vacuum entanglement entropies in 1+1 dimension and we find a transition between logarithmic and linear scaling of the entropy, which is the expected 1+1 dimensional analog of the transition from an area to a volume law. We also use entanglement entropy and mutual information as measures to probe in detail the localizability of the field degrees of freedom. We find that, even though neither translation nor rotation invariance are broken, each field degree of freedom occupies an incompressible volume of space, indicating a finite information density.
23 pages, 13 figures.

 

[marcus]

http://arxiv.org/abs/1508.06286
Distribution function of the Atoms of Spacetime and the Nature of Gravity
T. Padmanabhan
(Submitted on 25 Aug 2015)
The fact that the equations of motion for matter remain invariant when a constant is added to the Lagrangian, suggests postulating that the field equations of gravity should also respect this symmetry. This principle implies that: (a) The metric cannot be varied in any extremum principle to obtain the field equations. (b) The stress-tensor of matter should appear in the variational principle through the combination T_abn^an^b where n_a is an auxiliary null vector field which should be varied to get the field equations. This procedure uniquely selects the Lanczos-Lovelock models of gravity in D-dimensions and Einstein's theory in D=4. Identifying n_a with the normals to the null surfaces in the spacetime in the macroscopic limit leads to a thermodynamic interpretation for gravity. Several geometrical variables and the equation describing the spacetime evolution acquire a thermodynamic interpretation.
Extending these ideas one level deeper, I show how this variational principle can be obtained from a distribution function for the number density of the "atoms of spacetime". This is based on the curious fact that the renormalized spacetime endows each event with zero volume but finite area!
32 pages.

http://arxiv.org/abs/1508.06572
Quantum information erasure inside black holes
David A. Lowe, Larus Thorlacius
(Submitted on 26 Aug 2015)
An effective field theory for infalling observers in the vicinity of a quasi-static black hole is given in terms of a freely falling lattice discretization. The lattice model successfully reproduces the thermal spectrum of outgoing Hawking radiation, as was shown by Corley and Jacobson, but can also be used to model observations made by a typical low-energy observer who enters the black hole in free fall at a prescribed time. The explicit short distance cutoff ensures that, from the viewpoint of the infalling observer, any quantum information that entered the black hole more than a scrambling time earlier has been erased by the black hole singularity. This property, combined with the requirement that outside observers need at least of order the scrambling time to extract quantum information from the black hole, ensures that a typical infalling observer does not encounter drama upon crossing the black hole horizon in a theory where black hole information is preserved for asymptotic observers.
19 pages, 3 figures.

light reading:
http://arxiv.org/abs/1508.06478
Light is Heavy
M.B. van der Mark, G.W. 't Hooft
(Submitted on 26 Aug 2015)
Einstein's relativity theory appears to be very accurate, but at times equally puzzling. On the one hand, electromagnetic radiation must have zero rest mass in order to propagate at the speed of light, but on the other hand, since it definitely carries momentum and energy, it has non-zero inertial mass. Hence, by the principle of equivalence, it must have non-zero gravitational mass, and so, light must be heavy. In this paper, no new results will be derived, but a possibly surprising perspective on the above paradox is given.
6 pages, 2 figures. Originally written as a contribution to the 25th aniversary of the "Sectie Atoomfysica en Quantum Electronica" of the Dutch Physical Society (NNV), November 2000.

 

[marcus]

http://arxiv.org/abs/1508.06786
Primordial scalar power spectrum from the Euclidean bounce of loop quantum cosmology
Susanne Schander, Aurélien Barrau, Boris Bolliet, Linda Linsefors, Julien Grain
(Submitted on 27 Aug 2015)
In effective models of loop quantum cosmology, the holonomy corrections lead to a deformed algebra of constraints. Among other consequences of this new spacetime structure is the emergence of an Euclidean phase around the bounce. In this article, we explicitly compute the resulting primordial power spectrum for scalar modes by setting initial conditions in the contracting phase.
10 pages, 4 figures

http://arxiv.org/abs/1508.05533
An argument against the realistic interpretation of the wave function
Carlo Rovelli
(Submitted on 22 Aug 2015)
Testable predictions of quantum mechanics are invariant under time reversal. But the change of the quantum state in time is not so, neither in the collapse nor in the no-collapse interpretations of the theory. This fact challenges the realistic interpretation of the quantum state. On the other hand, this fact follows easily if we interpret the quantum state as a mere calculation device, bookkeeping past real quantum events. The same conclusion follows from the analysis of the meaning of the wave function in the semiclassical regime.
4 pages, 3 figures

http://arxiv.org/abs/1508.06895
Comment on Carlo Rovelli's "An argument against the realistic interpretation of the wave function"
H. Dieter Zeh
(Submitted on 27 Aug 2015)
Rovelli's argument against the realistic interpretation of quantum states and in favor of an ontology of quantum events is refuted.
3 pages

http://arxiv.org/abs/1508.06951
Mathematical Foundations of Quantum Mechanics: An Advanced Short Course
Valter Moretti (Trento U.)
(Submitted on 27 Aug 2015)
This paper collects and extends the lectures given by the author at the "XXIV International Fall Workshop on Geometry and Physics" held in Zaragoza (Spain) during September 2015. Within these lectures I review the formulation of Quantum Mechanics, and quantum theories in general, from a mathematically advanced viewpoint, essentially based on the orthomodular lattice of elementary propositions, discussing some fundamental ideas, mathematical tools and theorems also related to the representation of physical symmetries. The final step consists of an elementary introduction the so-called (C*-) algebraic formulation of quantum theories.
95 pages.

 

[marcus]

http://arxiv.org/abs/1509.00458
Four-dimensional Quantum Gravity with a Cosmological Constant from Three-dimensional Holomorphic Blocks
Hal M. Haggard, Muxin Han, Wojciech Kamiński, Aldo Riello
(Submitted on 1 Sep 2015)
Prominent approaches to quantum gravity struggle when it comes to incorporating a positive cosmological constant in their models. Using quantization of a complex SL(2,ℂ) Chern-Simons theory we include a cosmological constant, of either sign, into a model of quantum gravity.
5 pages and 2 figures

http://arxiv.org/abs/1509.00466
4d Quantum Geometry from 3d Supersymmetric Gauge Theory and Holomorphic Block
Muxin Han
(Submitted on 31 Aug 2015)
A class of 3d N=2 supersymmetric gauge theories are constructed and shown to encode the simplicial geometries in 4-dimensions. The gauge theories are defined by applying the Dimofte-Gaiotto-Gukov construction in 3d/3d correspondence to certain graph complement 3-manifolds. Given a gauge theory in this class, the massive supersymmetric vacua of the theory contain the classical geometries on a 4d simplicial complex. The corresponding 4d simplicial geometries are locally constant curvature (either dS or AdS), in the sense that they are made by gluing geometrical 4-simplices of the same constant curvature. When the simplicial complex is sufficiently refined, the simplicial geometries can approximate all possible smooth geometries on 4-manifold. At the quantum level, we propose that a class of holomorphic blocks defined in arXiv:1211.1986 from the 3d N=2 gauge theories are wave functions of quantum 4d simplicial geometries. In the semiclassical limit, the asymptotic behavior of holomorphic block reproduces the classical action of 4d Einstein-Hilbert gravity in the simplicial context.
40+7 pages, 9 figures

http://arxiv.org/abs/1509.00640
Minisuperspace models as infrared contributions
Martin Bojowald, Suddhasattwa Brahma
(Submitted on 2 Sep 2015)
A direct correspondence of quantum mechanics as a minisuperspace model for a self-interacting scalar quantum-field theory is established by computing, in several models, the infrared contributions to 1-loop effective potentials of Coleman--Weinberg type. A minisuperspace approximation rather than truncation is thereby obtained. By this approximation, the spatial averaging scale of minisuperspace models is identified with an infrared scale (but not a regulator or cut-off) delimiting the modes included in the minisuperspace model. Some versions of the models studied here have discrete space or modifications of the Hamiltonian expected from proposals of loop quantum gravity. They shed light on the question of how minisuperspace models of quantum cosmology can capture features of full quantum gravity. While it is shown that modifications of the Hamiltonian can well be described by minisuperspace truncations, some related phenomena such as signature change, confirmed and clarified here for modified scalar field theories, require at least a perturbative treatment of inhomogeneity beyond a strict minisuperspace model. The new methods suggest a systematic extension of minisuperspace models by a canonical effective formulation of perturbative inhomogeneity.
31 pages

http://arxiv.org/abs/1508.07961
Quantum Cuboids and the EPRL-FK path integral for quantum gravity
Benjamin Bahr, Sebastian Steinhaus
(Submitted on 31 Aug 2015)
In this work, we investigate the 4d path integral for Euclidean quantum gravity on a hypercubic lattice, as given by the EPRL-FK model. To tackle the problem, we restrict to a set of quantum geometries that reflects the large amount of lattice symmetries. In particular, the sum over intertwiners is restricted to quantum cuboids, i.e. coherent intertwiners which describe a cuboidal geometry in the large-j limit.
Using asymptotic expressions for the vertex amplitude, we find several interesting properties of the state sum. First of all, the value of coupling constants in the amplitude functions determines whether geometric or non-geometric configurations dominate the path integral. Secondly, there is a critical value of the coupling constant α, which separates two phases. In one, the main contribution comes from very irregular and crumpled states. In the other, the dominant contribution comes from a highly regular configuration, which can be interpreted as flat Euclidean space, with small non-geometric perturbations around it.
Thirdly, we use the state sum to compute the physical norm of kinematical states, i.e. their norm in the physical Hilbert space. We find that states which describe boundary geometry with high torsion have exponentially suppressed physical norm. We argue that this allows one to exclude them from the state sum in calculations.
15 pages, 15 figures

http://arxiv.org/abs/1509.00076
Alternative derivation of Krasnov's action for general relativity
Mariano Celada, Diego Gonzalez, Merced Montesinos
(Submitted on 31 Aug 2015)
Starting from Plebanski's action for general relativity with cosmological constant, we show that by integrating out all the auxiliary fields Krasnov's action immediately emerges. We also perform the Hamiltonian analysis of the latter and show that the constraints are those of the Ashtekar formalism.
4 pages

http://arxiv.org/abs/1509.00833
Gravitational collapse of thin shells of dust in Shape Dynamics
Henrique Gomes, Tim Koslowski, Flavio Mercati, Andrea Napoletano
(Submitted on 2 Sep 2015)
Shape dynamics is a theory of gravity whose physical configuration space is composed of spatial conformal equivalence classes of 3-dimensional geometries. This physical configuration space is not a priori related to the one given by 4-dimensional space-time geometries, familiar to general relativists. Although one can largely match dynamical solutions arising in each theory, this is not always the case. One expects such differences in collapse situations that lead to the formation of black holes. In this paper we study spherical collapse of thin-shells of pressureless dust as a fully back-reacting dynamical system, in a context related to both shape dynamics and general relativity in ADM form in Constant Mean Curvature (CMC) gauge -- the particular time slicing where any correspondence between ADM and shape dynamics is manifest. To better accommodate the relational setting of shape dynamics, we also begin a study of collapse of two such shells in a compact Universe.
26 + 17 page appendix, 11 figures.

possibly of general interest:
http://arxiv.org/abs/1509.01147
The Information Paradox for Black Holes
S. W. Hawking
(Submitted on 3 Sep 2015)
I propose that the information loss paradox can be resolved by considering the supertranslation of the horizon caused by the ingoing particles. Information can be recovered in principle, but it is lost for all practical purposes.
3 pages. Talk given on 28 August 2015 at Hawking Radiation, a conference held at KTH Royal Institute of Technology, Stockholm

odds and ends:
http://arxiv.org/abs/1508.07488
http://arxiv.org/abs/1508.07664
http://arxiv.org/abs/1508.07507

 

[marcus]

http://arxiv.org/abs/1509.01695
Diagonalizing the Black Hole Information Retrieval Process
Gerard t Hooft
(Submitted on 5 Sep 2015)
The mechanism by which black holes return the absorbed information to the outside world is reconsidered, and described in terms of a set of mutually non-interacting modes. Our mechanism is based on the mostly classical gravitational back-reaction. The diagonalized formalism is particularly useful for further studies of this process. Although no use is made of string theory, our analysis appears to point towards an ensuing string-like interaction. It is shown how black hole entropy can be traced down to classical gravitational back-reaction.
10 pages

http://arxiv.org/abs/1509.01312
Analog of the Peter-Weyl Expansion for Lorentz Group
Leonid Perlov
(Submitted on 4 Sep 2015)

19pages.

http://arxiv.org/abs/1509.02036
A note on quantum supergravity and AdS/CFT
Norbert Bodendorfer
(Submitted on 7 Sep 2015)
We note that the non-perturbative quantisation of supergravity as recently investigated using loop quantum gravity techniques provides an opportunity to probe an interesting sector of the AdS/CFT correspondence, which is usually not considered in conventional treatments. In particular, assuming a certain amount of convergence between the quantum supergravity sector of string theory and quantum supergravity constructed via loop quantum gravity techniques, we argue that the large quantum number expansion in loop quantum supergravity corresponds to the 1/N_c² expansion in the corresponding gauge theory. In order to argue that we are indeed dealing with an appropriate quantum supergravity sector of string theory, high energy (α′) corrections are being neglected, leading to a gauge theory at strong coupling, yet finite N_c. The arguments given in this paper are mainly of qualitative nature, with the aim of serving as a starting point for a more in depth interaction between the string theory and loop quantum gravity communities.
8 pages.

http://arxiv.org/abs/1509.01772
100 Years of General Relativity
George F. R. Ellis
(Submitted on 6 Sep 2015)
This is Chapter 1 in the book General Relativity and Gravitation: A Centennial Perspective, Edited by Abhay Ashtekar (Editor in Chief), Beverly Berger, James Isenberg, Malcolm MacCallum. Publisher: Cambridge University Press (June, 2015). It gives a survey of themes that have been developed during the 100 years of progress in general relativity theory.
36 pages, 4 figures.

http://arxiv.org/abs/1509.03304
Are black holes in an ekpyrotic phase possible?
J. C. S. Neves
(Submitted on 10 Sep 2015)
The ekpyrotic phase (a slow contraction cosmic phase before the current expansion phase) manages to solve the main problems of the standard cosmology by means of a scalar field interpreted as an isotropic cosmic fluid in the Friedmann equation. Moreover, this phase generates a nearly scale-invariant spectrum of perturbations in agreement with the latest data. Then, the ekpyrotic mechanism is a serious [alternative] possibility to the inflationary model. In this work, we point out that it is impossible to generate a black hole with spherical symmetry supported by an isotropic fluid in this scenario. Using the approach of deforming metrics to obtain solutions with an isotropic energy-momentum tensor, we show that the stiff fluid, dominant in the ekpyrotic phase, does not support these black holes.
4 pages, 1 figure.

http://arxiv.org/abs/1509.01833
(Loop) quantum gravity and the inflationary scenario
Martin Bojowald
(Submitted on 6 Sep 2015)
Quantum gravity, as a fundamental theory of space-time, is expected to reveal how the universe may have started, perhaps during or before an inflationary epoch. It may then leave a potentially observable (but probably minuscule) trace in cosmic large-scale structures that seem to match well with predictions of inflation models. A systematic quest to derive such tiny effects using one approach, loop quantum gravity, has, however, led to unexpected obstacles. Such models remain incomplete, and it is not clear whether loop quantum gravity can be consistent as a full theory. But some surprising effects appear to be generic and would drastically alter our understanding of space-time at large density. These new high-curvature phenomena are a consequence of a widening gap between quantum gravity and ordinary quantum-field theory on a background.
10 pages, 1 figure, invited contribution to a special issue published by the French Academy of Sciences

possibly of general interest:
http://arxiv.org/abs/1509.03637
Origin of Cosmic Chemical Abundances
Umberto Maio, Edoardo Tescari
(Submitted on 11 Sep 2015)
Cosmological N-body hydrodynamic computations following atomic and molecular chemistry (e−, H, H+, H−, He, He+, He++, D, D+, H2, H+2, HD, HeH+), gas cooling, star formation and production of heavy elements (C, N, O, Ne, Mg, Si, S, Ca, Fe, etc.) from stars covering a range of mass and metallicity are used to explore the origin of several chemical abundance patterns and to study both the metal and molecular content during simulated galaxy assembly. The resulting trends show a remarkable similarity to up-to-date observations of the most metal-poor damped Lyman-α absorbers at redshift z≳2. These exhibit a transient nature and represent collapsing gaseous structures captured while cooling is becoming effective in lowering the temperature below ∼10⁴K, before they are disrupted by episodes of star formation or tidal effects. Our theoretical results agree with the available data for typical elemental ratios, such as [C/O], [Si/Fe], [O/Fe], [Si/O], [Fe/H], [O/H] at redshifts z∼2−7. Correlations between HI and H2 abundances show temporal and local variations and large spreads as a result of the increasing cosmic star formation activity from z∼6 to z∼3. The scatter we find in the abundance ratios is compatible with the observational data and is explained by simultaneous enrichment by sources from different stellar phases or belonging to different stellar populations. Simulated synthetic spectra support the existence of metal-poor cold clumps with large optical depth at z∼6 that could be potential population~III sites at low or intermediate redshift. The expected dust content is in line with recent determinations.
24 pages, 17 figures

http://arxiv.org/abs/1509.03222
Entropic Dynamics
Ariel Caticha
(Submitted on 10 Sep 2015)
Entropic Dynamics is a framework in which dynamical laws are derived as an application of entropic methods of inference. No underlying action principle is postulated. Instead, the dynamics is driven by entropy subject to the constraints appropriate to the problem at hand. In this paper we review three examples of entropic dynamics. First we tackle the simpler case of a standard diffusion process which allows us to address the central issue of the nature of time. Then we show that imposing the additional constraint that the dynamics be non-dissipative leads to Hamiltonian dynamics. Finally, considerations from information geometry naturally lead to the type of Hamiltonian that describes quantum theory.
31 pages. Invited contribution to the Entropy special volume on Dynamical Equations and Causal Structures from Observations

http://arxiv.org/abs/1509.03155
One-loop contribution to the matter-driven expansion of the Universe
Bogusław Broda
(Submitted on 10 Sep 2015)
6 pages.

http://arxiv.org/abs/1509.02542
Proof of the Quantum Null Energy Condition
Raphael Bousso, Zachary Fisher, Jason Koeller, Stefan Leichenauer, Aron C. Wall
(Submitted on 8 Sep 2015)
We prove the Quantum Null Energy Condition (QNEC), a lower bound on the stress tensor in terms of the second variation in a null direction of the entropy of a region. The QNEC arose previously as a consequence of the Quantum Focussing Conjecture, a proposal about quantum gravity. The QNEC itself does not involve gravity, so a proof within quantum field theory is possible. Our proof is somewhat nontrivial, suggesting that there may be alternative formulations of quantum field theory that make the QNEC more manifest.
Our proof applies to free and superrenormalizable bosonic field theories, and to any points that lie on stationary null surfaces. ...
...
...
32 pages, 3 figures

 

[marcus]

http://arxiv.org/abs/1509.05297
Ising Spin Network States for Loop Quantum Gravity: a Toy Model for Phase Transitions
Alexandre Feller, Etera R. Livine
(Submitted on 17 Sep 2015)
Non-perturbative approaches to quantum gravity call for a deep understanding of the emergence of geometry and locality from the quantum state of the gravitational field. Without background geometry, the notion of distance should entirely emerge from the correlations between the gravity fluctuations. In the context of loop quantum gravity, quantum states of geometry are defined as spin networks. These are graphs decorated with spin and intertwiners, which represent quantized excitations of areas and volumes of the space geometry. Here, we develop the condensed matter point of view on extracting the physical and geometrical information out of spin network states: we introduce new Ising spin network states, both in 2d on a square lattice and in 3d on a hexagonal lattice, whose correlations map onto the usual Ising model in statistical physics. We construct these states from the basic holonomy operators of loop gravity and derive a set of local Hamiltonian constraints which entirely characterize our states. We discuss their phase diagram and show how the distance can be reconstructed from the correlations in the various phases. Finally, we propose generalizations of these Ising states, which open the perspective to study the coarse graining and dynamics of spin network states using well-known condensed matter techniques and results.
17 pages

possibly of wider interest:
http://arxiv.org/abs/1509.04645
Non-Thermal Corrections to Hawking Radiation Versus the Information Paradox
Gia Dvali
(Submitted on 15 Sep 2015)
We provide a model-independent argument indicating that for a black hole of entropy N the non-thermal deviations from Hawking radiation, per each emission time, are of order 1/N, as opposed to exp(-N). This fact abolishes the standard a priori basis for the information paradox.
5 pages

http://arxiv.org/abs/1509.04282
Scalar Dark Matter: Direct vs. Indirect Detection
Michael Duerr, Pavel Fileviez Perez, Juri Smirnov
(Submitted on 14 Sep 2015)
We revisit the simplest model for dark matter. In this context the dark matter candidate is a real scalar field which interacts with the Standard Model particles through the Higgs portal. We discuss the relic density constraints as well as the predictions for direct and indirect detection. The final state radiation processes are investigated in order to understand the visibility of the gamma lines from dark matter annihilation. We find two regions where one could observe the gamma lines at gamma-ray telescopes. We point out that the region where the dark matter mass is between 100 and 300 GeV can be tested in the near future at direct and indirect detection experiments.
Comments: 27 pages, 13 figures

http://arxiv.org/abs/1509.04325
Anomalies of the Entanglement Entropy in Chiral Theories
Nabil Iqbal, Aron C. Wall
(Submitted on 14 Sep 2015)
We study entanglement entropy in theories with gravitational or mixed U(1) gauge-gravitational anomalies in two, four and six dimensions. In such theories there is an anomaly in the entanglement entropy: it depends on the choice of reference frame in which the theory is regulated. We discuss subtleties regarding regulators and entanglement entropies in anomalous theories. We then study the entanglement entropy of free chiral fermions and self-dual bosons and show that in sufficiently symmetric situations this entanglement anomaly comes from an imbalance in the flux of modes flowing through the boundary, controlled by familiar index theorems. In two and four dimensions we use anomalous Ward identities to find general expressions for the transformation of the entanglement entropy under a diffeomorphism. (In the case of a mixed anomaly there is an alternative presentation of the theory in which the entanglement entropy is not invariant under a U(1) gauge transformation. The free-field manifestation of this phenomenon involves a novel kind of fermion zero mode on a gravitational background with a twist in the normal bundle to the entangling surface.) We also study d-dimensional anomalous systems as the boundaries of d + 1 dimensional gapped Hall phases. Here the full system is non-anomalous, but the boundary anomaly manifests itself in a change in the entanglement entropy when the boundary metric is sheared relative to the bulk.
45 pages + appendices, 5 figures

http://arxiv.org/abs/1509.04497
General relativity and cosmology
Martin Bucher (APC, U. Paris 7/CNRS, Paris, France & University of KwaZulu-Natal, Durban, South Africa), Wei-Tou Ni (National Tsing Hua University, Hsinchu, Taiwan)
(Submitted on 15 Sep 2015)
15 pages, 4 figures, to appear as introductory chapter in "One Hundred Years of General Relativity: From Genesis and Empirical Foundations to Gravitational Waves, Cosmology and Quantum Gravity"

 

[marcus]

http://arxiv.org/abs/1509.05693
Detailed analysis of the predictions of loop quantum cosmology for the primordial power spectra
Ivan Agullo, Noah A. Morris
(Submitted on 18 Sep 2015)
We provide an exhaustive numerical exploration of the predictions of loop quantum cosmology (LQC) with a post-bounce phase of inflation for the primordial power spectrum of scalar and tensor perturbations. We extend previous analysis by characterizing the phenomenologically relevant parameter space and by constraining it using observations. Furthermore, we characterize the shape of LQC-corrections to observable quantities across this parameter space. Our analysis provides a framework to contrast more accurately the theory with forthcoming polarization data, and it also paves the road for the computation of other observables beyond the power spectra, such as non-Gaussianity.
24 pages, 5 figures

possible wider interest:
http://arxiv.org/abs/1509.07767
Brief reply to "Can gravity account for the emergence of classicality?"
Igor Pikovski, Magdalena Zych, Fabio Costa, Časlav Brukner
(Submitted on 24 Sep 2015)
In a series of comments, Bonder et al. criticized our work on decoherence due to time dilation [Nature Physics 11, 668-672 (2015)]. First the authors erroneously claimed that our results contradict the equivalence principle, only to "resolve" the alleged conflict in a second note. The resolution - relativity of simultaneity - was already explained in our reply [arXiv:1508.03296], which Bonder et al. now essentially reiterate. The newly raised points were also already extensively clarified in our note. The physical prediction of our work remains valid: systems with internal dynamics decohere if the superposed paths have different proper times.
1 page

http://arxiv.org/abs/1509.07666
Summary of the XXVIIth Rencontres de Blois: Particle Physics and Cosmology
Gavin P. Salam
(Submitted on 25 Sep 2015)
This writeup summarises some of the highlights from the 2015 Rencontres de Blois, with a compression ratio of about 100:1 relative to the original presentations.
22 pages, 24 figures, presented at XXVIIth Rencontres de Blois, France, 31 May - 5 June 2015

http://arxiv.org/abs/1509.07506
Density Jumps Near the Virial Radius of Galaxy Clusters
Anna Patej, Abraham Loeb
(Submitted on 24 Sep 2015)
Recent simulations have indicated that the dark matter halos of galaxy clusters should feature steep density jumps near the virial radius. Since the member galaxies are expected to follow similar collisionless dynamics as the dark matter, the galaxy density profile should show such a feature as well. We examine the potential of current datasets to test this prediction by selecting cluster members for a sample of 56 low-redshift (0.1<z<0.3) galaxy clusters, constructing their projected number density profiles, and fitting them with two profiles, one with a steep density jump and one without. Additionally, we investigate the presence of a jump using a non-parametric spline approach. We find that some of these clusters show strong evidence for a model with a density jump. We discuss avenues for further analysis of the density jump with future datasets.
11 pages, 8 figures

http://arxiv.org/abs/1509.07501
Cosmological Hints of Modified Gravity ?
Eleonora Di Valentino, Alessandro Melchiorri, Joseph Silk
(Submitted on 24 Sep 2015)
The recent measurements of Cosmic Microwave Background temperature and polarization anisotropies made by the Planck satellite have provided impressive confirmation of the ΛCDM cosmological model. However interesting hints of slight deviations from ΛCDM have been found, including a 95% c.l. preference for a "modified gravity" structure formation scenario. In this paper we confirm the preference for a modified gravity scenario from Planck 2015 data, find that modified gravity solves the so-called Alensanomaly in the CMB angular spectrum, and constrains the amplitude of matter density fluctuations to σ₈=0.815+0.032−0.048, in better agreement with weak lensing constraints. Moreover, we find a lower value for the reionization optical depth of τ=0.059±0.020 (to be compared with the value of τ=0.079±0.017obtained in the standard scenario), more consistent with recent optical and UV data. We check the stability of this result by considering possible degeneracies with other parameters, including the neutrino effective number, the running of the spectral index and the amount of primordial helium. The indication for modified gravity is still present at about 95% c.l., and could become more significant if lower values of τ were to be further confirmed by future cosmological and astrophysical data.
10 pages, 5 figures

 

[marcus]

http://arxiv.org/abs/1509.08899
Generalized effective description of loop quantum cosmology
Abhay Ashtekar, Brajesh Gupt
(Submitted on 29 Sep 2015)
The effective description of loop quantum cosmology (LQC) has proved to be a convenient platform to study phenomenological implications of the quantum bounce that resolves the classical big-bang singularity. Originally, this description was derived using Gaussian quantum states with small dispersions. In this paper we present a generalization to incorporate states with large dispersions. Specifically, we derive the generalized effective Friedmann and Raychaudhuri equations and propose a generalized effective Hamiltonian which are being used in an ongoing study of the phenomenological consequences of a broad class of quantum geometries. We also discuss an interesting interplay between the physics of states with larger dispersions in standard LQC, and of sharply peaked states in (hypothetical) LQC theories with larger area gap.
21 pages, 4 figures

http://arxiv.org/abs/1509.09182
Loop quantum cosmology and the fate of cosmological singularities
Parampreet Singh
(Submitted on 30 Sep 2015)
Singularities in general relativity such as the big bang and big crunch, and exotic singularities such as the big rip are the boundaries of the classical spacetimes. These events are marked by a divergence in the curvature invariants and the breakdown of the geodesic evolution. Recent progress on implementing techniques of loop quantum gravity to cosmological models reveals that such singularities may be generically resolved because of the quantum gravitational effects. Due to the quantum geometry, which replaces the classical differential geometry at the Planck scale, the big bang is replaced by a big bounce without any assumptions on the matter content or any fine tuning. In this manuscript, we discuss some of the main features of this approach and the results on the generic resolution of singularities for the isotropic as well as anisotropic models. Using effective spacetime description of the quantum theory, we show the way quantum gravitational effects lead to the universal bounds on the energy density, the Hubble rate and the anisotropic shear. We discuss the geodesic completeness in the effective spacetime and the resolution of all of the strong singularities. It turns out that despite the bounds on energy density and the Hubble rate, there can be divergences in the curvature invariants. However such events are geodesically extendible, with tidal forces not strong enough to cause inevitable destruction of the in-falling objects.
26 pages, 1 figure. Invited review based on M K Vainu Bappu gold medal award lecture. Published in Bulletin of Astronomical Society of India

http://arxiv.org/abs/1509.08788
Recent results in CDT quantum gravity
Jan Ambjorn, Daniel Coumbe, Jakub Gizbert-Studnicki, Jerzy Jurkiewicz
(Submitted on 29 Sep 2015)
We review some recent results from the causal dynamical triangulation (CDT) approach to quantum gravity. We review recent observations of dimensional reduction at a number of previously undetermined points in the parameter space of CDT, and discuss their possible relevance to the asymptotic safety scenario. We also present an updated phase diagram of CDT, discussing properties of a newly discovered phase and its possible relation to a signature change of the metric.
6 pages, 3 figures, 1 table. Contribution to the proceedings of MG14, Rome July 2015

http://arxiv.org/abs/1509.09122
On selfdual spin-connections and Asymptotic Safety
Ulrich Harst, Martin Reuter
(Submitted on 30 Sep 2015)
We explore Euclidean quantum gravity using the tetrad field together with a selfdual or anti-selfdual spin-connection as the basic field variables. Setting up a functional renormalization group (RG) equation of a new type which is particularly suitable for the corresponding theory space we determine the non-perturbative RG flow within a two-parameter truncation suggested by the Holst action. We find that the (anti-)selfdual theory is likely to be asymptotically safe. The existing evidence for its non-perturbative renormalizability is comparable to that of Einstein-Cartan gravity without the selfduality condition.
14 pages, 4 fgures

possibly of wider interest:
http://arxiv.org/abs/1509.08772
Bounce Inflation Cosmology with Standard Model Higgs Boson
Youping Wan, Taotao Qiu, Fa Peng Huang, Yi-Fu Cai, Hong Li, Xinmin Zhang
(Submitted on 28 Sep 2015)
It is of great interest to connect cosmology in the early universe to the Standard Model of particle physics. In this paper, we try to construct a bounce inflation model with the standard model Higgs boson, where the one loop correction is taken into account in the effective potential of Higgs field. In this model, a Galileon term has been introduced to eliminate the ghost mode when bounce happens. Moreover, due to the fact that the Fermion loop correction can make part of the Higgs potential negative, one naturally obtains a large equation of state(EoS) parameter in the contracting phase, which can eliminate the anisotropy problem. After the bounce, the model can drive the universe into the standard higgs inflation phase, which can generate nearly scale-invariant power spectrum.
13 pages, 9 figures

 

[marcus]

http://arxiv.org/abs/1510.01925
Projective Loop Quantum Gravity II. Searching for Semi-Classical States
Suzanne Lanéry, Thomas Thiemann
(Submitted on 7 Oct 2015)
In [arXiv:1411.3592] an extension of the Ashtekar-Lewandowski (AL) state space of Loop Quantum Gravity was set up with the help a projective formalism introduced by Kijowski [Kijowski 1977; see also: arXiv:1304.6330, arXiv:1411.3590]. The motivation for this work was to achieve a more balanced treatment of the position and momentum variables (aka. holonomies and fluxes). Indeed, states in the AL Hilbert spaces describe discrete quantum excitations on top of a vacuum which is an eigenstate of the flux variables (a `no-geometry' state): in such states, most holonomies are totally spread, making it difficult to approximate a smooth, classical 4-geometry. However, going beyond the AL sector does not fully resolve this difficulty: one uncovers a deeper issue hindering the construction of states semi-classical with respect to a full set of observables. In the present article, we analyze this issue in the case of real-valued holonomies (we will briefly comment on the heuristic implications for other gauge groups, eg. (2)). Specifically, we show that, in this case, there does not exist any state on the holonomy-flux algebra in which the variances of the holonomies and fluxes observables would all be finite, let alone small. It is important to note that this obstruction cannot be bypassed by further enlarging the quantum state space, for it arises from the structure of the algebra itself: as there are too many (uncountably many) non-vanishing commutators between the holonomy and flux operators, the corresponding Heisenberg inequalities force the quantum uncertainties to blow up uncontrollably. A way out would be to suitably restrict the algebra of observables. In a companion paper we take the first steps in this direction by developing a general framework to perform such a restriction without giving up the universality and diffeomorphism invariance of the theory.
51 pages, 1 figure

http://arxiv.org/abs/1510.01926
Projective Limits of State Spaces IV. Fractal Label Sets
Suzanne Lanéry, Thomas Thiemann
(Submitted on 7 Oct 2015)
Instead of formulating the state space of a quantum field theory over one big Hilbert space, it has been proposed by Kijowski [Kijowski 1977] to represent quantum states as projective families of density matrices over a collection of smaller, simpler Hilbert spaces. One can thus bypass the need to select a vacuum state for the theory, and still be provided with an explicit and constructive description of the quantum state space, at least as long as the label set indexing the projective structure is countable. Because uncountable label sets are much less practical in this context, we develop in the present article a general procedure to trim an originally uncountable label set down to countable cardinality. In particular, we investigate how to perform this tightening of the label set in a way that preserves both the physical content of the algebra of observables and its symmetries. This work is notably motivated by applications to the holonomy-flux algebra underlying Loop Quantum Gravity. Building on earlier work by Okolow [arXiv:1304.6330], a projective state space was introduced for this algebra in [arXiv:1411.3592]. However, the non-trivial structure of the holonomy-flux algebra prevents the construction of satisfactory semi-classical states. Implementing the general procedure just mentioned in the case of a one-dimensional version of this algebra, we show how a discrete subalgebra can be extracted without destroying universality nor diffeomorphism invariance. On this subalgebra, states can then be constructed whose semi-classicality is enforced step by step, starting from collective, macroscopic degrees of freedom and going down progressively toward smaller and smaller scales.
42 pages, 1 figure

For convenience here are the previous four papers in this series:
arXiv:1411.3592
Projective Loop Quantum Gravity I. State Space
Suzanne Lanéry, Thomas Thiemann
81 pages, many figures

arXiv:1411.3589
Projective Limits of State Spaces I. Classical Formalism
Suzanne Lanéry, Thomas Thiemann
51 pages, many figures

arXiv:1411.3590
Projective Limits of State Spaces II. Quantum Formalism
Suzanne Lanéry, Thomas Thiemann
56 pages, 2 figures

arXiv:1411.3591
Projective Limits of State Spaces III. Toy-Models
Suzanne Lanéry, Thomas Thiemann
40 pages