Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1501.06270
Matter Bounce Scenario in F(T) gravity
Jaume Haro, Jaume Amorós
(Submitted on 26 Jan 2015)
It is shown that teleparallel F(T) theories of gravity combined with holonomy corrected Loop Quantum Cosmology (LQC) support a Matter Bounce Scenario (MBS) which is a potential alternative to the inflationary paradigm. The Matter Bounce Scenario is reviewed and, according to the current observational data provided by PLANCK's team, we have summarized all the conditions that it has to satisfy in order to be a viable alternative to inflation, such as to provide a theoretical value of the spectral index and its running compatible with the latest PLANCK data, to have a reheating process via gravitational particle production, or to predict some signatures in the non-gaussianities of the power spectrum. The calculation of the power spectrum for scalar perturbations and the ratio of tensor to scalar perturbations has been done, in the simplest case of an exact matter dominated background, for both holonomy corrected LQC and teleparallel F(T) gravity. Finally, we have discussed the challenges (essentially, dealing with non-gaussianities, the calculation of the 3-point function in flat spatial geometries for theories beyond General Relativity) and problems (Jeans instabilities in the case of holonomy corrected LQC or local Lorentz dependence in teleparallelism) that arise in either bouncing scenario.
6 pages. Communication to the FFP2014 (Frontiers in Fundamental Physics, Marseille 2014). To appear in Proceedings of Science

http://arxiv.org/abs/1501.06591
Superbounce and Loop Quantum Ekpyrotic Cosmologies from Modified Gravity: F(R), F(G) and F(T) Theories
S.D. Odintsov, V.K. Oikonomou, Emmanuel N. Saridakis
(Submitted on 26 Jan 2015)
We investigate the realization of two bouncing paradigms, namely of the superbounce and the loop quantum cosmological ekpyrosis, in the framework of various modified gravities. In particular, we focus on the F(R), F(G) and F(T) gravities, and we reconstruct their specific subclasses which lead to such universe evolutions. These subclasses constitute from power laws, polynomials, or hypergeometric ansatzes, which can be approximated by power laws. The qualitative similarity of different effective gravities which realize the above two bouncing cosmologies, indicates to some universality lying behind such a bounce. Finally, performing a linear perturbation analysis, we show that the obtained solutions are conditionally or fully stable.
31 pages.http://arxiv.org/abs/1501.05682
The Quantum Echo of the Early Universe
Ana Blasco, Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez
(Submitted on 22 Jan 2015)
We show that the fluctuations of quantum fields as seen by late comoving observers are significantly influenced by the history of the early Universe, and therefore they transmit information about the nature of spacetime in timescales when quantum gravitational effects were non-negligible. We discuss how this may be observable even nowadays, and thus used to build falsifiability tests of quantum gravity theories.
3 pages. 2 Figures. Proceedings of Theory Canada 9. Published in Canadian Journal of Physics.

http://arxiv.org/abs/1501.06282
Intrinsic Time Quantum Geometrodynamics
Eyo Eyo Ita III, Chopin Soo, Hoi-Lai Yu
(Submitted on 26 Jan 2015)
Quantum Geometrodynamics with intrinsic time development and momentric variables is presented. An underlying SU(3) group structure at each spatial point regulates the theory. The intrinsic time behavior of the theory is analyzed, together with its ground state and primordial quantum fluctuations. Cotton-York potential dominates at early times when the universe was small; the ground state naturally resolves Penrose's Weyl Curvature Hypothesis, and thermodynamic and gravitational `arrows of time' point in the same direction. Ricci scalar potential corresponding to Einstein's General Relativity emerges as a zero-point energy contribution. A new set of fundamental canonical commutation relations without Planck's constant emerges from the unification of Gravitation and Quantum Mechanics.
6 pages

possible general interest:
http://arxiv.org/abs/1501.05969
Quantum Superpositions Cannot be Epistemic
John-Mark A. Allen
(Submitted on 23 Jan 2015)
Quantum superposition states are behind many of the curious phenomena exhibited by quantum systems, including Bell non-locality, quantum interference, quantum computational speed-up, and the measurement problem. However, many qualitative properties of quantum superpositions can also be observed in classical probability distributions leading to a suspicion that superpositions may be explicable as probability distributions over less problematic states; that is, a suspicion that superpositions are epistemic. Here, it is proved that, for any quantum system of dimension d > 3, this cannot be the case for almost all superpositions. Equivalently, any underlying ontology must contain ontic superposition states. A related question concerns general possibility of non-orthogonal quantum states |ψ⟩,|ϕ⟩ being ontologically indistinct. A similar method proves that if |⟨ϕ|ψ⟩|²∈(0,14) then |ψ⟩,|ϕ⟩ must approach ontological distinctness as d→∞. The robustness of these results to small experimental error is also discussed.
5 + 7 pages

 

[marcus]

http://arxiv.org/abs/1502.00278
Compact phase space, cosmological constant, discrete time
Carlo Rovelli, Francesca Vidotto
(Submitted on 1 Feb 2015)
We study the quantization of geometry in the presence of a cosmological constant, using a discretization with constant-curvature simplices. Phase space turns out to be compact and the Hilbert space finite dimensional for each link. Not only the intrinsic, but also the extrinsic geometry turns out to be discrete, pointing to discreetness of time, in addition to space. We work in 2+1 dimensions, but these results may be relevant also for the physical 3+1 case.
6 pages

General interest, the tenuous possibility of a theory with a becoming, a "now". In contrast to the static block universe, which has no representation of the present moment.
http://arxiv.org/abs/1502.00018
What becomes of a causal set
Christian Wuthrich, Craig Callender
(Submitted on 30 Jan 2015)
Unlike the relativity theory it seeks to replace, causal set theory has been interpreted to leave space for a substantive, though perhaps 'localized', form of 'becoming'. The possibility of fundamental becoming is nourished by the fact that the analogue of Stein's theorem from special relativity does not hold in causal set theory. Despite this, we find that in many ways, the debate concerning becoming parallels the well-rehearsed lines it follows in the domain of relativity. We present, however, some new twists and challenges. In particular, we show that a novel and exotic notion of becoming is compatible with causal sets. In contrast to the 'localized' becoming considered compatible with the dynamics of causal set theory by its advocates, our novel kind of becoming, while not answering to the typical A-theoretic demands, is 'global' and objective.
22 pages, 3 figures; forthcoming in the British Journal for the Philosophy of Science

http://arxiv.org/abs/1502.01225
Schrodinger Evolution for the Universe: Reparametrization
Sean Gryb, Karim Thebault
(Submitted on 4 Feb 2015)
Starting from a generalized Hamilton-Jacobi formalism, we develop a new framework for constructing observables and their evolution in theories invariant under global time reparametrizations. Our proposal relaxes the usual Dirac prescription for the observables of a totally constrained system (`perennials') and allows one to recover the influential partial and complete observables approach in a particular limit. Difficulties such as the non-unitary evolution of the complete observables in terms of certain partial observables are explained as a breakdown of this limit. Identification of our observables (`mutables') relies upon a physical distinction between gauge symmetries that exist at the level of histories and states (`Type 1'), and those that exist at the level of histories and not states (`Type 2'). This distinction resolves a tension in the literature concerning the physical interpretation of the partial observables and allows for a richer class of observables in the quantum theory. There is the potential for the application of our proposal to the quantization of gravity when understood in terms of the Shape Dynamics formalism.
25 pages, 1 figure

http://arxiv.org/abs/1502.01907
Black holes: Their large interiors
Ingemar Bengtsson, Emma Jakobsson
(Submitted on 6 Feb 2015)
Christodoulou and Rovelli have remarked on the large interiors possessed by static black holes. We amplify their remarks, and extend them to the spinning case.
6 pages, 2 figures

 

[marcus]

http://arxiv.org/abs/1502.02431
Comparison of primordial tensor power spectra from the deformed algebra and dressed metric approaches in loop quantum cosmology
B. Bolliet, J. Grain, C. Stahl, L. Linsefors, A. Barrau
(Submitted on 9 Feb 2015)
Loop quantum cosmology tries to capture the main ideas of loop quantum gravity and to apply them to the Universe as a whole. Two main approaches within this framework have been considered to date for the study of cosmological perturbations: the dressed metric approach and the deformed algebra approach. They both have advantages and drawbacks. In this article, we accurately compare their predictions. In particular, we compute the associated primordial tensor power spectra. We show -- numerically and analytically -- that the large scale behavior is similar for both approaches and compatible with the usual prediction of general relativity. The small scale behavior is, the other way round, drastically different. Most importantly, we show that in a range of wavenumbers explicitly calculated, both approaches do agree on predictions that, in addition, differ from standard general relativity and do not depend on unknown parameters. These features of the power spectrum at intermediate scales might constitute a universal loop quantum cosmology prediction that can hopefully lead to observational tests and constraints. We also present a complete analytical study of the background evolution for the bouncing universe that can be used for other purposes.
14 pages, 4 figures

http://arxiv.org/abs/1502.02342
Loop quantum cosmology of Bianchi IX: Effective dynamics
Alejandro Corichi, Edison Montoya
(Submitted on 9 Feb 2015)
We study numerically the solutions to the effective equations of Bianchi IX spacetimes within Loop Quantum Cosmology. We consider Bianchi IX models with and without inverse triad corrections whose matter content is a scalar field without mass. The solutions are classified using the classical observables. We show that both effective theories --with lapse N=V and N=1-- solve the big bang singularity and reproduce the classical dynamics far from the bounce. Moreover, due to the spatial compactness, there is an infinity number of bounces and recollapses. We study the limit of large volume and show that both effective theories reproduce the same dynamics, thus recovering general relativity. We implement a procedure to identify amongst the Bianchi IX solutions, those that behave like k=0,1 FLRW as well as Bianchi I, II, and VII_0 models. The effective solutions exhibit Bianchi I phases with Bianchi II transitions and also Bianchi VII_0 phases, which had not been studied before, at the quantum nor effective level. We comment on the possible implications of these results for a quantum modification to the classical BKL behaviour.
24 pages, 7 figures

http://arxiv.org/abs/1502.02919
Noncommutative spectral geometry, Bogoliubov transformations and neutrino oscillations
Maria Vittoria Gargiulo, Mairi Sakellariadou, Giuseppe Vitiello
(Submitted on 10 Feb 2015)
In this report we show that neutrino mixing is intrinsically contained in Connes' noncommutative spectral geometry construction, thanks to the introduction of the doubling of algebra, which is connected to the Bogoliubov transformation. It is known indeed that these transformations are responsible for the mixing, turning the mass vacuum state into the flavor vacuum state, in such a way that mass and flavor vacuum states are not unitary equivalent. There is thus a red thread that binds the doubling of algebra of Connes' model to the neutrino mixing.
9 pages, DICE 14 proceeding

Not QG but possibly of general interest:
http://arxiv.org/abs/1502.02480
Entangled History
Jordan Cotler, Frank Wilczek
(Submitted on 9 Feb 2015)
We introduce quantum history states and their mathematical framework, thereby reinterpreting and extending the consistent histories approach to quantum theory. Through thought experiments, we demonstrate that our formalism allows us to analyze a quantum version of history in which we reconstruct the past by observations. In particular, we can pass from measurements to inferences about "what happened" in a way that is sensible and free of paradox. Our framework allows for a richer understanding of the temporal structure of quantum theory, and we construct history states that embody peculiar, non-classical correlations in time.
16 pages, 1 figure

http://arxiv.org/abs/1502.02087
How to include fermions into General relativity by exotic smoothness
T. Asselmeyer-Maluga, C.H. Brans
(Submitted on 7 Feb 2015)
This paper is two-fold. At first we will discuss the generation of source terms in the Einstein-Hilbert action by using (topologically complicated) compact 3-manifolds. There is a large class of compact 3-manifolds with boundary: a torus given as the complement of a (thickened) knot admitting a hyperbolic geometry, denoted as hyperbolic knot complements in the following. We will discuss the fermionic properties of this class of 3-manifolds, i.e. we are able to identify a fermion with a hyperbolic knot complement. Secondly we will construct a large class of space-times, the exotic ℝ4, containing this class of 3-manifolds naturally. We begin with a topological trivial space, the ℝ4, and change only the differential structure to obtain many nontrivial 3-manifolds. It is known for a long time that exotic ℝ4's generate extra sources of gravity (Brans conjecture) but here we will analyze the structure of these source terms more carefully. Finally we will state that adding a hyperbolic knot complement will result in the appearance of a fermion as source term in the Einstein-Hilbert action.
27 pages, 4 figures, accepted in Gen. Rel. Grav

Oddity:
http://arxiv.org/abs/1502.02429
Brian Josephson

 

[marcus]

http://arxiv.org/abs/1502.03230
An Extended Matter Bounce Scenario: current status and challenges
Jaume de Haro, Yi-Fu Cai
(Submitted on 11 Feb 2015)
As an alternative to the paradigm of slow roll inflation, we propose an extended scenario of the matter bounce cosmology in which the Universe has experienced a quasi-matter contracting phase with a variable background equation of state parameter. This extended matter bounce scenario can be realized by considering a single scalar field evolving along an approximately exponential potential. Our result reveals that the rolling of the scalar field in general leads to a running behavior on the spectral index of primordial cosmological perturbations and a negative running can be realized in this model. We constrain the corresponding parameter space by using the newly released Planck data. To apply this scenario, we revisit bouncing cosmologies within the context of modified gravity theories, in particular, the holonomy corrected loop quantum cosmology and teleparallel F(T) gravity. A gravitational process of reheating is presented in such a matter bounce scenario to demonstrate the condition of satisfying current observations. We also comment on several unresolved issues that often appear in matter bounce models.
31 pages, 2 figures.

http://arxiv.org/abs/1502.03410
The Montevideo Interpretation of Quantum Mechanics: a short review
Rodolfo Gambini, Jorge Pullin
(Submitted on 11 Feb 2015)
The Montevideo interpretation of quantum mechanics, which consists in supplementing environmental decoherence with fundamental limitations in measurement stemming from gravity, has been described in several publications. However, some of them appeared before the full picture provided by the interpretation was developed. As such it can be difficult to get a good understanding via the published literature. Here we summarize it in a self contained brief presentation including all its principal elements.
10 pages

brief mention:
http://arxiv.org/abs/1502.03292
Is there a C-function in 4D Quantum Einstein Gravity?
Daniel Becker, Martin Reuter

http://arxiv.org/abs/1502.03129
Firewalls as artefacts of inconsistent truncations of quantum geometries
Cristiano Germani, Debajyoti Sarkar

 

[John86]

http://arxiv.org/abs/1502.03424
Is dark energy an artifact of decoherence?
Chris Fields
(Submitted on 11 Feb 2015)
Within the quantum Darwinist framework introduced by W. H. Zurek ({\em Nat. Phys.}, 5:181-188, 2009), observers obtain pointer-state information about quantum systems by interacting with the surrounding environment, e.g. the ambient photon field. This framework is applied to the observation of stellar center-of-mass positions, which are assumed to be encoded in a way that is uniformly accessible to all observers regardless of their location. Assuming Landauer's Principle, constructing such environmental encodings requires ∼ kT per bit. For 1025 stars and a binary encoding of center-of-mass positions into 10 km3 voxels, the free energy required at T = 2.7 K is ∼ 5 ⋅ 10−27 kg ⋅ m−3, in striking agreement with the observed value of ΩΛρc. Decreasing the voxel size to l3P results in a free energy requirement 10117 times larger.

http://arxiv.org/abs/1502.02198
Emergence of Four Dimensions in the Causal Set Approach to Discrete Quantum Gravity
Stan Gudder
(Submitted on 8 Feb 2015)
One could begin a study like the present one by simply postulating that our universe is four-dimensional. There are ample reasons for doing this. Experience, observation and experiment all point to the fact that we inhabit a four-dimensional universe. Another approach would be to show that four-dimensions arise naturally from a reasonable model of the universe or multiverse. After reviewing the causal set approach to discrete quantum gravity in Section~1, we shall discuss the emergence of four-dimensions in Section~2. We shall see that certain patterns of four arise that suggest the introduction of a 4-dimensional discrete manifold. In the later sections we shall discuss some consequences of this introduced framework. In particular, we will show that quantum amplitudes can be employed to describe a multiverse dynamics. Moreover, a natural unitary operator together with energy, position and momentum operators will be introduced and their properties studied.

http://arxiv.org/abs/1502.02833
On the initial singularity problem in rainbow cosmology
Grasiele dos Santos, Giulia Gubitosi, Giovanni Amelino-Camelia
(Submitted on 10 Feb 2015)
It has been recently claimed that the initial singularity might be avoided in the context of rainbow cosmology, where one attempts to account for quantum-gravitational corrections through an effective-theory description based on an energy-dependent ("rainbow") spacetime metric. We here scrutinize this exciting hypothesis much more in depth than ever done before. In particular, we take into account all requirements for singularity avoidance, while previously only a subset of these requirements had been considered. Moreover, we show that the implications of a rainbow metric for thermodynamics are more significant than previously appreciated. Through the analysis of two particularly meaningful examples of rainbow metrics we find that our concerns are not merely important conceptually, but actually change in quantitatively significant manner the outcome of the analysis. Notably we only find examples where the singularity is not avoided, though one can have that in the regime where our semi-classical picture is still reliable the approach to the singularity is slowed down when compared to the standard classical scenario. We conclude that the study of rainbow metrics provides tantalizing hints of singularity avoidance but is inconclusive, since some key questions remain to be addressed just when the scale factor is very small, a regime which, as here argued, cannot be reliably described by an effective rainbow-metric picture.

http://arxiv.org/abs/1502.02758
Conformally Friedmann-Lemaitre-Robertson-Walker cosmologies
Matt Visser (Victoria University of Wellington)
(Submitted on 10 Feb 2015)
In a universe where, according to the standard cosmological models, some 97% of the total mass-energy is still "missing in action" it behooves us to spend at least a little effort critically assessing and exploring radical alternatives. Among possible, (dare we say plausible), nonstandard but superficially viable models, those spacetimes conformal to the standard Friedmann-Lemaitre-Robertson-Walker class of cosmological models play a very special role --- these models have the unique and important property of permitting large non-perturbative geometric deviations from Friedmann-Lemaitre-Robertson-Walker cosmology without unacceptably distorting the cosmic microwave background. Performing a "cosmographic" analysis, (that is, temporarily setting aside the Einstein equations, since the question of whether or not the Einstein equations are valid on galactic and cosmological scales is essentially the same question as whether or not dark matter/dark energy actually exist), and using both supernova data and information about galactic structure, one can nevertheless place some quite significant observational constraints on any possible conformal mode --- however there is still an extremely rich range of phenomenological possibilities for both cosmologists and astrophysicists to explore.

 

[marcus]

Not Loop-and-allied quantum gravity, but possibly of general interest:
http://arxiv.org/abs/1502.03410
The Montevideo Interpretation of Quantum Mechanics: a short review
Rodolfo Gambini, Jorge Pullin
(Submitted on 11 Feb 2015)
The Montevideo interpretation of quantum mechanics, which consists in supplementing environmental decoherence with fundamental limitations in measurement stemming from gravity, has been described in several publications. However, some of them appeared before the full picture provided by the interpretation was developed. As such it can be difficult to get a good understanding via the published literature. Here we summarize it in a self contained brief presentation including all its principal elements.
10 pages,

http://arxiv.org/abs/1502.03831
Quantum mechanics, strong emergence and ontological non-reducibility
Rodolfo Gambini, Lucia Lewowicz, Jorge Pullin
(Submitted on 12 Feb 2015)
We show that a new interpretation of quantum mechanics, in which the notion of event is defined without reference to measurement or observers, allows to construct a quantum general ontology based on systems, states and events. Unlike the Copenhagen interpretation, it does not resort to elements of a classical ontology. The quantum ontology in turn allows us to recognize that a typical behavior of quantum systems exhibits strong emergence and ontological non-reducibility. Such phenomena are not exceptional but natural, and are rooted in the basic mathematical structure of quantum mechanics.
8 pages, to appear in Foundations of Chemistry

http://arxiv.org/abs/1502.04640
The Lorentzian proper vertex amplitude: Classical analysis and quantum derivation
Jonathan Engle, Antonia Zipfel
(Submitted on 16 Feb 2015)
Spin foam models, an approach to defining the dynamics of loop quantum gravity, make use of the Plebanski formulation of gravity, in which gravity is recovered from a topological field theory via certain constraints called simplicity constraints. However, the simplicity constraints in their usual form select more than just one gravitational sector as well as a degenerate sector. This was shown, in previous work, to be the reason for the "extra" terms appearing in the semiclassical limit of the Euclidean EPRL amplitude. In this previous work, a way to eliminate the extra sectors, and hence terms, was developed, leading to the what was called the Euclidean proper vertex amplitude. In the present work, these results are extended to the Lorentzian signature, establishing what is called the Lorentzian proper vertex amplitude. This extension is non-trivial and involves a number of new elements since, for Lorentzian bivectors, the split into self-dual and anti-self-dual parts, on which the Euclidean derivation was based, is no longer available. In fact, the classical parts of the present derivation provide not only an extension to the Lorentzian case, but also, with minor modifications, provide a new, more four dimensionally covariant derivation for the Euclidean case. The new elements in the quantum part of the derivation are due to the different structure of unitary representations of the Lorentz group.
36 pages

http://arxiv.org/abs/1502.05619
Gravitational Lensing by Self-Dual Black Holes in Loop Quantum Gravity
Satyabrata Sahu, Kinjalk Lochan, D. Narasimha
(Submitted on 19 Feb 2015)
We study gravitational lensing by a recently proposed black hole solution in Loop Quantum Gravity. We highlight the fact that the quantum gravity corrections to the Schwarzschild metric in this model evade the `mass suppression' effects (that the usual quantum gravity corrections are susceptible to) by virtue of one of the parameters in the model being dimensionless, which is unlike any other quantum gravity motivated parameter. Gravitational lensing in the strong and weak deflection regimes is studied and a sample consistency relation is presented which could serve as a test of this model. We discuss that though the consistency relation for this model is qualitatively similar to what would have been in Brans-Dicke, in general it can be a good discriminator between many alternative theories. Although the observational prospects do not seem to be very optimistic even for a galactic supermassive black hole case, time delay between relativistic images for billion solar mass black holes in other galaxies might be within reach of future relativistic lensing observations.
13 pages; 4 figures; accepted for publication in Physical Review D

http://arxiv.org/abs/1502.05388
Boundary Terms for Causal Sets
Michel Buck, Fay Dowker, Ian Jubb, Sumati Surya
(Submitted on 18 Feb 2015)
We propose a family of boundary terms for the action of a causal set with a spacelike boundary. We show that in the continuum limit one recovers the Gibbons-Hawking-York boundary term in the mean. We also calculate the continuum limit of the mean causal set action for an Alexandrov interval in flat spacetime. We find that it is equal to the volume of the codimension-2 intersection of the two light-cone boundaries of the interval.
23 pages, 4 figures

http://arxiv.org/abs/1502.05296
Lectures on the Cosmological Constant Problem
Antonio Padilla
(Submitted on 18 Feb 2015)
These lectures on the cosmological constant problem were prepared for the X Mexican School on Gravitation and Mathematical Physics. The problem itself is explained in detail, emphasising the importance of radiative instability and the need to repeatedly fine tune as we change our effective description. Weinberg's no go theorem is worked through in detail. I review a number of proposals including Linde's universe multiplication, Coleman's wormholes, the fat graviton, and SLED, to name a few. Large distance modifications of gravity are also discussed, with causality considerations pointing towards a global modification as being the most sensible option. The global nature of the cosmological constant problem is also emphasized, and as a result, the sequestering scenario is reviewed in some detail, demonstrating the cancellation of the Standard Model vacuum energy through a global modification of General Relativity.
31 pages

 

[marcus]

http://arxiv.org/abs/1502.06125
ΛCDM Bounce Cosmology without ΛCDM: the case of modified gravity
S.D. Odintsov, V.K. Oikonomou
(Submitted on 21 Feb 2015)
We provide an F(R) gravity description of a ΛCDM bouncing model, without the need for matter fluids or for cosmological constant. As we explicitly demonstrate, the two cosmological eras that constitute the ΛCDM bouncing model, can be generated by F(R) gravity which can lead to accelerating cosmologies. The resulting F(R) gravity has Einstein frame inflationary properties that have concordance to the latest Planck observational data. Both the F(R) gravity stability properties are thoroughly investigated and also, the gravitational particle production, a feature necessary for the viability of the ΛCDM bounce scenario, is also addressed. As we will show, the ΛCDM bounce model can be successfully described by pure F(R) gravity, with appealing phenomenological attributes, which we extensively discuss.
31 pages, accepted by PRD

http://arxiv.org/abs/1502.05875
Perturbations in Bouncing and Cyclic Models, a General Study
Tirthabir Biswas, Riley Mayes, Colleen Lattyak
(Submitted on 18 Feb 2015)
Being able to reliably track perturbations across bounces and turnarounds in cyclic and bouncing cosmology lies at the heart of being able to compare the predictions of these models with the Cosmic Microwave Background observations. This has been a challenging task due to the unknown nature of the physics involved during the bounce as well as the technical challenge of matching perturbations precisely between the expansion and contraction phases. In this paper, we will present general techniques (analytical and numerical) that can be applied to understand the physics of the fluctuations, especially those with "long" wavelengths, and test its validity in some simple bouncing/cyclic toy models where the physics is well understood. We will then apply our techniques to more interesting cosmological models such as the bounce inflation and cyclic inflation.
21 pages, 12 figures

possible wider interest:
http://arxiv.org/abs/1502.06539
Fields as Bodies: a unified presentation of spacetime and internal gauge symmetry
David Wallace
(Submitted on 23 Feb 2015)
Using the parametrised representation of field theory (in which the location in spacetime of a part of a field is itself represented by a map from the base manifold to Minkowski spacetime) I demonstrate that in both local and global cases, internal (Yang-Mills-type) and spacetime (Poincaré) symmetries can be treated precisely on a par, so that gravitational theories may be regarded as gauge theories in a completely standard sense.
10 pages

http://arxiv.org/abs/1502.06141
ℏ as a Physical Constant of Classical Optics and Electrodynamics
Real Tremblay, Nicolas Doyon, Claudine Ni Allen
(Submitted on 21 Feb 2015)
The Planck constant (ℏ) plays a pivotal role in quantum physics. Historically, it has been proposed as postulate, part of a genius empirical relationship E=ℏω in order to explain the intensity spectrum of the blackbody radiation for which classical electrodynamic theory led to an unacceptable prediction: The ultraviolet catastrophe. While the usefulness of the Planck constant in various fields of physics is undisputed, its derivation (or lack of) remains unsatisfactory from a fundamental point of view. In this paper, the analysis of the blackbody problem is performed with a series expansion of the electromagnetic field in terms of TE, TM modes in a metallic cavity with small losses, that leads to developing the electromagnetic fields in a \textit{complete set of orthonormal functions}. This expansion, based on coupled power theory, maintains both space and time together enabling modeling of the blackbody's evolution toward equilibrium. Reaching equilibrium with a multimodal waveguide analysis brings into consideration the coupling between modes in addition to absorption and emission of radiation. The properties of the modes, such as spectral broadening, losses and lifetime, then progressively become independent of frequency and explains how equilibrium is allowed in good conductor metallic cavities. Based on the free electron relaxation time in gold, a value of ℏ=1.02×10⁻³⁴ J⋅s for the reduced Planck constant is found and the uncertainty principle is also emerging from this a priori classical study. The Planck constant is then obtained no longer as an ad hoc addition but as a natural consequence of the analysis taking boundary conditions into account as into optical resonators. That analysis based on finite-spacetime paradigm, also shine new light on the notion of decoherence in classical optics and electrodynamics.
26 pages, 4 figures.

 

[wabbit]

Hi marcus interesting pointers as always, just wanted to let you know your very first link is misdirected.

 

[marcus]

Thanks for catching that, wabbit! Fixed.

http://arxiv.org/abs/1502.06770
Quantum Transitions Between Classical Histories: Bouncing Cosmologies
James Hartle, Thomas Hertog
(Submitted on 24 Feb 2015)
In a quantum theory of gravity spacetime behaves classically when quantum probabilities are high for histories of geometry and field that are correlated in time by the Einstein equation. Probabilities follow from the quantum state. This quantum perspective on classicality has important implications:
(a) Classical histories are generally available only in limited patches of the configuration space on which the state lives.
(b) In a given patch states generally predict relative probabilities for an ensemble of possible classical histories.
(c) In between patches classical predictability breaks down and is replaced by quantum evolution connecting classical histories in different patches.
(d) Classical predictability can break down on scales well below the Planck scale, and with no breakdown in the classical equations of motion.
We support and illustrate (a)-(d) by calculating the quantum transition across the de Sitter like throat connecting asymptotically classical, inflating histories in the no-boundary quantum state. This supplies probabilities for how a classical history on one side transitions and branches into a range of classical histories on the opposite side. We also comment on the implications of (a)-(d) for the dynamics of black holes and eternal inflation.
36 pages, 6 figures

 

[John86]

http://arxiv.org/abs/1502.08005
Metastring Theory and Modular Space-time
Laurent Freidel, Robert G. Leigh, Djordje Minic
(Submitted on 27 Feb 2015)
String theory is canonically accompanied with a space-time interpretation which determines S-matrix-like observables, and connects to the standard physics at low energies in the guise of local effective field theory. Recently, we have introduced a reformulation of string theory which does not rely on an {\it a priori} space-time interpretation or a pre-assumption of locality. This \hlt{metastring theory} is formulated in such a way that stringy symmetries (such as T-duality) are realized linearly. In this paper, we study metastring theory on a flat background and develop a variety of technical and interpretational ideas. These include a formulation of the moduli space of Lorentzian worldsheets, a careful study of the symplectic structure and consequently consistent closed and open boundary conditions, and the string spectrum and operator algebra. What emerges from these studies is a new quantum notion of space-time that we refer to as a quantum Lagrangian or equivalently a \hlt{modular space-time}. This concept embodies the standard tenets of quantum theory and implements in a precise way a notion of {relative locality}. The usual string backgrounds (non-compact space-time along with some toroidally compactified spatial directions) are obtained from modular space-time by a limiting procedure that can be thought of as a correspondence limit.

 

[marcus]

http://arxiv.org/abs/1502.07789
Uniqueness of Measures in Loop Quantum Cosmology
Maximilian Hanusch
(Submitted on 26 Feb 2015)
In a paper of Ashtekar and Campiglia, residual diffeomorphisms have been used to single out the standard representation of the reduced holonomy-flux algebra in homogeneous loop quantum cosmology (LQC). We show that, in the homogeneous isotropic case, unitarity of the translations w.r.t. the extended ℝ-action (exponentiated reduced fluxes in the standard approach) singles out the Bohr measure on both the standard quantum configuration space ℝBohr as well as on the Fleischhack one. Thus, leads to the standard kinematical Hilbert space of LQC in both situations.
4 pages

http://arxiv.org/abs/1503.00442
Inflationary cosmology in modified gravity theories
Kazuharu Bamba, Sergei D. Odintsov
(Submitted on 2 Mar 2015)
We review inflationary cosmology in modified gravity such as R² gravity with its extensions in order to generalize the Starobinsky inflation model. In particular, we explore inflation realized by three kinds of effects: modification of gravity, the quantum anomaly, and the R² term in loop quantum cosmology. It is explicitly demonstrated that in these inflationary models, the spectral index of scalar modes of the density perturbations and the tensor-to-scalar ratio can be consistent with the Planck results. Bounce cosmology in F(R) gravity is also explained.
24 pages, invited review to appear in Symmetry

http://arxiv.org/abs/1503.01671
Aspects of the Bosonic Spectral Action
Mairi Sakellariadou (King's College London)
(Submitted on 5 Mar 2015)
A brief description of the elements of noncommutative spectral geometry as an approach to unification is presented. The physical implications of the doubling of the algebra are discussed. Some high energy phenomenological as well as various cosmological consequences are presented. A constraint in one of the three free parameters, namely the one related to the coupling constants at unification, is obtained, and the possible role of scalar fields is highlighted. A novel spectral action approach based upon zeta function regularisation, in order to address some of the issues of the traditional bosonic spectral action based on a cutoff function and a cutoff scale, is discussed.
16 pages, Invited talk in the Fourth Symposium on Prospects in the Physics of Discrete Symmetries, DISCRETE 2014, King's College London,2-6 December 2014

http://arxiv.org/abs/1503.01636
The microscopic structure of 2D CDT coupled to matter
J. Ambjorn, A. Goerlich, J. Jurkiewicz, H. Zhang
(Submitted on 5 Mar 2015)
We show that for 1+1 dimensional Causal Dynamical Triangulations (CDT) coupled to 4 massive scalar fields one can construct an effective transfer matrix if the masses squared is larger than or equal to 0.05. The properties of this transfer matrix can explain why CDT coupled to matter can behave completely different from "pure" CDT. We identify the important critical exponent in the effective action, which may determine the universality class of the model.
14 pages,lot of figures

http://arxiv.org/abs/1503.00359
Creation of quantized particles, gravitons and scalar perturbations by the expanding universe
Leonard Parker
(Submitted on 1 Mar 2015)
Quantum creation processes during the very rapid early expansion of the universe are believed to give rise to temperature anisotropies and polarization patterns in the CMB radiation. These have been observed by satellites such as COBE, WMAP, and PLANCK, and by bolometric instruments placed near the South Pole by the BICEP collaborations. The expected temperature anisotropies are well-confirmed. The B-mode polarization patterns in the CMB are currently under measurement jointly by the PLANCK and BICEP groups to determine the extent to which the B-modes can be attributed to gravitational waves from the creation of gravitons in the earliest universe. It was during 1962 that I proved that quanta of the minimally-coupled scalar field were created by the general expanding FLRW universe. This was relevant also to the creation of quantized perturbations of the gravitational field, since these perturbations satisfied linear field equations that could be quantized in the same way as the minimally-coupled scalar field equation. In fact, in 1946, E.M. Lifshitz had considered the classical Einstein gravitational field in FLRW expanding universes and had shown that the classical linearized Einstein field equations reduced, in what is now known as the Lifshitz gauge, to two separate classical minimally-coupled massless scalar field equations. These field equations of Lifshitz, when quantized, correspond to the field equations for massless gravitons, one equation for each of the two independent polarization components of the spin-2 massless graviton. I will discuss this further in this article.1 1 Plenary Lecture given September 2, 2014 at the ERE2014 Conference in Valencia, Spain
Plenary Lecture given September 2, 2014 at the ERE2014 Conference in Valencia, Spain To appear in the Proceedings of the ERE2014 Conference

http://arxiv.org/abs/1503.01567
Coherent Quantum Dynamics: What Fluctuations Can Tell
John Schliemann
(Submitted on 5 Mar 2015)
Coherent states provide a natural connection of quantum systems to their classical limit and are employed in various fields of physics. Here we derive general systematic expansions, with respect to quantum parameters, of expectation values of products of arbitrary operators within both oscillator coherent states and SU(2) coherent states. In particular, we generally prove that the energy fluctuations of an arbitrary Hamiltonian are in leading order entirely due to the time dependence of the classical variables. These results add to the list of wellknown properties of coherent states and are applied here to the Lipkin-Meshkov-Glick model, the Dicke model, and to coherent intertwiners in spin networks as considered in Loop Quantum Gravity.
13 pages.

possible interest:
http://arxiv.org/abs/1503.02366
Generating Luminous and Dark Matter During Inflation
Neil D. Barrie, Archil Kobakhidze
(Submitted on 9 Mar 2015)
We propose a new mechanism for generating both luminous and dark matter during cosmic inflation. ...
14 pages

 

[marcus]

http://arxiv.org/abs/1503.02916
Inhomogeneous Dark Fluid and Dark Matter, Leading to a Bounce Cosmology
Iver Brevik, Alexander Timoshkin
(Submitted on 10 Mar 2015)
The purpose of this short review is to describe cosmological models with a linear inhomogeneous time-dependent equation of state (EoS) for the dark energy, when the dark fluid is coupled with dark matter. This may lead to a bounce cosmology. We consider equivalent descriptions in terms of the EoS parameters for an exponential, a power-law, or a double-exponential law for the scale factor a. Stability issues are discussed by considering small perturbations around the critical points for the bounce, in the early as well as in the late, universe. The latter part of the paper is concerned with dark energy coupled with dark matter in viscous fluid cosmology. We allow the bulk viscosity ζ=ζ(H,t) to be a function of the Hubble parameter and the time, and consider the Little Rip, the Pseudo Rip, and the bounce universe. Analytic expressions for characteristic properties of these cosmological models are obtained.
13 pages. Mini-review, to appear in the MDPI journal Universe

http://arxiv.org/abs/1503.02981
Four-Dimensional Entropy from Three-Dimensional Gravity
S. Carlip
(Submitted on 10 Mar 2015)
At the horizon of a black hole, the action of (3+1)-dimensional loop quantum gravity acquires a boundary term that is formally identical to an action for three-dimensional gravity. I show how to use this correspondence to obtain the entropy of the (3+1)-dimensional black hole from well-understood conformal field theory computations of the entropy in (2+1)-dimensional de Sitter space.
8 pages

http://arxiv.org/abs/1503.03030
Energy in first order 2+1 gravity
Alejandro Corichi, Irais Rubalcava-Garcia
(Submitted on 10 Mar 2015)
We consider Lambda=0 three dimensional gravity with asymptotically flat boundary conditions. This system was studied by Ashtekar and Varadarajan within the second order formalism -with metric variables- who showed that the Regge-Teitelboim formalism yields a consistent Hamiltonian description where, surprisingly, the energy is bounded from below and from above. The energy of the spacetime is, however, determined up to an arbitrary constant. The natural choice was to fix that freedom such that Minkowski spacetime has zero energy. More recently, Marolf and Patiño started from the Einstein-Hilbert action supplemented with the Gibbons-Hawking term and showed that, in the 2+1 decomposition of the theory, the energy is shifted from the Ashtekar-Varadarajan analysis in such a way that Minkowski spacetime possesses a negative energy. In this contribution we consider the first order formalism, where the fundamental variables are a so(2,1) connection ωaIJ and a triad eIa. We consider two actions. A natural extension to 3 dimensions of the consistent action in 4D Palatini gravity is shown to be finite and differentiable. For this action, the 2+1 decomposition (that we perform using two methods) yields a Hamiltonian boundary term that corresponds to energy. It assigns zero energy to Minkowski spacetime. We then put forward a totally gauge invariant action, and show that it is also well defined and differentiable. Interestingly, it turns out to be related, on shell, to the 3D Palatini action by an additive constant in such a way that its associated energy is given by the Marolf-Patiño expression. Thus, we conclude that, from the perspective of the first order formalism, Minkowski spacetime can consistently have either, zero, or a negative energy equal to -1/4G, depending on the choice of consistent action employed as starting point.
36 pages

 

[John86]

http://arxiv.org/abs/1503.01774
A quantum peek inside the black hole event horizon
Sumanta Chakraborty, Suprit Singh, T. Padmanabhan
(Submitted on 5 Mar 2015)
We solve the Klein-Gordon equation for a scalar field, in the background geometry of a dust cloud collapsing to form a black hole, everywhere in the (1+1) spacetime: that is, both inside and outside the event horizon and arbitrarily close to the curvature singularity. This allows us to determine the regularized stress tensor expectation value, everywhere in the appropriate quantum state (viz., the Unruh vacuum) of the field. We use this to study the behaviour of energy density and the flux measured in local inertial frames for the radially freely falling observer at any given event. Outside the black hole, energy density and flux lead to the standard results expected from the Hawking radiation emanating from the black hole, as the collapse proceeds. Inside the collapsing dust ball, the energy densities of both matter and scalar field diverge near the singularity in both (1+1) and (1+3) spacetime dimensions; but the energy density of the field dominates over that of classical matter. In the (1+3) dimensions, the total energy (of both scalar field and classical matter) inside a small spatial volume around the singularity is finite (and goes to zero as the size of the region goes to zero) but the total energy of the quantum field still dominates over that of the classical matter. Inside the event horizon, but \textit{outside} the collapsing matter, freely falling observers find that the energy density and the flux diverge close to the singularity. In this region, even the integrated energy inside a small spatial volume enclosing the singularity diverges. This result holds in both (1+1) and (1+3) spacetime dimensions with a \emph{milder} divergence for the total energy inside a small region in (1+3) dimensions. These results suggest that the back-reaction effects are significant even in the region \emph{outside the matter but inside the event horizon}, close to the singularity.

 

[marcus]

http://arxiv.org/abs/1503.03407
Unitarity and ultraviolet regularity in cosmology
Ivan Agullo, Abhay Ashtekar
(Submitted on 11 Mar 2015)
Quantum field theory in curved space-times is a well developed area in mathematical physics which has had important phenomenological applications to the very early universe. However, it is not commonly appreciated that on time dependent space-times ---including the simplest cosmological models--- dynamics of quantum fields is not unitary in the standard sense. This issue is first explained with an explicit example and it is then shown that a generalized notion of unitarity does hold. The generalized notion allows one to correctly pass to the Schrödinger picture starting from the Heisenberg picture used in the textbook treatments. Finally, we indicate how these considerations can be extended from simple cosmological models to general globally hyperbolic space-times
30 pages

http://arxiv.org/abs/1503.03907
Gauge-Invariant Perturbations in Hybrid Quantum Cosmology
Laura Castelló Gomar, Mercedes Martín-Benito, Guillermo A. Mena Marugán
(Submitted on 12 Mar 2015)
We consider cosmological perturbations around homogeneous and isotropic spacetimes minimally coupled to a scalar field and present a formulation which is designed to preserve covariance. We truncate the action at quadratic perturbative order and particularize our analysis to flat compact spatial sections and a field potential given by a mass term, although the formalism can be extended to other topologies and potentials. The perturbations are described in terms of Mukhanov-Sasaki gauge invariants, linear perturbative constraints, and variables canonically conjugate to them. This set is completed into a canonical one for the entire system, including the homogeneous degrees of freedom. We find the global Hamiltonian constraint of the model, in which the contribution of the homogeneous sector is corrected with a term quadratic in the perturbations, that can be identified as the Mukhanov-Sasaki Hamiltonian in our formulation. We then adopt a hybrid approach to quantize the model, combining a quantum representation of the homogeneous sector with a more standard field quantization of the perturbations. Covariance is guaranteed in this approach inasmuch as no gauge fixing is adopted. Next, we adopt a Born-Oppenheimer ansatz for physical states and show how to obtain a Schrödinger-like equation for the quantum evolution of the perturbations. This evolution is governed by the Mukhanov-Sasaki Hamiltonian, with the dependence on the homogeneous geometry evaluated at quantum expectation values, and with a time parameter defined also in terms of suitable expectation values on that geometry. Finally, we derive effective equations for the dynamics of the Mukhanov-Sasaki gauge invariants, that include quantum contributions, but have the same ultraviolet limit as the classical equations. They provide the master equation to extract predictions about the power spectrum of primordial scalar perturbations.
35 pages, prepared for submission to JCAP

possible general interest:
http://arxiv.org/abs/1503.05270
Efficiently Controllable Graphs
Can Gokler, Kevin Thompson, Peter Shor, Seth Lloyd
(Submitted on 18 Mar 2015)
We show that universal quantum computation can be performed efficiently on quantum networks while the fraction of controlled subsystems vanishes as the network grows larger. We provide examples of quantum spin network families admitting polynomial quantum gate complexity with a vanishing fraction of controlled spins. We define a new family of graphs, the efficiently controllable family, which admits efficient computation with vanishing fraction of controls. We explore generalizations to percolation clusters, fractals and random graphs. We show that the classical computational complexity of estimating the ground state of Hamiltonians described by controllable graphs is polynomial in the number of subsystems/qubits.
5 pages.
My comment is it sounds interesting but I don't understand quantum computation at all well so I can't judge.

http://arxiv.org/abs/1503.05007
The Evolution of Quantum Field Theory, From QED to Grand Unification
Gerard 't Hooft
(Submitted on 17 Mar 2015)
In the early 1970s, after a slow start, and lots of hurdles, Quantum Field Theory emerged as the superior doctrine for understanding the interactions between relativistic sub-atomic particles. After the conditions for a relativistic field theoretical model to be renormalizable were established, there were two other developments that quickly accelerated acceptance of this approach: first the Brout-Englert-Higgs mechanism, and then asymptotic freedom. Together, these gave us a complete understanding of the perturbative sector of the theory, enough to give us a detailed picture of what is now usually called the Standard Model. Crucial for this understanding were the strong indications and encouragements provided by numerous experimental findings. Subsequently, non-perturbative features of the quantum field theories were addressed, and the first proposals for completely unified quantum field theories were launched. Since the use of continuous symmetries of all sorts, together with other topics of advanced mathematics, were recognised to be of crucial importance, many new predictions were pointed out, such as the Higgs particle, supersymmetry and baryon number violation. There are still many challenges ahead.
25 pages in total. A contribution to: The Standard Theory up to the Higgs discovery - 60 years of CERN - L. Maiani and G. Rolandi, eds

My comment: might reviewing the history of QFT help increase one's general understanding?

 

[marcus]

http://arxiv.org/abs/1503.05943
Curvatures and discrete Gauss-Codazzi equation in (2+1)-dimensional loop quantum gravity
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen
(Submitted on 19 Mar 2015)
We derive the Gauss-Codazzi equation in the holonomy and plane-angle representations and we use the result to write a Gauss-Codazzi equation for a discrete (2+1)-dimensional manifold, triangulated by isosceles tetrahedra. This allows us to write operators acting on spin network states in (2+1)-dimensional loop quantum gravity, representing the 3-dimensional intrinsic, 2-dimensional intrinsic, and 2-dimensional extrinsic curvatures.
16 pages, 10 figures

http://arxiv.org/abs/1503.06085
Time asymmetric extensions of general relativity
Marina Cortes, Henrique Gomes, Lee Smolin
(Submitted on 20 Mar 2015)
We describe a class of modified gravity theories that deform general relativity in a way that breaks time reversal invariance and, very mildly, locality. The algebra of constraints, local physical degrees of freedom, and their linearized equations of motion, are unchanged, yet observable effects may be present on cosmological scales, which have implications for the early history of the universe.
This is achieved in the Hamiltonian framework, in a way that requires the constant mean curvature gauge conditions and is, hence, inspired by shape dynamics.
19 pages.

http://arxiv.org/abs/1503.06761
Detecting quantum gravitational effects of loop quantum cosmology in the early universe
Tao Zhu, Anzhong Wang, Gerald Cleaver, Klaus Kirsten, Qin Sheng, Qiang Wu
(Submitted on 23 Mar 2015)
We derive the primordial power spectra and spectral indexes of the density fluctuations and gravitational waves in the framework of loop quantum cosmology (LQC) with holonomy and inverse-volume corrections, by using the uniform asymptotic approximation method to its third-order, at which the upper error bounds are ≲0.15%, accurate enough for the current and forthcoming cosmological observations. Then, using the Planck, BAO and SN data we obtain new constraints on quantum gravitational effects from LQC corrections, and find that such effects could be well within the detection of the current and forthcoming experiments.
5 pages, 2 figures and 1 table

http://arxiv.org/abs/1503.06294
Midisuperspace quantization: possibilities for fractional and emergent spacetime dimensions
Rakesh Tibrewala
(Submitted on 21 Mar 2015)
Recently, motivated by certain loop quantum gravity (LQG) inspired corrections, it was shown that for spherically symmetric midisuperspace models infinitely many second derivative theories of gravity exist (as revealed by the presence of three arbitrary functions in the corresponding Lagrangian/Hamiltonian) and not just those allowed by spherically symmetric general relativity. This freedom can be interpreted as the freedom to accommodate certain quantum gravity corrections in these models even in the absence of higher curvature terms (at a semi-classical level, at least). For a particular choice of the arbitrary functions it is shown that the new theories map to spherically symmetric general relativity in arbitrary number of (integer) dimensions thus explicitly demonstrating that when working with midisuperspace models, one loses the information about the dimensionality of the full spacetime. In addition, it is shown that these new theories can accommodate scenarios of fractional spacetime dimensions as well as those of emergent spacetime dimensions - a possibility suggested by various approaches to quantum gravity.
13 pages.

http://arxiv.org/abs/1503.06233
Critical scaling in quantum gravity from the renormalisation group
Kevin Falls
(Submitted on 20 Mar 2015)
The scaling behaviour of euclidean quantum gravity at an asymptotically safe critical point is studied by means of the exact renormalisation group. Gauge independence is ensured via a specific parameterisation of metric fluctuations introduced in a recent paper. Within a non-perturbative approximation the beta function for Newton's constant takes a simple form to all orders in ℏ. A UV fixed point is found to exist for d≤7 spacetime dimensions at which the critical scaling can be assessed. The critical exponent for the Newton's constant ν is found to be regulator independent close to two dimensions. Applying Litim's optimisation criteria we find ν≈1/3 in four spacetime dimensions. This value is in agreement with lattice studies supporting the existence of a second order phase transition between strongly and weakly coupled phases.
5 pages, 1 figure

http://arxiv.org/abs/1503.06472
Black holes in Asymptotically Safe Gravity
Frank Saueressig, Natalia Alkofer, Giulio D'Odorico, Francesca Vidotto
(Submitted on 22 Mar 2015)
Black holes are among the most fascinating objects populating our universe. Their characteristic features, encompassing spacetime singularities, event horizons, and black hole thermodynamics, provide a rich testing ground for quantum gravity ideas. In this note we observe that the renormalization group improved Schwarzschild black holes constructed by Bonanno and Reuter within Weinberg's asymptotic safety program constitute a prototypical example of a Hayward geometry used to model non-singular black holes within quantum gravity phenomenology. Moreover, they share many features of a Planck star: their effective geometry naturally incorporates the one-loop corrections found in the effective field theory framework, their Kretschmann scalar is bounded, and the black hole singularity is replaced by a regular de Sitter patch. The role of the cosmological constant in the renormalization group improvement process is briefly discussed.
6 pages, 3 figures; prepared for the proceedings of the conference "Frontiers of Fundamental Physics 14"

http://arxiv.org/abs/1503.07438
Addendum: Observables for General Relativity related to geometry
Paweł Duch, Wojciech Kamiński, Jerzy Lewandowski, Jedrzej Świeżewski
(Submitted on 24 Mar 2015)
In this addendum we clarify a point which strengthens one of the results from [the original paper]. Namely, we show that the algebra of the observables F(r,θ) is yet simpler then it was described in [the original paper]. This is an important point, because with this simplification an important subalgebra becomes canonical, allowing for a natural reduction of the phase space.
4 pages, addendum to http://arxiv.org/abs/1403.8062

briefly noted, possible side interest:
http://arxiv.org/abs/1503.06109
On the partner particles for moving mirror radiation and black hole evaporation
M. Hotta, R. Schützhold, W. G. Unruh
(Submitted on 20 Mar 2015)
...
...The idea that black holes emit huge amounts of energy in their last stages because of all the information which must be emitted under the assumption of black-hole unitarity is found not necessarily to be the case.
10 pages.

http://arxiv.org/abs/1503.06254
What Chern-Simons theory assigns to a point
Andre Henriques
(Submitted on 21 Mar 2015)
In this note, we answer the questions "What does Chern-Simons theory assign to a point?" and "What kind of mathematical object does Chern-Simons theory assign to a point?".
Our answer to the first question is representations of the based loop group. More precisely, we identify a certain class of projective unitary representations of the based loop group that we call positive energy representations...
...
Our answer to the second question is bicommutant categories. The latter are a sort of categorification of the notion a von Neumann algebras: they are tensor categories that are equivalent to their bicommutant inside a certain fixed tensor category...

http://arxiv.org/abs/1503.07548
Twisted spectral geometry for the standard model
Pierre Martinetti
(Submitted on 25 Mar 2015)
The Higgs field is a connection one-form as the other bosonic fields, provided one describes space no more as a manifold M but as a slightly non-commutative generalization of it. ...
...Applied to the standard model, a similar twist yields in addition the extra scalar field needed to stabilize the electroweak vacuum, and to make the computation of the Higgs mass in noncommutative geometry compatible with its experimental value.

http://arxiv.org/abs/1503.08794
From Causal Dynamical Triangulations To Astronomical Observations
Jakub Mielczarek
(Submitted on 30 Mar 2015)
This essay discusses phenomenological aspects of the diffusion time dependence of the spectral dimension predicted by the Causal Dynamical Triangulations (CDT) approach to quantum gravity. The deformed form of the dispersion relation for the fields defined on the CDT space-time is reconstructed. Using the Fermi satellite observations of the GRB 090510 source we find that the energy scale of the dimensional reduction is E∗>6.7⋅10¹⁰ GeV at (95 % CL).
By applying the deformed dispersion relation to the cosmological perturbations it is shown that, for a scenario when the primordial perturbations are formed in the UV region, the scalar power spectrum P_S∝k^n_S−1 where n_S−1≈3r(d_UV−2)/r+48(d_UV−3). Here, d_UV≈2 is obtained from the CDT value of the spectral dimension in the UV limit and r is the tensor-to-scalar ratio. We find that, the predicted deviation from the scale-invariance (n_S=1) is in contradiction with the up to date Planck and BICEP2 results.
Comments: 10 pages, 1 figure

 

[marcus]

http://inspirehep.net/record/1356275
http://arxiv.org/abs/1503.07855
Loop quantum cosmology with self-dual variables
Edward Wilson-Ewing
(Submitted on 26 Mar 2015)
Using the complex-valued self-dual connection variables, the loop quantum cosmology of a closed Friedmann universe coupled to a massless scalar field is studied. It is shown how the reality conditions can be imposed in the quantum theory by choosing a particular measure for the inner product in the kinematical Hilbert space. While holonomies of the self-dual Ashtekar connection are not well-defined in the kinematical Hilbert space, it is possible to introduce a family of generalized holonomy-like operators, some of which are well-defined; these operators in turn are used in the definition of a Hamiltonian constraint operator where the scalar field can be used as a relational clock. The resulting quantum dynamics are similar, although not identical, to standard loop quantum cosmology constructed from the Ashtekar-Barbero variables with a real Immirzi parameter. Effective Friedmann equations are derived, which provide a good approximation to the full quantum dynamics for sharply-peaked states whose volume remains much larger than the Planck volume, and they show that for these states quantum gravity effects resolve the big-bang and big-crunch singularities and replace them by a non-singular bounce. Finally, the loop quantization in self-dual variables of a flat Friedmann space-time is recovered in the limit of zero spatial curvature and is identical to the standard loop quantization in terms of the real-valued Ashtekar-Barbero variables.
10 pages

http://arxiv.org/abs/1503.09154
Some implications of signature-change in cosmological models of loop quantum gravity
Martin Bojowald, Jakub Mielczarek
(Submitted on 31 Mar 2015)
Signature change at high density has been obtained as a possible consequence of deformed space-time structures in models of loop quantum gravity. This article provides a conceptual discussion of implications for cosmological scenarios, based on an application of mathematical results for mixed-type partial differential equations (the Tricomi problem). While the effective equations from which signature change has been derived are shown to be locally regular and therefore reliable, the underlying theory of loop quantum gravity may face several global problems in its semiclassical solutions.
35 pages, 5 figures

http://arxiv.org/abs/1504.00867
Can Back-Reaction Prevent Eternal Inflation?
Robert Brandenberger, Renato Costa, Guilherme Franzmann (McGill University)
(Submitted on 3 Apr 2015)
We study the effects which the back-reaction of long wavelength fluctuations exert on stochastic inflation. In the cases of power-law and Starobinsky inflation these effects are too weak to terminate the stochastic growth of the inflaton field. However, in the case of the cyclic Ekpyrotic scenario, the back-reaction effects prevent the unlimited growth of the scalar field.
8 pages, 2 figures

 

[John86]

http://arxiv.org/abs/1503.08814
Long-lived resonances at mirrors
Friedemann Queisser, William G. Unruh
(Submitted on 30 Mar 2015)
Motivated by realistic scattering processes of composite systems, we study the dynamics of a two-particle bound system which is scattered at a mirror. The physics of the scattering process will be discussed in the cases when only one particle interacts directly with the mirror and when both particles are scattered directly. It is shown that the coherence between the transmitted and the reflected wave-packet becomes reduced due to the scattering process. When both particles interact directly with the mirror, the system exhibits long-lived resonances. The results should be of interest for interference experiments with composite systems.

http://arxiv.org/abs/1503.08207
Hilbert space structure in quantum gravity: an algebraic perspective
Steven B. Giddings
(Submitted on 27 Mar 2015)
If quantum gravity respects the principles of quantum mechanics, suitably generalized, it may be that a more viable approach to the theory is through identifying the relevant quantum structures rather than by quantizing classical spacetime. This viewpoint is supported by difficulties of such quantization, and by the apparent lack of a fundamental role for locality. In finite or discrete quantum systems, important structure is provided by tensor factorizations of the Hilbert space. However, even in local quantum field theory properties of the generic type III von Neumann algebras and of long range gauge fields indicate that factorization of the Hilbert space is problematic. Instead it is better to focus on on the structure of the algebra of observables, and in particular on its subalgebras corresponding to regions. This paper suggests that study of analogous algebraic structure in gravity gives an important perspective on the nature of the quantum theory. Significant departures from the subalgebra structure of local quantum field theory are found, working in the correspondence limit of long-distances/low-energies. Particularly, there are obstacles to identifying commuting algebras of localized operators. In addition to suggesting important properties of the algebraic structure, this and related observations pose challenges to proposals of a fundamental role for entanglement.

http://arxiv.org/abs/1503.08580
A Spontaneous Signature Change of the Metric in CDT Quantum Gravity?
Jan Ambjørn, Daniel N. Coumbe, Jakub Gizbert-Studnicki, Jerzy Jurkiewicz
(Submitted on 30 Mar 2015 (v1), last revised 1 Apr 2015 (this version, v2))
We study the effective transfer matrix within the semiclassical and bifurcation phases of CDT quantum gravity. We find that for sufficiently large lattice volumes the kinetic term of the effective transfer matrix has a different sign in each of the two phases. We argue that this sign change can be viewed as a Wick rotation of the metric. We discuss the likely microscopic mechanism responsible for the bifurcation phase transition, and propose an order parameter that can potentially be used to determine the precise location and order of the transition. Using the effective transfer matrix we approximately locate the position of the bifurcation transition in some region of coupling constant space, allowing us to present an updated version of the CDT phase diagram.

http://arxiv.org/abs/1503.08911
Variations on an aethereal theme
Ted Jacobson, Antony J. Speranza
(Submitted on 31 Mar 2015)
We consider a class of Lorentz-violating theories of gravity involving a timelike unit vector field (the aether) coupled to a metric, two examples being Einstein-aether theory and Ho\v{r}ava gravity. The action always includes the Ricci scalar of the metric and the invariants quadratic in covariant derivatives of the aether, but the theories differ in how the aether is constructed from other fields, and whether those fields are varied in the action. Fields that are not varied define background structures breaking diffeomorphsim invariance, including threadings, folations, and clocks, which generally produce novel degrees of freedom arising from the violation of what would otherwise be initial value constraints. The principal aims of this paper are to survey the nature of the theories that arise, and to understand the consequences of breaking diffeomorphism invariance in this setting. In a companion paper [1], we address some of the phenomenology of the "ponderable aether" case in which the presence of a background clock endows the aether with a variable internal energy density that behaves in some respects like dark matter.

http://arxiv.org/abs/1503.08794
From Causal Dynamical Triangulations To Astronomical Observations
Jakub Mielczarek
(Submitted on 30 Mar 2015)
This essay discusses phenomenological aspects of the diffusion time dependence of the spectral dimension predicted by the Causal Dynamical Triangulations (CDT) approach to quantum gravity. The deformed form of the dispersion relation for the fields defined on the CDT space-time is reconstructed. Using the \emph{Fermi} satellite observations of the GRB 090510 source we find that the energy scale of the dimensional reduction is E∗>6.7⋅1010 GeV at (95 % CL).
By applying the deformed dispersion relation to the cosmological perturbations it is shown that, for a scenario when the primordial perturbations are formed in the UV region, the scalar power spectrum PS∝knS−1 where nS−1≈3r(dUV−2)r+48(dUV−3). Here, dUV≈2 is obtained from the CDT value of the spectral dimension in the UV limit and r is the tensor-to-scalar ratio. We find that, the predicted deviation from the scale-invariance (nS=1) is in contradiction with the up to date \emph{Planck} and \emph{BICEP2} results.

http://arxiv.org/abs/1503.08770
What can quantum cosmology say about the inflationary universe?
Gianluca Calcagni, Claus Kiefer, Christian F. Steinwachs
(Submitted on 30 Mar 2015)
We propose a method to extract predictions from quantum cosmology for inflation that can be confronted with observations. Employing the tunneling boundary condition in quantum geometrodynamics, we derive a probability distribution for the inflaton field. A sharp peak in this distribution can be interpreted as setting the initial conditions for the subsequent phase of inflation. In this way, the peak sets the energy scale at which the inflationary phase has started. This energy scale must be consistent with the energy scale found from the inflationary potential and with the scale found from a potential observation of primordial gravitational waves. Demanding a consistent history of the universe from its quantum origin to its present state, which includes decoherence, we derive a condition that allows one to constrain the parameter space of the underlying model of inflation. We demonstrate our method by applying it to two models: Higgs inflation and natural inflation.

http://arxiv.org/abs/1503.08725
Statistical mechanics of reparametrization invariant systems. Takes Three to Tango
Thibaut Josset, Goffredo Chirco, Carlo Rovelli
(Submitted on 30 Mar 2015)
It is notoriously difficult to apply statistical mechanics to generally covariant systems, because the notions of time, energy and equilibrium are seriously modified in this context. We discuss the conditions under which weaker versions of these notions can be defined, sufficient for statistical mechanics. We focus on reparametrization invariant systems without additional gauges. The key idea is to reconstruct statistical mechanics from the ergodic theorem. We find that a suitable split of the system into two non-interacting components is sufficient for generalizing statistical mechanics. While equilibrium acquires sense only when the system admits a suitable split into three weakly interacting components ---roughly: a clock and two systems among which a generalization of energy is equi-partitioned. The key property that allows the application of statistical mechanics and thermodynamics is an additivity condition of such generalized energy.

http://arxiv.org/abs/1504.00187
Autonomous quantum thermal machine for generating steady-state entanglement
Jonatan Bohr Brask, Nicolas Brunner, Géraldine Haack, Marcus Huber
(Submitted on 1 Apr 2015)
We discuss a simple quantum thermal machine for the generation of steady-state entanglement between two interacting qubits. The machine is autonomous in the sense that it uses only incoherent interactions with thermal baths, but no source of coherence or external control. By weakly coupling the qubits to thermal baths at different temperatures, inducing a heat current through the system, steady-state entanglement is generated far from thermal equilibrium. Finally, we discuss two possible implementations, using superconducting flux qubits or a semiconductor double quantum dot. Experimental prospects for steady-state entanglement are promising in both systems.

 

[marcus]

http://arxiv.org/abs/1504.01065
Wilson loops in CDT quantum gravity
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
(Submitted on 4 Apr 2015)
By explicit construction, we show that one can in a simple way introduce and measure gravitational holonomies and Wilson loops in lattice formulations of nonperturbative quantum gravity based on (Causal) Dynamical Triangulations. We use this set-up to investigate a class of Wilson line observables associated with the world line of a point particle coupled to quantum gravity, and deduce from their expectation values that the underlying holonomies cover the group manifold of SO(4) uniformly.
30 pages, 5 figures

http://arxiv.org/abs/1504.02068
Hamiltonian operator for loop quantum gravity coupled to a scalar field
E. Alesci, M. Assanioussi, J. Lewandowski, I. Mäkinen
(Submitted on 8 Apr 2015)
We present the construction of a physical Hamiltonian operator in the deparametrized model of loop quantum gravity coupled to a free scalar field. This construction is based on the use of the recently introduced curvature operator, and on the idea of so-called "special loops". We discuss in detail the regularization procedure and the assignment of the loops, along with the properties of the resulting operator. We compute the action of the squared Hamiltonian operator on spin network states, and close with some comments and outlooks.
31 pages, numerous graph diagrams

 

[atyy]

http://arxiv.org/abs/1504.02169
Coherent states, quantum gravity and the Born-Oppenheimer approximation, I: General considerations
Alexander Stottmeister, Thomas Thiemann
(Submitted on 9 Apr 2015)
This article, as the first of three, aims at establishing the (time-dependent) Born-Oppenheimer approximation, in the sense of space adiabatic perturbation theory, for quantum systems constructed by techniques of the loop quantum gravity framework, especially the canonical formulation of the latter. The analysis presented here fits into a rather general framework, and offers a solution to the problem of applying the usual Born-Oppenheimer ansatz for molecular (or structurally analogous) systems to more general quantum systems (e.g. spin-orbit models) by means of space adiabatic perturbation theory. The proposed solution is applied to a simple, finite dimensional model of interacting spin systems, which serves as a non-trivial, minimal model of the aforesaid problem. Furthermore, it is explained how the content of this article, and its companion, affect the possible extraction of quantum field theory on curved spacetime from loop quantum gravity (including matter fields).

http://arxiv.org/abs/1504.02170
Coherent states, quantum gravity and the Born-Oppenheimer approximation, II: Compact Lie Groups
Alexander Stottmeister, Thomas Thiemann
(Submitted on 9 Apr 2015)
In this article, the second of three, we discuss and develop the basis of a Weyl quantisation for compact Lie groups aiming at loop quantum gravity-type models. This Weyl quantisation may serve as the main mathematical tool to implement the program of space adiabatic perturbation theory in such models. As we already argued in our first article, space adiabatic perturbation theory offers an ideal framework to overcome the obstacles that hinder the direct implementation of the conventional Born-Oppenheimer approach in the canonical formulation of loop quantum gravity. Additionally, we conjecture the existence of a new form of the Segal-Bargmann-Hall "coherent state" transform for compact Lie groups $G$, which we prove for $G=U(1)^{n}$ and support by numerical evidence for $G=SU(2)$. The reason for conjoining this conjecture with the main topic of this article originates in the observation, that the coherent state transform can be used as a basic building block of a coherent state quantisation (Berezin quantisation) for compact Lie groups $G$. But, as Weyl and Berezin quantisation for $\mathbb{R}^{2d}$ are intimately related by heat kernel evolution, it is natural to ask, whether a similar connection exists for compact Lie groups, as well. Moreover, since the formulation of space adiabatic perturbation theory requires a (deformation) quantisation as minimal input, we analyse the question to what extent the coherent state quantisation, defined by the Segal-Bargmann-Hall transform, can serve as basis of the former.

http://arxiv.org/abs/1504.02171
Coherent states, quantum gravity and the Born-Oppenheimer approximation, III: Applications to loop quantum gravity
Alexander Stottmeister, Thomas Thiemann
(Submitted on 9 Apr 2015)
In this article, the third of three, we analyse how the Weyl quantisation for compact Lie groups presented in the second article of this series fits with the projective-phase space structure of loop quantum gravity-type models. Thus, the proposed Weyl quantisation may serve as the main mathematical tool to implement the program of space adiabatic perturbation theory in such models. As we already argued in our first article, space adiabatic perturbation theory offers an ideal framework to overcome the obstacles that hinder the direct implementation of the conventional Born-Oppenheimer approach in the canonical formulation of loop quantum gravity.

 

[marcus]

http://arxiv.org/abs/1504.02822
Duality between Spin networks and the 2D Ising model
Valentin Bonzom, Francesco Costantino, Etera R. Livine
(Submitted on 11 Apr 2015)
The goal of this paper is to exhibit a deep relation between the partition function of the Ising model on a planar trivalent graph and the generating series of the spin network evaluations on the same graph. We provide respectively a fermionic and a bosonic Gaussian integral formulation for each of these functions and we show that they are the inverse of each other (up to some explicit constants) by exhibiting a supersymmetry relating the two formulations. We investigate three aspects and applications of this duality. First, we propose higher order supersymmetric theories which couple the geometry of the spin networks to the Ising model and for which supersymmetric localization still holds. Secondly, after interpreting the generating function of spin network evaluations as the projection of a coherent state of loop quantum gravity onto the flat connection state, we find the probability distribution induced by that coherent state on the edge spins and study its stationary phase approximation. It is found that the stationary points correspond to the critical values of the couplings of the 2D Ising model, at least for isoradial graphs. Third, we analyze the mapping of the correlations of the Ising model to spin network observables, and describe the phase transition on those observables on the hexagonal lattice. This opens the door to many new possibilities, especially for the study of the coarse-graining and continuum limit of spin networks in the context of quantum gravity.
35 pages

possible wider interest:
http://arxiv.org/abs/1504.02818
Path integrals in configuration space and the emergence of classical behavior for closed systems
Henrique Gomes
(Submitted on 11 Apr 2015)
Traditionally, the field of quantum foundations has been preoccupied with different forms of the question "How can an observer be in a state of quantum superposition?". In this paper, I approach this question by exploring a timeless interpretation of quantum mechanics of closed systems, solely in terms of path integrals in non-relativistic timeless configuration space. What prompts a fresh look at the foundational problems in this context, is the advent of multiple gravitational models in which Lorentz symmetry is only emergent. In this setting, I propose a new understanding of records as certain relations between two configurations, the recorded one and the record-holding one. These relations are formalized through a factorization of the amplitude kernel, which forbids unwanted `recoherence' of branches. On this basis, I show that in simple cases the Born rule emerges from counting the relative density of observers with the same records. Furthermore, unlike what occurs in consistent histories, in this context there is indeed a preferred notion of coarse-grainings: those centered around piece-wise classical paths in configuration space. Thus, this new understanding claims to resolve aspects of the measurement problem which are still deemed controversial in the standard approaches.
26 pages, 10 page appendix, 1 figure

http://arxiv.org/abs/1504.02797
Foundations of statistical mechanics from symmetries of entanglement
Sebastian Deffner, Wojciech H. Zurek
(Submitted on 10 Apr 2015)
Envariance - entanglement assisted invariance - is a recently discovered symmetry of composite quantum systems. In this work, it is shown that thermodynamic equilibrium states are fully characterized by their envariance. In particular, the microcanonical equilibrium of a system S with Hamiltonian H_S can be described as an "even", i.e., envariant under any unitary transformation, fully energetically degenerate quantum state. The representation of the canonical equilibrium then follows from simply counting degenerate energy states. Our conceptually novel approach is free of mathematically ambiguous notions such as probability, ensemble, randomness, etc. .
15 pages

http://arxiv.org/abs/1504.04005
Symbolic Dynamics, Modular Curves, and Bianchi IX Cosmologies
Yuri Manin, Matilde Marcolli
(Submitted on 15 Apr 2015)
It is well known that the so called Bianchi IX spacetimes with SO(3)-symmetry in a neighbourhood of the Big Bang exhibit a chaotic behaviour of typical trajectories in the backward movement of time. This behaviour (Mixmaster Model of the Universe) can be encoded by the shift of two-sided continued fractions. Exactly the same shift encodes the sequences of intersections of hyperbolic geodesics with purely imaginary axis in the upper complex half-plane, that is geodesic flow on an appropriate modular surface. A physical interpretation of this coincidence was suggested in arXiv:1402.2158: namely, that Mixmaster chaos is an approximate description of the passage from a hot quantum Universe at the Big Bang moment to the cooling classical Universe. Here we discuss and elaborate this suggestion, looking at the Mixmaster Model from the perspective of the second class of Bianchi IX spacetimes: those with SU(2)-symmetry (self-dual Einstein metrics). We also extend it to the more general context related to Painlevé VI equations.
26 pages

since this paper continues the development begun in a February 2014 paper which I don't recall getting logged on this bibliography, I'll add the abstract to that one here as well:
http://arxiv.org/abs/1402.2158
Big Bang, Blowup, and Modular Curves: Algebraic Geometry in Cosmology
Yuri I. Manin, Matilde Marcolli
(Submitted on 10 Feb 2014)
We introduce some algebraic geometric models in cosmology related to the "boundaries" of space-time: Big Bang, Mixmaster Universe, Penrose's crossovers between aeons. We suggest to model the kinematics of Big Bang using the algebraic geometric (or analytic) blow up of a point x. This creates a boundary which consists of the projective space of tangent directions to x and possibly of the light cone of x. We argue that time on the boundary undergoes the Wick rotation and becomes purely imaginary. The Mixmaster (Bianchi IX) model of the early history of the universe is neatly explained in this picture by postulating that the reverse Wick rotation follows a hyperbolic geodesic connecting imaginary time axis to the real one. Penrose's idea to see the Big Bang as a sign of crossover from "the end of previous aeon" of the expanding and cooling Universe to the "beginning of the next aeon" is interpreted as an identification of a natural boundary of Minkowski space at infinity with the Big Bang boundary.
20 pages. Published in SIGMA (2014)

 

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http://arxiv.org/abs/1504.05352
Black hole spectroscopy from Loop Quantum Gravity models
Aurelien Barrau, Xiangyu Cao, Karim Noui, Alejandro Perez
(Submitted on 21 Apr 2015)
Using Monte Carlo simulations, we compute the integrated emission spectra of black holes in the framework of Loop Quantum Gravity (LQG). The black hole emission rates are governed by the entropy whose value, in recent holographic loop quantum gravity models, was shown to agree at leading order with the Bekenstein-Hawking entropy. Quantum corrections depend on the Barbero-Immirzi parameter γ. Starting with black holes of initial horizon area A∼10² in Planck units, we present the spectra for different values of γ. Each spectrum clearly decomposes in two distinct parts: a continuous background which corresponds to the semi-classical stages of the evaporation and a series of discrete peaks which constitutes a signature of the deep quantum structure of the black hole. We show that γ has an effect on both parts that we analyze in details. Finally, we estimate the number of black holes and the instrumental resolution required to experimentally distinguish between the considered models.
11 pages, 9 figures

http://arxiv.org/abs/1503.08640
New first order Lagrangian for General Relativity
Yannick Herfray, Kirill Krasnov
(Submitted on 30 Mar 2015)
We describe a new BF-type first-order in derivatives Lagrangian for General Relativity. The Lagrangian depends on a connection field as well as a Lie-algebra valued two-form field, with no other fields present. There are two free parameters, which translate into the cosmological constant and the coefficient in front of a topological term. When one of the parameters is set to zero, the theory becomes topological. When the other parameter is zero, the theory reduces to the (anti-) self-dual gravity. Thus, our new Lagrangian interpolates between the topological and anti-self-dual gravities. It also interprets GR as the (anti-) self-dual gravity with an extra quadratic in the auxiliary two-form field term added to the Lagrangian, precisely paralleling the situation in Yang-Mills theory.
4 pages

possibly of general interest:
http://arxiv.org/abs/1412.8462
An operational approach to spacetime symmetries: Lorentz transformations from quantum communication
Philipp A. Hoehn, Markus P. Mueller
(Submitted on 29 Dec 2014 (v1), last revised 23 Apr 2015 (this version, v2))
In most approaches to fundamental physics, spacetime symmetries are postulated a priori and then explicitly implemented in the theory. This includes Lorentz covariance in quantum field theory and diffeomorphism invariance in quantum gravity, which are seen as fundamental principles to which the final theory has to be adjusted. In this paper, we suggest, within a much simpler setting, that this kind of reasoning can actually be reversed, by taking an operational approach inspired by quantum information theory. We consider observers in distant laboratories, with local physics described by the laws of abstract quantum theory, and without presupposing a particular spacetime structure. We ask what information-theoretic effort the observers have to spend to synchronize their descriptions of local physics. If there are "enough" observables that can be measured jointly on different types of systems, we show that the observers' descriptions are related by an element of the orthochronous Lorentz group O^+(3,1), together with a global scaling factor. This operational derivation of the Lorentz transformations correctly describes the physics of relativistic Stern-Gerlach measurements in the WKB approximation, and predicts representations of different spin and Wigner little groups. This result also hints at a novel information-theoretic perspective on spacetime.
Comments: 37 pages, 6 figures, added two new sections, additional explanations and motivations, updated references, corrected typos

 

[marcus]

http://arxiv.org/abs/1504.07559
Loop quantum cosmology: From pre-inflationary dynamics to observations
Abhay Ashtekar, Aurelien Barrau
(Submitted on 28 Apr 2015)
The Planck collaboration has provided us rich information about the early universe, and a host of new observational missions will soon shed further light on the `anomalies' that appear to exist on the largest angular scales. From a quantum gravity perspective, it is natural to inquire if one can trace back the origin of such puzzling features to Planck scale physics. Loop quantum cosmology provides a promising avenue to explore this issue because of its natural resolution of the big bang singularity. Thanks to advances over the last decade, the theory has matured sufficiently to allow concrete calculations of the phenomenological consequences of its pre-inflationary dynamics. In this article we summarize the current status of the ensuing two-way dialog between quantum gravity and observations.
20 pages, 5 figures. Invited review article for the "focus issue" of Classical and Quantum Gravity : "Planck and the fundamentals of cosmology"

http://arxiv.org/abs/1504.07100
Quantum Holonomy Theory
Johannes Aastrup, Jesper M. Grimstrup
(Submitted on 27 Apr 2015)
We present quantum holonomy theory, which is a non-perturbative theory of quantum gravity coupled to fermionic degrees of freedom. The theory is based on a C*-algebra that involves holonomy-diffeomorphisms on a 3-dimensional manifold and which encodes the canonical commutation relations of canonical quantum gravity formulated in terms of Ashtekar variables. Employing a Dirac type operator on the configuration space of Ashtekar connections we obtain a semi-classical state and a kinematical Hilbert space via its GNS construction. We use the Dirac type operator, which provides a metric structure over the space of Ashtekar connections, to define a scalar curvature operator, from which we obtain a candidate for a Hamilton operator. We show that the classical Hamilton constraint of general relativity emerges from this in a semi-classical limit and we then compute the operator constraint algebra. Also, we find states in the kinematical Hilbert space on which the expectation value of the Dirac type operator gives the Dirac Hamiltonian in a semi-classical limit and thus provides a connection to fermionic quantum field theory. Finally, an almost-commutative algebra emerges from the holonomy-diffeomorphism algebra in the same limit.
76 pages, 6 figures

 

[marcus]

http://arxiv.org/abs/1504.08152
Modified FRW cosmologies arising from states of the hybrid quantum Gowdy model
Beatriz Elizaga Navascués, Mercedes Martín-Benito, Guillermo A. Mena Marugán
(Submitted on 30 Apr 2015)
We construct approximate solutions of the hybrid quantum Gowdy cosmology with three-torus topology, linear polarization, and local rotational symmetry, in the presence of a massless scalar field. More specifically, we determine some families of states for which the complicated inhomogeneous and anisotropic Hamiltonian constraint operator of the Gowdy model is approximated by a much simpler one. Our quantum states follow the dynamics governed by this simpler constraint, while being at the same time also approximate solutions of the full Gowdy model. This is so thanks to the quantum correlations that the considered states present between the isotropic and anisotropic sectors of the model. Remarkably, this simpler constraint can be regarded as that of a flat Friedmann-Robertson-Walker universe filled with different kinds of perfect fluids and geometrically corrected by homogeneous and isotropic curvature-like terms. Therefore, our quantum states, which are intrinsically inhomogeneous, admit approximate homogeneous and isotropic effective descriptions similar to those considered in modified theories of gravity.
24 pages.

 

[marcus]

http://arxiv.org/abs/1505.00223
Graphical method in loop quantum gravity: I. Derivation of the closed formula for the matrix element of the volume operator
Jinsong Yang, Yongge Ma
(Submitted on 1 May 2015)
To adopt a practical method to calculate the action of geometrical operators on quantum states is a crucial task in loop quantum gravity. In the series of papers, we will introduce a graphical method, developed by Yutsis and Brink, to loop quantum gravity. The graphical method provides a very powerful technique for simplifying complicated calculations. In this first paper, the closed formula of volume operator is derived via the graphical method. By employing suitable and non-ambiguous graphs to represent the acting of operators as well as the spin network states, we use the simple rules for transforming graphs to yield the resulting formula. Comparing with the complicated algebraic derivation in some literatures, our procedure is more concise, intuitive and visual. The resulting matrix elements of volume operator is compact and uniform, fitting for both gauge-invariant and gauge-variant spin network states.
40 pages

http://arxiv.org/abs/1505.00225
Graphical method in loop quantum gravity: II. The Hamiltonian constraint and inverse volume operators
Jinsong Yang, Yongge Ma
(Submitted on 1 May 2015)
This is the second paper in the series to introduce a graphical method to loop quantum gravity. We employ the graphical method as a powerful tool to calculate the actions of the Hamiltonian constraint operator and the so-called inverse volume operator on spin network states with trivalent vertices. Both of the operators involve the co-triad operator which contains holonomies by construction. The non-ambiguous, concise and visual characters of our graphical method ensure the rigour for our calculations. Our results indicate some corrections to the existing results in literatures for both operators.
19 pages

 

[marcus]

http://arxiv.org/abs/1505.01125
Is Time's Arrow Perspectival?
Carlo Rovelli
(Submitted on 4 May 2015)
We observe entropy decrease towards the past. Does this imply that in the past the world was in a non-generic microstate? I point out an alternative. The subsystem to which we belong interacts with the universe via a relatively small number of quantities, which define a coarse graining. Entropy happens to depends on coarse-graining. Therefore the entropy we ascribe to the universe depends on the peculiar coupling between us and the rest of the universe. Low past entropy may be due to the fact that this coupling (rather than microstate of the universe) is non-generic. I argue that for any generic microstate of a sufficiently rich system there are always special subsystems defining a coarse graining for which the entropy of the rest is low in one time direction (the "past"). These are the subsystems allowing creatures that "live in time" ---such as those in the biosphere--- to exist. I reply to some objections raised to an earlier presentation of this idea, in particular by Bob Wald, David Albert and Jim Hartle.
Comments: 6 pages, 4 figures.

 

[marcus]

The above paper by Rovelli cites an interesting 2012 paper by Deutsch, Li, Sharma that I didn't know about but now think could belong in QG biblio.
http://arxiv.org/abs/1202.2403
The microscopic origin of thermodynamic entropy in isolated systems
Joshua M. Deutsch, Haibin Li, Auditya Sharma
(Submitted on 11 Feb 2012)
A microscopic understanding of the thermodynamic entropy in quantum systems has been a mystery ever since the invention of quantum mechanics. In classical physics, this entropy is believed to be the logarithm of the volume of phase space accessible to an isolated system [1]. There is no quantum mechanical analog to this. Instead, Von Neumann's hypothesis for the entropy [2] is most widely used. However this gives zero for systems with a known wave function, that is a pure state. This is because it measures the lack of information about the system rather than the flow of heat as obtained from thermodynamic experiments. Many arguments attempt to sidestep these issues by considering the system of interest coupled to a large external one, unlike the classical case where Boltzmann's approach for isolated systems is far more satisfactory. With new experimental techniques, probing the quantum nature of thermalization is now possible [3, 4]. Here, using recent advances in our understanding of quantum thermalization [5-10] we show how to obtain the entropy as is measured from thermodynamic experiments, solely from the self-entanglement of the wavefunction, and find strong numerical evidence that the two are in agreement for non-integrable systems. It is striking that this entropy, which is closely related to the concept of heat, and generally thought of as microscopic chaotic motion, can be determined for systems in energy eigenstates which are stationary in time and therefore not chaotic, but instead have a very complex spatial dependence.
5 pages, 2 figures, plus supplementary materials of 8 pages and 5 figures
Published Physical Review E 87, 30 April 2013, with a briefer abstract:
The quantum entropy is usually defined using von Neumann's formula, which measures lack of information and vanishes for pure states. In contrast, we obtain a formula for the entropy of a pure state as it is measured from thermodynamic experiments, solely from the self-entanglement of the wave function, and find strong numerical evidence that the two are in agreement for nonintegrable systems, both for energy eigenstates and for states that are obtained at long times under the evolution of more general initial conditions. This is an extension of Boltzmann's hypothesis for classical systems, relating microscopic motion to thermodynamics.
Josh Deutsch home page at UCSC
http://deutsch.physics.ucsc.edu/
http://deutsch.physics.ucsc.edu/research.html
http://deutsch.physics.ucsc.edu/publications.html
http://deutsch.physics.ucsc.edu/entropy.html
I guess Deutsch was born around 1958 or 1959
PhD Cambridge 1983.

possible side interest:
http://arxiv.org/abs/1505.01445
Why We Observe Large Expansion
David Sloan
(Submitted on 6 May 2015)
Today, observers find that the universe is large, broadly isotropic and appears to have undergone a period of expansion characterised by w = -1. We show that such observations are typical for any system whereby physical parameters are distributed at a high energy scale, due to the conservation of the Liouville measure and the gauge nature of volume. This inverts the usual problem of fine-tuning in initial conditions; it is hard to avoid large, isotropic universes which undergo a period of slow-roll inflation.
4 pages

http://arxiv.org/abs/1505.01403
Dynamical and Hamiltonian formulation of General Relativity
Domenico Giulini
(Submitted on 6 May 2015)
This is a substantially expanded version of a chapter-contribution to "The Springer Handbook of Spacetime", edited by Abhay Ashtekar and Vesselin Petkov, published by Springer Verlag in 2014. This contribution introduces the reader to the reformulation of Einstein's field equations of General Relativity as a constrained evolutionary system of Hamiltonian type and discusses some of its uses, together with some technical and conceptual aspects. Attempts were made to keep the presentation self contained and accessible to first-year graduate students. This implies a certain degree of explicitness and occasional reviews of background material.
76 pages, 5 figures, index. Chapter 17 of A. Ashtekar and V. Petkov (Eds.): Springer Handbook of Spacetime, Springer Verlag, 2014

http://arxiv.org/abs/1505.01456 (?)

 

[marcus]

http://arxiv.org/abs/1505.02089
The ADM papers and part of their modern legacy: loop quantum gravity
Jorge Pullin
(Submitted on 8 May 2015)
We present a summary for non-specialists of loop quantum gravity as part of the modern legacy of the series of papers by Arnowitt, Deser and Misner circa 1960.
7 pages, prepared for Classical and Quantum Gravity for its "Milestones of General Relativity" focus issue to be published during the Centenary Year of GR

http://arxiv.org/abs/1505.03119
Computing the Effective Action with the Functional Renormalization Group
Alessandro Codello, Roberto Percacci, Leslaw Rachwal, Alberto Tonero
(Submitted on 12 May 2015)
The "exact" or "functional" renormalization group equation describes the renormalization group flow of the effective average action Γ_k. The ordinary effective action Γ₀ can be obtained by integrating the flow equation from an ultraviolet scale k=Λ down to k=0. We give several examples of such calculations at one-loop, both in renormalizable and in effective field theories. We use the results of Barvinsky, Vilkovisky and Avramidi on the non-local heat kernel coefficients to reproduce the four point scattering amplitude in the case of a real scalar field theory with quartic potential and in the case of the pion chiral lagrangian. In the case of gauge theories, we reproduce the vacuum polarization of QED and of Yang-Mills theory. We also compute the two point functions for scalars and gravitons in the effective field theory of scalar fields minimally coupled to gravity.
40 pages

http://arxiv.org/abs/1505.03106
Algebraic approach to quantum theory: a finite-dimensional guide
Cédric Bény, Florian Richter
(Submitted on 12 May 2015)
This document is meant as a pedagogical introduction to the modern language used to talk about quantum theories, especially in the field of quantum information. It assumes that the reader has taken a first traditional course on quantum mechanics, and is familiar with the concept of Hilbert space and elementary linear algebra.
As in the popular textbook on quantum information by Nielsen and Chuang, we introduce the generalised concept of states (density matrices), observables (POVMs) and transformations (channels), but we also go further and characterise these structures from an algebraic standpoint, which provides many useful technical tools, and clarity as to their generality. This approach also makes it manifest that quantum theory is a direct generalisation of probability theory, and provides a unifying formalism for both fields.
Although this algebraic approach dates back, in parts, to John von Neumann, we are not aware of any presentation which focuses on finite-dimensional systems. This simplifcation allows us to have a self-contained presentation which avoids many of the technicalities inherent to the most general C∗-algebraic approach, while being perfectly appropriate for the quantum information literature.
44 pages, 1 figure

http://arxiv.org/abs/1505.02821
Cosmological Structure Formation
Joel R. Primack
(Submitted on 11 May 2015)
LCDM is remarkably successful in predicting the cosmic microwave background and large-scale structure, and LCDM parameters have been determined with only mild tensions between different types of observations. Hydrodynamical simulations starting from cosmological initial conditions are increasingly able to capture the complex interactions between dark matter and baryonic matter in galaxy formation. Simulations with relatively low resolution now succeed in describing the overall galaxy population. For example, the EAGLE simulation in volumes up to 100 cubic Mpc reproduces the observed local galaxy mass function nearly as well as semi-analytic models. It once seemed that galaxies are pretty smooth, that they generally grow in size as they evolve, and that they are a combination of disks and spheroids. But recent HST observations combined with high-resolution hydrodynamic simulations are showing that most star-forming galaxies are very clumpy; that galaxies often undergo compaction which reduces their radius and increases their central density; and that most lower-mass star-forming galaxies are not spheroids or disks but are instead elongated when their centers are dominated by dark matter. We also review LCDM challenges on smaller scales: cusp-core, "too big to fail," and substructure issues. Although starbursts can rapidly drive gas out of galaxy centers and thereby reduce the dark matter density, it remains to be seen whether this or other baryonic physics can explain the observed rotation curves of the entire population of dwarf and low surface brightness galaxies. If not, perhaps more complicated physics such as self-interacting dark matter may be needed. But standard LCDM appears to be successful in predicting the dark matter halo substructure that is now observed via gravitational lensing and breaks in cold stellar streams, and any alternative theory must do at least as well.
31 pages, 6 figures, invited lecture at Philosophy of Cosmology conference in Tenerife, September 2014, for proceedings to be published by Cambridge University Press; lecture slides are at this https URL, video is at this https URL

http://arxiv.org/abs/1505.01995
Quantum Corrections to Unimodular Gravity
Enrique Álvarez, Sergio González-Martín, Mario Herrero-Valea, Carmelo P. Martín
(Submitted on 8 May 2015)
The problem of the cosmological constant appears in a new light in Unimodular Gravity. In particular, the zero momentum piece of the potential (that is, the constant piece independent of the matter fields) does not automatically produce a cosmological constant proportional to it. The aim of this paper is to give some details on a calculation showing that quantum corrections do not renormalize the classical value of this observable.
33 pages

 

[marcus]

This is a duplicate. The paper was already logged in post #2310. It was added to the bibliography a second time by mistake.
http://arxiv.org/abs/1503.08640
New first order Lagrangian for General Relativity
Yannick Herfray, Kirill Krasnov
(Submitted on 30 Mar 2015)
We describe a new BF-type first-order in derivatives Lagrangian for General Relativity. The Lagrangian depends on a connection field as well as a Lie-algebra valued two-form field, with no other fields present. There are two free parameters, which translate into the cosmological constant and the coefficient in front of a topological term. When one of the parameters is set to zero, the theory becomes topological. When the other parameter is zero, the theory reduces to the (anti-) self-dual gravity. Thus, our new Lagrangian interpolates between the topological and anti-self-dual gravities. It also interprets GR as the (anti-) self-dual gravity with an extra quadratic in the auxiliary two-form field term added to the Lagrangian, precisely paralleling the situation in Yang-Mills theory.
4 pages.

MTd2 identified the last paragraph of the conclusions section, on page 4, as suggesting a link to Spinfoam QG:
"Our work is also of relevance for the spin foam approach to quantum gravity [15]. The spin foam description of the topological theory, whose Lagrangian is (2) with α = 0, is considered to be understood. Thus, if it was possible to give a spin foam description of the ASD gravity (2) with λ = 0, then it would perhaps be also possible to combine the two and obtain full GR. Given that the theory with α = 0 is believed to give rise to the quantum group SU_q(2), our description thus points in the direction of full GR being about ”q-deformed instantons”, whatever that may be."

 

[marcus]

http://arxiv.org/abs/1505.04088
Gravitational crystal inside the black hole
H. Nikolic
(Submitted on 15 May 2015)
Crystals, as quantum objects typically much larger than their lattice spacing, are a counterexample to a frequent prejudice that quantum effects should not be pronounced at macroscopic distances. We propose that the Einstein theory of gravity only describes a fluid phase and that a phase transition of crystallization can occur under extreme conditions such as those inside the black hole. Such a crystal phase with lattice spacing of the order of the Planck length offers a natural mechanism for pronounced quantum-gravity effects at distances much larger than the Planck length. A resolution of the black-hole information paradox is proposed, according to which all information is stored in a crystal-phase remnant with size and mass much above the Planck scale.
6 pages

A recent PI conference:
http://pirsa.org/C15026
Information theoretic foundations of physics.
Around 26 video talks including ones by:
Gerald 't Hooft
Robert Oeckl
Ryszard Kostecki
Tim Koslowski
Rob Myers
Ariel Caticha
Steve Giddings
Philipp Hoehn
Lee Smolin
Markus Mueller
Achim Kempf

general interest:
http://arxiv.org/abs/1505.04753
Entanglement equilibrium and the Einstein equation
Ted Jacobson
(Submitted on 18 May 2015)
We show that the semiclassical Einstein equation holds if and only if the entanglement entropy in small causal diamonds is stationary at constant volume, when varied from a maximally symmetric vacuum state of geometry and quantum fields. The argument hinges on a conjecture about the variation of the conformal boost energy of quantum fields in small diamonds.
7 pages

http://arxiv.org/abs/1505.04974
The Equivalence Principle in a Quantum World
N. E. J. Bjerrum-Bohr, John F. Donoghue, Basem Kamal El-Menoufi, Barry R. Holstein, Ludovic Planté, Pierre Vanhove
(Submitted on 19 May 2015)
We show how modern methods can be applied to quantum gravity at low energy. We test how quantum corrections challenge the classical framework behind the Equivalence Principle, for instance through introduction of non-locality from quantum physics, embodied in the Uncertainty Principle. When the energy is small we now have the tools to address this conflict explicitly. Despite the violation of some classical concepts, the EP continues to provide the core of the quantum gravity framework through the symmetry - general coordinate invariance - that is used to organize the effective field theory.
5 pages, Honorable Mention in the Gravity Research Foundation Essay Competition 2015

http://arxiv.org/abs/1505.05021
Vacuum fluctuations in theories with deformed dispersion relations
Michele Arzano, Giulia Gubitosi, Joao Magueijo, Giovanni Amelino-Camelia
(Submitted on 19 May 2015)
We examine vacuum fluctuations in theories with modified dispersion relations which represent dimensional reduction at high energies. By changing units of energy and momentum we can obtain a description rendering the dispersion relations undeformed and transferring all the non-trivial effects to the integration measure in momentum space. Using this description we propose a general quantization procedure, which should be applicable whether or not the theory explicitly introduces a preferred frame. Based on this scheme we evaluate the power spectrum of quantum vacuum fluctuations. We find that in {\it all} theories which run to 2 dimensions in the ultraviolet the vacuum fluctuations, in the ultraviolet regime, are scale-invariant. This is true in flat space but also for "inside the horizon" modes in an expanding universe. We spell out the conditions upon the gravity theory for this scale-invariance to be preserved as the modes are frozen-in outside the horizon. We also digress on the meaning of dimensionality (in momentum and position space) and suggest that the spectral index could itself provide an operational definition of dimensionality.
13 pages.

 

[marcus]

http://arxiv.org/abs/1505.05727
Emergence of product of constant curvature spaces in loop quantum cosmology
Naresh Dadhich, Anton Joe, Parampreet Singh
(Submitted on 21 May 2015)
The loop quantum dynamics of Kantowski-Sachs spacetime and the interior of higher genus black hole spacetimes with a cosmological constant has some peculiar features not shared by various other spacetimes in loop quantum cosmology. As in the other cases, though the quantum geometric effects resolve the physical singularity and result in a non-singular bounce, after the bounce a spacetime with small spacetime curvature does not emerge in either the subsequent backward or the forward evolution. Rather, in the asymptotic limit the spacetime manifold is a product of two constant curvature spaces. Interestingly, though the spacetime curvature of these asymptotic spacetimes is very high, their effective metric is a solution to the Einstein's field equations. Analysis of the components of the Ricci tensor shows that after the singularity resolution, the Kantowski-Sachs spacetime leads to an effective metric which can be interpreted as the `charged' Nariai, while the higher genus black hole interior can similarly be interpreted as anti Bertotti-Robinson spacetime with a cosmological constant. These spacetimes are `charged' in the sense that the energy momentum tensor that satisfies the Einstein's field equations is formally the same as the one for the uniform electromagnetic field, albeit it has a purely quantum geometric origin. The asymptotic spacetimes also have an emergent cosmological constant which is different in magnitude, and sometimes even its sign, from the cosmological constant in the Kantowski-Sachs and the interior of higher genus black hole metrics. With a fine tuning of the latter cosmological constant, we show that `uncharged' Nariai, and anti Bertotti-Robinson spacetimes with a vanishing emergent cosmological constant can also be obtained.
21 pages, 16 figures

http://arxiv.org/abs/1505.07828
Thermodynamics of asymptotically safe theories
Dirk H. Rischke, Francesco Sannino
(Submitted on 28 May 2015)
We investigate the thermodynamic properties of a novel class of gauge-Yukawa theories that have recently been shown to be completely asymptotically safe, because their short-distance behaviour is determined by the presence of an interacting fixed point. Not only do all the coupling constants freeze at a constant and calculable value in the ultraviolet, their values can even be made arbitrarily small for an appropriate choice of the ratio Nc/Nf of fermion colours and flavours in the Veneziano limit. Thus, a perturbative treatment can be justified. We compute the pressure, entropy density, and thermal degrees of freedom of these theories to next-to-next-to-leading order in the coupling constants.
16 pages, 7 figures

possibly of general interest:
http://arxiv.org/abs/1505.06787
State of matter at high density and entropy bounds
Ali Masoumi
(Submitted on 26 May 2015)
Entropy of all systems that we understand well is proportional to their volumes except for black holes given by their horizon area. This makes the microstates of any quantum theory of gravity drastically different from the ordinary matter. Because of the assumption that black holes are the maximum entropy states there have been many conjectures that put the area, defined one way or another, as a bound on the entropy in a given region of spacetime. Here we construct a simple model with entropy proportional to volume which exceeds the entropy of a single black hole. We show that a homogeneous cosmology filled with this gas exceeds one of the tightest entropy bounds, the covariant entropy bound and discuss the implications.
6 pages, 2 figures. This essay was written for the 2015 Gravity Research Foundation essay competition and received an honorable mention

http://arxiv.org/abs/1505.05679
General relativity as a two-dimensional CFT
Tim Adamo
(Submitted on 21 May 2015)
The tree-level scattering amplitudes of general relativity encode the full non-linearity of the Einstein field equations. Yet remarkably compact expressions for these amplitudes have been found which seem unrelated to a perturbative expansion of the Einstein-Hilbert action. This suggests an entirely different description of GR which makes this on-shell simplicity manifest. Taking our cue from the tree-level amplitudes, we discuss how such a description can be found. The result is a formulation of GR in terms of a solvable two-dimensional CFT, with the Einstein equations emerging as quantum consistency conditions.
6 pages. Honorable Mention in the 2015 Gravity Research Foundation Essay Competition

another honorable mention in the 2015 GRF essay competition:
http://arxiv.org/abs/1505.05863
The cosmological constant and entropy problems: mysteries of the present with profound roots in the past
Joan Sola