Loop-and-allied QG bibliography

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https://arxiv.org/abs/1611.07849
General Relativity from Three-Forms in Seven Dimensions
Kirill Krasnov
(Submitted on 23 Nov 2016)
We consider a certain theory of 3-forms in 7 dimensions, and study its dimensional reduction to 4D, compactifying the 7-dimensional manifold on the 3-sphere of a fixed radius. We show that the resulting 4D theory is General Relativity (GR) in Plebanski formulation, modulo corrections that are negligible for curvatures smaller than Planckian. Possibly the most interesting point of this construction is that the dimensionally reduced theory is GR with a non-zero cosmological constant, and the value of the cosmological constant is directly related to the size of S^3. Realistic values of Lambda correspond to S^3 of Planck size.

 

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https://arxiv.org/abs/1611.08915
Uncolored Random Tensors, Melon Diagrams, and the SYK Models
Igor R. Klebanov, Grigory Tarnopolsky
Comments: 23 pages, 18 figures
Certain models with rank-$3$ tensor degrees of freedom have been shown by Gurau and collaborators to possesses a novel large $N$ limit, where $g^(2)N^{3}$ is held fixed. In this limit the perturbative expansion in the quartic coupling constant, $g$, is dominated by a special class of "melon" diagrams. We study "uncolored" models of this type, which contain a single copy of real rank-$3$ tensor. Its three indexes are distinguishable; therefore, the models possesses $O(N)^{3}$ symmetry with the tensor field transforming in the tri-fundamental representation. Such uncolored models also possesses the large $N$ limit dominated by the melon diagrams. The quantum mechanics of a real anti-commuting tensor therefore has a similar large N limit to the model recently introduced by Witten as an implementation of the Sachdev-Ye-Kitaev (SYK) model which does not require disorder. Gauging the $O(N)^{3}$ symmetry in our quantum mechanical model removes the non-singlet states; therefore, one can search for its well-defined gravity dual. We point out, however, that the model possesses a vast number of gauge-invariant operators involving higher powers of the tensor field, suggesting that the complete gravity dual will be intricate. We also discuss the quantum mechanics of a complex 3-index anti-commuting tensor, which has $U(N)^{2}×O(N)$ symmetry and argue that it is equivalent in the large $N$ limit to a version of SYK model with complex fermions. Finally, we discuss similar models of a commuting tensor in dimension $d$. While the quartic interaction is not positive definite, we construct the large N Schwinger-Dyson equation for the two-point function and show that its solution is consistent with conformal invariance. We carry out a perturbative check of this result using the $4−\epsilon$ expansion.

 

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https://arxiv.org/abs/1611.09810
Phenomenology with fluctuating quantum geometries in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, Brajesh Gupt
(Submitted on 29 Nov 2016)
The goal of this paper is to probe phenomenological implications of large fluctuations of quantum geometry in the Planck era, using cosmology of the early universe. For the background (Friedmann, Lema\^{i}tre, Robertson, Walker) \emph{quantum} geometry, we allow `widely spread' states in which the \emph{relative} dispersions are as large as 168% in the Planck regime. By introducing suitable methods to overcome the ensuing conceptual and computational issues, we calculate the power spectrum PR(k) and the spectral index ns(k) of primordial curvature perturbations. These results generalize the previous work in loop quantum cosmology which focused on those states which were known to remain sharply peaked throughout the Planck regime. Surprisingly, even though the fluctuations we now consider are large, their presence does not add new features to the final PR(k) and ns(k): Within observational error bars, their effect is degenerate with a different freedom in the theory, namely the number of \emph{pre-inflationary} e-folds NB⋆ between the bounce and the onset of inflation. Therefore, with regard to observational consequences, one can simulate the freedom in the choice of states with large fluctuations in the Planck era using the simpler, sharply peaked states, simply by allowing for different values of NB⋆.

 

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https://arxiv.org/abs/1611.10281
Entanglement Entropy in Causal Set Theory
Rafael D. Sorkin, Yasaman K. Yazdi
(Submitted on 30 Nov 2016)
Entanglement entropy is now widely accepted as having deep connections with quantum gravity. It is therefore desirable to understand it in the context of causal sets, especially since they provide in a natural manner the UV cutoff needed to render entanglement entropy finite. Defining entropy in a causal set is not straightforward because the type of canonical hypersurface-data on which definitions of entanglement typically rely is not available in a causal set. Instead, we will appeal to a more global expression given in arXiv:1205.2953 which, for a gaussian scalar field, expresses the entropy of a spacetime region in terms of the field's correlation function within that region. Carrying this formula over to the causal set, one obtains an entanglement entropy which is both finite and of a Lorentz invariant nature. Herein we evaluate this entropy for causal sets of 1+1 dimensions, and specifically for order-intervals ("causal diamonds") within the causal set, finding in the first instance an entropy that obeys a (spacetime) volume law instead of the expected (spatial) area law. We find, however, that one can obtain an area law by truncating the eigenvalues of a certain "Pauli-Jordan" operator that enters into the entropy formula. In connection with these results, we also study the "entropy of coarse-graining" generated by thinning out the causal set, and we compare it with what one obtains by similarly thinning out a chain of harmonic oscillators, finding the same, "universal" behaviour in both cases.

 

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https://arxiv.org/abs/1612.00266
Echoes from the Abyss: Evidence for Planck-scale structure at black hole horizons
Jahed Abedi, Hannah Dykaar, Niayesh Afshordi
(Submitted on 1 Dec 2016)
In classical General Relativity (GR), an observer falling into an astrophysical black hole is not expected to experience anything dramatic as she crosses the event horizon. However, tentative resolutions to problems in quantum gravity, such as the cosmological constant problem, or the black hole information paradox, invoke significant departures from classicality in the vicinity of the horizon. It was recently pointed out that such near-horizon structures can lead to late-time echoes in the black hole merger gravitational wave signals that are otherwise indistinguishable from GR. We search for observational signatures of these echoes in the gravitational wave data released by advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), following the three black hole merger events GW150914, GW151226, and LVT151012. In particular, we look for repeating damped echoes with time-delays of 8MlogM (+spin corrections, in Planck units), corresponding to Planck-scale departures from GR near their respective horizons. Accounting for the "look elsewhere" effect due to uncertainty in the echo template, we find tentative evidence for Planck-scale structure near black hole horizons at 2.9σ significance level (corresponding to false detection probability of 1 in 270). Future data releases from LIGO collaboration, along with more physical echo templates, will definitively confirm (or rule out) this finding, providing possible empirical evidence for alternatives to classical black holes, such as in firewall or fuzzball paradigms.

https://arxiv.org/abs/1612.00324
Quantum reduced loop gravity: extension to gauge vector field
Jakub Bilski, Emanuele Alesci, Francesco Cianfrani, Pietro Donà, Antonino Marciano
(Submitted on 1 Dec 2016)
Within the framework of Quantum Reduced Loop Gravity we quantize the Hamiltonian for a gauge vector field. The regularization can be performed using tools analogous to the ones adopted in full Loop Quantum Gravity, while the matrix elements of the resulting operator between basis states are analytic coefficients. This analysis is the first step towards deriving the full quantum gravity corrections to the vector field semiclassical dynamics.

https://arxiv.org/abs/1612.00353
Time in quantum cosmology
Martin Bojowald, Theodore Halnon
(Submitted on 1 Dec 2016)
A cosmological model with two global internal times shows that time reparameterization invariance, and therefore covariance, is not guaranteed by deparameterization. In particular, it is impossible to derive proper-time effective equations from a single deparameterized model if quantum corrections from fluctuations and higher moments are included. The framework of effective constraints shows how proper-time evolution can consistently be defined in quantum cosmological systems, such that it is time reparameterization invariant when compared with other choices of coordinate time. At the same time, it allows transformations of moment corrections in different deparameterizations of the same model, indicating partial time reparameterization of internal-time evolution. However, in addition to corrections from moments such as quantum fluctuations, also factor ordering corrections may appear. The latter generically break covariance in internal-time formulations. Fluctuation effects in quantum cosmology are therefore problematic, in particular if derivations are made with a single choice of internal time or a fixed physical Hilbert space.

 

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https://arxiv.org/abs/1612.00551
A Classical and Spinorial Description of the Relativistic Spinning Particle
Trevor Rempel, Laurent Freidel
(Submitted on 2 Dec 2016)
In a previous work we showed that spin can be envisioned as living in a phase space that is dual to the standard phase space of position and momentum. In this work we demonstrate that the second class constraints inherent in this "Dual Phase Space" picture can be solved by introducing a spinorial parameterization of the spinning degrees of freedom. This allows for a purely first class formulation that generalizes the usual relativistic description of spinless particles and provides several insights into the nature of spin and its relationship with spacetime and locality. In particular, we find that the spin motion acts as a Lorentz contraction on the four-velocity and that, in addition to proper time, spinning particles posses a second gauge invariant observable which we call proper angle. Heuristically, this proper angle represents the amount of Zitterbewegung necessary for a spin transition to occur. Additionally, we show that the spin velocity satisfies a causality constraint, and even more stringently, that it is constant along classical trajectories. This leads to the notion of "half-quantum" states which violate the classical equations of motion, and yet do not experience an exponential suppression in the path integral. Finally we give a full analysis of the Poisson bracket structure of this new parametrization.

 

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https://arxiv.org/abs/1612.01084
Causal structures in cosmology
George Ellis, Jean-Philippe Uzan
(Submitted on 4 Dec 2016)
This article reviews the properties and limitations associated with the existence of particle, visual, and event horizons in cosmology in general and in inflationary universes in particular, carefully distinguishing them from `Hubble horizons'. It explores to what extent one might be able to probe conditions beyond the visual horizon (which is close in size to the present Hubble radius), thereby showing that visual horizons place major limits on what are observationally testable aspects of a multiverse, if such exists. Indeed these limits largely prevent us from observationally proving a multiverse either does or does not exist. We emphasize that event horizons play no role at all in observational cosmology, even in the multiverse context, despite some claims to the contrary in the literature.

https://arxiv.org/abs/1612.01236
Loop Quantum Cosmology: A brief review
Ivan Agullo, Parampreet Singh
(Submitted on 5 Dec 2016)
In the last decade, progress on quantization of homogeneous cosmological spacetimes using techniques of loop quantum gravity has led to insights on various fundamental questions and has opened new avenues to explore Planck scale physics. These include the problem of singularities and their possible generic resolution, constructing viable non-singular models of the very early universe, and bridging quantum gravity with cosmological observations. These results, which emerge from an interplay of sophisticated analytical and numerical techniques, has also led to valuable hints on loop quantization of black hole and inhomogeneous spacetimes. In this review, we provide a summary of this progress while focusing on concrete examples of the quantization procedure and phenomenology of cosmological perturbations.

https://arxiv.org/abs/1612.01296
Conformal anomalies and the Einstein Field Equations
Hadi Godazgar, Krzysztof A. Meissner, Hermann Nicolai
(Submitted on 5 Dec 2016)
We compute corrections to the Einstein field equations which are induced by the anomalous effective actions associated to the type A conformal anomaly, both for the (non-local) Riegert action, as well as for the local action with dilaton. In all cases considered we find that these corrections can be very large.

 

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https://arxiv.org/abs/1612.01952
Why Our Universe is Comprehensible
James B. Hartle
(Submitted on 6 Dec 2016)
Einstein wrote memorably that `The eternally incomprehensible thing about the world is its comprehensibility.' This paper argues that the universe must be comprehensible at some level for information gathering and utilizing subsystems such as human observers to evolve and function.

 

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https://arxiv.org/abs/1612.03851
A Supersymmetric SYK-like Tensor Model
Cheng Peng, Marcus Spradlin, Anastasia Volovich
(Submitted on 12 Dec 2016)
We consider a supersymmetric SYK-like model without quenched disorder that is built by coupling two kinds of fermionic N=1 tensor-valued superfields, "quarks" and "mesons". We prove that the model has a well-defined large-N limit in which the (s)quark 2-point functions are dominated by mesonic "melon" diagrams. We sum these diagrams to obtain the Schwinger-Dyson equations and show that in the IR, the solution agrees with that of the supersymmetric SYK model.

 

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https://arxiv.org/abs/1612.04002
Self-Dual Gravity and the Immirzi parameter
Javier Chagoya, M. Sabido
(Submitted on 13 Dec 2016)
Working in the first order formalism of gravity, we propose an action that combines the self and anti-self-dual parts of the curvature and comprises all the diffeomorphism invariant Lagrangians that one can consider in this formalism. The action that we propose is motivated by (A)dS gauge theories of gravity. We use this action to derive the (2+1)-dimensional version of the Immirzi parameter. Our derivation relates explicitly the Immirzi parameter to the existence of two classically equivalent actions for the description of gravity in (2+1) dimensions, namely the standard and exotic actions introduced by Witten in the description of (2+1) gravity as a gauge theory. This relation had been conjectured previously in the literature, but not derived.

https://arxiv.org/abs/1612.04334
Echoes of chaos from string theory black holes
Vijay Balasubramanian, Ben Craps, Bartłomiej Czech, Gábor Sárosi
(Submitted on 13 Dec 2016)
The strongly coupled D1-D5 conformal field theory is a microscopic model of black holes which is expected to have chaotic dynamics. Here, we study the weak coupling limit of the theory where it is integrable rather than chaotic. In this limit, the operators creating microstates of the lowest mass black hole are known exactly. We consider the time-ordered two-point function of light probes in these microstates, normalized by the same two-point function in vacuum. These correlators display a universal early-time decay followed by late-time sporadic behavior. To find a prescription for temporal coarse-graining of these late fluctuations we appeal to random matrix theory, where we show that a progressive time-average smooths the spectral form factor (a proxy for the 2-point function) in a typical draw of a random matrix. This coarse-grained quantity reproduces the matrix ensemble average to a good approximation. Employing this coarse-graining in the D1-D5 system, we find that the early-time decay is followed by a dip, a ramp and a plateau, in remarkable qualitative agreement with recent studies of the Sachdev-Ye-Kitaev (SYK) model. We study the timescales involved, comment on similarities and differences between our integrable model and the chaotic SYK model, and suggest ways to extend our results away from the integrable limit.

 

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https://arxiv.org/abs/1612.04506
Towards a phase diagram for spin foams
Clement Delcamp, Bianca Dittrich
(Submitted on 14 Dec 2016)
One of the most pressing issues for loop quantum gravity and spin foams is the construction of the continuum limit. In this paper, we propose a systematic coarse-graining scheme for three-dimensional lattice gauge models including spin foams. This scheme is based on the concept of decorated tensor networks, which have been introduced recently. Here we develop an algorithm applicable to gauge theories with non-Abelian groups, which for the first time allows for the application of tensor network coarse-graining techniques to proper spin foams. The procedure deals efficiently with the large redundancy of degrees of freedom resulting from gauge symmetry. The algorithm is applied to 3D spin foams defined on a cubical lattice which, in contrast to a proper triangulation, allows for non--trivial simplicity constraints. This mimics the construction of spin foams for 4D gravity. For lattice gauge models based on a finite group we use the algorithm to obtain phase diagrams, encoding the continuum limit of a wide range of these models. We find phase transitions for various families of models carrying non--trivial simplicity constraints.

https://arxiv.org/abs/1612.04551
Testing loop quantum cosmology
Edward Wilson-Ewing
(Submitted on 14 Dec 2016)
Loop quantum cosmology predicts that quantum gravity effects resolve the big-bang singularity and replace it by a cosmic bounce. Furthermore, loop quantum cosmology can also modify the form of primordial cosmological perturbations, for example by reducing power at large scales in inflationary models or by suppressing the tensor-to-scalar ratio in the matter bounce scenario; these two effects are potential observational tests for loop quantum cosmology. In this article, I review these predictions and others, and also briefly discuss three open problems in loop quantum cosmology: its relation to loop quantum gravity, the trans-Planckian problem, and a possible transition from a Lorentzian to a Euclidean space-time around the bounce point.

 

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https://arxiv.org/abs/1612.05364
Semi-classical analysis of black holes in Loop Quantum Gravity: Modelling Hawking radiation with volume fluctuations
Pierre Heidmann, Hongguang Liu, Karim Noui
(Submitted on 16 Dec 2016)
We introduce the notion of fluid approximation of a quantum spherical black hole in the context of Loop Quantum Gravity. In this limit, the microstates of the black hole are intertwiners between "large" representations $s_{i}$ which typically scale as $s_{i}∼\sqrt{a_{H}}$ where $a_{H}$ denotes the area of the horizon in Planck units. The punctures with large colors are, for the black hole horizon, similar to what are the fluid parcels for a classical fluid. We dub them puncels. Hence, in the fluid limit, the horizon is composed by puncels which are themselves interpreted as composed (in the sense of the tensor product) by a large number of more fundamental intertwiners. We study the spectrum of the euclidean volume acting on puncels and we compute its quantum fluctuations. Then, we propose an interpretation of black holes radiation based on the properties of the quantum fluctuations of the euclidean volume operator. We estimate a typical temperature of the black hole and we show that it scales as the Hawking temperature.

 

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https://arxiv.org/abs/1612.06330
Quantum Chaos and Holographic Tensor Models
Chethan Krishnan, Sambuddha Sanyal, P. N. Bala Subramanian
(Submitted on 19 Dec 2016)
A class of tensor models were recently outlined as potentially calculable examples of holography: their perturbative large-N behavior is similar to the Sachdev-Ye-Kitaev (SYK) model, but they are fully quantum mechanical (in the sense that there is no quenched disorder averaging). These facts make them intriguing tentative models for quantum black holes. In this note, we explicitly diagonalize the simplest non-trivial Gurau-Witten tensor model and study its spectral and late-time properties. We find parallels to (a single sample of) SYK where some of these features were recently attributed to random matrix behavior and quantum chaos. In particular, the spectral form factor exhibits a dip-ramp-plateau structure after a running time average, in qualitative agreement with SYK. But we also observe that even though the spectrum has a unique ground state, it has a huge (quasi-?)degeneracy of intermediate energy states, not seen in SYK. If one ignores the delta function due to the degeneracies however, there is level repulsion in the unfolded spacing distribution hinting chaos. Furthermore, there are gaps in the spectrum. The system also has a spectral mirror symmetry which we trace back to the presence of a unitary operator with which the Hamiltonian anticommutes. We use it to argue that to the extent that the model exhibits random matrix behavior, it is controlled not by the Dyson ensembles, but by the BDI (chiral orthogonal) class in the Altland-Zirnbauer classification.

 

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First test of Verlinde's theory of Emergent Gravity using Weak Gravitational Lensing measurements
Margot M. Brouwer, Manus R. Visser, Andrej Dvornik, Henk Hoekstra, Konrad Kuijken, Edwin A. Valentijn, Maciej Bilicki, Chris Blake, Sarah Brough, Hugo Buddelmeijer, Thomas Erben, Catherine Heymans, Hendrik Hildebrandt, Benne W. Holwerda, Andrew M. Hopkins, Dominik Klaes, Jochen Liske, Jon Loveday, John McFarland, Reiko Nakajima, Cristóbal Sifón, Edward N. Taylor
(Submitted on 9 Dec 2016 (v1), last revised 19 Dec 2016 (this version, v2))
Verlinde (2016) proposed that the observed excess gravity in galaxies and clusters is the consequence of Emergent Gravity (EG). In this theory the standard gravitational laws are modified on galactic and larger scales due to the displacement of dark energy by baryonic matter. EG gives an estimate of the excess gravity (described as an apparent dark matter density) in terms of the baryonic mass distribution and the Hubble parameter. In this work we present the first test of EG using weak gravitational lensing, within the regime of validity of the current model. Although there is no direct description of lensing and cosmology in EG yet, we can make a reasonable estimate of the expected lensing signal of low redshift galaxies by assuming a background LambdaCDM cosmology. We measure the (apparent) average surface mass density profiles of 33,613 isolated central galaxies, and compare them to those predicted by EG based on the galaxies' baryonic masses. To this end we employ the ~180 square degrees overlap of the Kilo-Degree Survey (KiDS) with the spectroscopic Galaxy And Mass Assembly (GAMA) survey. We find that the prediction from EG, despite requiring no free parameters, is in good agreement with the observed galaxy-galaxy lensing profiles in four different stellar mass bins. Although this performance is remarkable, this study is only a first step. Further advancements on both the theoretical framework and observational tests of EG are needed before it can be considered a fully developed and solidly tested theory.
Comments: 14 pages, 3 figures. Accepted for publication in MNRAS. Added references for section 1 and 6
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Theory (hep-th)
DOI: 10.1093/mnras/stw3192
Cite as: arXiv:1612.03034 [astro-ph.CO]
(or arXiv:1612.03034v2 [astro-ph.CO] for this version)

 

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https://arxiv.org/abs/1612.06872
Diffeomorphism-invariant averaging in quantum gravity and cosmology
Anthony W. H. Preston
(Submitted on 20 Dec 2016)
This thesis concerns research undertaken in two related topics concerning high-energy gravitational physics. The first is the construction of a manifestly diffeomorphism-invariant Exact Renormalization Group (ERG). This is a procedure that constructs effective theories of gravity by integrating out high-energy modes down to an ultraviolet cutoff scale without gauge-fixing. The manifest diffeomorphism invariance enables us to construct a fully background-independent formulation. This thesis will explore both the fixed-background and background-independent forms of the manifestly diffeomorphism-invariant ERG. The second topic is cosmological backreaction, which concerns the effect of averaging over high-frequency metric perturbations to the gravitational field equations describing the universe at large scales. This has been much studied the context of the unmodified form of General Relativity, but has been much less studied in the context of higher-derivative effective theories obtained by integrating out the high-energy modes of some more fundamental (quantum) theory of gravity. The effective stress-energy tensor for backreaction can be used directly as a diffeomorphism-invariant effective stress-energy tensor for gravitational waves without specifying the background metric.
This thesis will construct the manifestly diffeomorphism-invariant ERG and compute the effective action at the classical level in two different schemes. We will then turn to cosmological backreaction in higher-derivative gravity, deriving the general form of the effective stress-energy tensor due to inhomogeneity for local diffeomorphism-invariant effective theories of gravity. This an exciting research direction, as it begins the construction of a quantum theory of gravity as well as investigating possible implications for cosmology.

https://arxiv.org/abs/1612.07315
Towards apparent convergence in asymptotically safe quantum gravity
Tobias Denz, Jan M. Pawlowski, Manuel Reichert
(Submitted on 21 Dec 2016)
The asymptotic safety scenario in gravity is accessed within the systematic vertex expansion scheme for functional renormalisation group flows put forward in \cite{Christiansen:2012rx,Christiansen:2014raa}, and implemented in \cite{Christiansen:2015rva} for propagators and three-point functions. In the present work this expansion scheme is extended to the dynamical graviton four-point function. For the first time, this provides us with a closed flow equation for the graviton propagator: all vertices and propagators involved are computed from their own flows.
In terms of a covariant operator expansion the current approximation gives access to Λ, R, R2 as well as R2μν and higher derivative operators. We find a UV fixed point with three attractive and two repulsive directions, thus confirming previous studies on the relevance of the first three operators. In the infrared we find trajectories that correspond to classical general relativity and further show non-classical behaviour in some fluctuation couplings.
We also find signatures for the apparent convergence of the systematic vertex expansion. This opens a promising path towards establishing asymptotically safe gravity in terms of apparent convergence.

https://arxiv.org/abs/1612.07615
New Hamiltonians for loop quantum cosmology with arbitrary spin representations
Jibril Ben Achour, Suddhasattwa Brahma, Marc Geiller
(Submitted on 22 Dec 2016)
In loop quantum cosmology, one has to make a choice of SU(2) irreducible representation in which to compute holonomies and regularize the curvature of the connection. The systematic choice made in the literature is to work in the fundamental representation, and very little is known about the physics associated with higher spin labels. This constitutes an ambiguity whose understanding, we believe, is fundamental for connecting loop quantum cosmology to full theories of quantum gravity like loop quantum gravity, its spin foam formulation, or cosmological group field theory. We take a step in this direction by providing here a new closed formula for the Hamiltonian of flat FLRW models regularized in a representation of arbitrary spin. This expression is furthermore polynomial in the basic variables which correspond to well-defined operators in the quantum theory, takes into account the so-called inverse-volume corrections, and treats in a unified way two different regularization schemes for the curvature. After studying the effective classical dynamics corresponding to single and multiple spin Hamiltonians, we study the behavior of the critical density when the number of representations is increased, and the stability of the difference equations in the quantum theory.

https://arxiv.org/abs/1612.07331
Can quantum probes satisfy the weak equivalence principle?
Luigi Seveso, Matteo G. A. Paris
(Submitted on 21 Dec 2016)
We address the question whether, as a matter of principle, a quantum probe in a gravitational field may be considered as a test particle obeying the weak equivalence principle (WEP). To this aim we put forward a quantitative generalization of the WEP, which applies also to quantum systems, while maintaining the physical content of its classical formulation. Our formulation of the WEP is information-theoretic in nature, and requires that information about the mass of a probe, as obtained from position measurements, cannot increase in the presence of a gravitational field. More precisely, the generalized WEP states that the Fisher information of position measurements cannot increase in the presence of gravity, compared to its value for a free probe. Our results show that while in a uniform field quantum probes satisfy the WEP exactly, gravity gradients may encode nontrivial information about the mass in the particle's wavefunction, thus leading to violations of the WEP. We conclude that the WEP is untenable for a quantum particle described by a wavefunction, which rather behaves in analogy with a classical extended object.

https://arxiv.org/abs/1612.07629
More on the Bending of Light in Quantum Gravity
Dong Bai, Yue Huang
(Submitted on 22 Dec 2016)
We reconsider the long-range effects of the scattering of massless scalars and photons from a massive scalar object in quantum gravity. At the one-loop level, the relevant quantum mechanical corrections could be sorted into the graviton double-cut contributions, massless-scalar double-cut contributions and photon double-cut contributions. In arXiv:1410.7590 and 1609.07477 N.E.J.~Bjerrum-Bohr et al.~have considered explicitly the implications of the graviton double-cut diagrams on the gravitational bending of light and some classical formulations of the equivalence principle, using the modern double-copy constructions and on-shell unitarity techniques. In this article, we redo the analysis using the traditional Feynman diagrammatic approach and consider all three contributions. Our results on the graviton double-cut contributions agree with the aforementioned references, which acts as a nontrivial check of previous computations. Furthermore, it turns out that the massless-scalar double cut contributions and the photon double-cut contributions do leave non-vanishing quantum effects on the scattering amplitudes and the gravitational bending of light. Yet, we find that the general structure of the gravitational amplitudes and the quantum discrepancy of the equivalence principle suggested in the aforementioned references remain intact.

 

[kodama]

Holographic signatures of resolved cosmological singularities
Norbert Bodendorfer, Andreas Schäfer, John Schliemann
(Submitted on 20 Dec 2016)
The classical gravity approximation is often employed in AdS/CFT to study the dual field theory, as it allows for many computations. A drawback is however the generic presence of singularities in classical gravity, which limits the applicability of AdS/CFT to regimes where the singularities are avoided by bulk probes, or some other form of regularisation is applicable. At the same time, quantum gravity is expected to resolve those singularities and thus to extend the range of applicability of AdS/CFT also in classically singular regimes. This paper provides a proof of principle that such computations are already possible. We use a quantum corrected Kasner-AdS metric inspired by results from loop quantum gravity to compute the 2-point correlator in the geodesic approximation for a negative Kasner exponent. The correlator derived in the classical gravity approximation has previously been shown to contain a pole at finite distance as a signature of the singularity. Using the quantum corrected metric, we show explicitly how the pole is resolved and that a new subdominant long-distance contribution to the correlator emerges, caused by geodesics passing arbitrarily close to the resolved classical singularity. We stress that these results rely on several choices in the quantum corrected metric which allow for an analytic computation and may not hold in general. An alternative choice is presented where the correlator may remain singular even though the bulk singularity is resolved.
Comments: 14 pages
Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); Mathematical Physics (math-ph)
Cite as: arXiv:1612.06679 [hep-th]

 

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https://arxiv.org/abs/1612.08800
Cosmological dynamics in spin-foam loop quantum cosmology: challenges and prospects
David A. Craig, Parampreet Singh
(Submitted on 28 Dec 2016)
We explore the structure of the spin foam-like vertex expansion in loop quantum cosmology and discuss properties of the corresponding amplitudes, with the aim of elucidating some of the expansion's useful properties and features. We find that the expansion is best suited for consideration of conceptual questions and for investigating short-time, highly quantum behavior. In order to study dynamics at cosmological scales, the expansion must be carried to very high order, limiting its direct utility as a calculational tool for such questions. Conversely, it is unclear that the expansion can be truncated at finite order in a controlled manner.

https://arxiv.org/abs/1612.09521
The universe as a quantum gravity condensate
Daniele Oriti
(Submitted on 30 Dec 2016)
This is an introduction to the approach to the extraction of cosmological dynamics from full quantum gravity based on group field theory condensates. We outline its general perspective, which sees cosmology as the hydrodynamics of the fundamental quantum gravity degrees of freedom, as well as its concrete implementation within the group field theory formalism. We summarise recent work showing the emergence of a bouncing cosmological dynamics from a fundamental group field theory model, and provide a brief but complete survey of other results in the literature. Finally, we discuss open issues and directions for further research.

 

[kodama]

Loop-Corrected Virasoro Symmetry of 4D Quantum Gravity
Temple He, Daniel Kapec, Ana-Maria Raclariu, Andrew Strominger
(Submitted on 2 Jan 2017)
Recently a boundary energy-momentum tensor Tzz has been constructed from the soft graviton operator for any 4D quantum theory of gravity in asymptotically flat space. Up to an "anomaly" which is one-loop exact, Tzz generates a Virasoro action on the 2D celestial sphere at null infinity. Here we show by explicit construction that the effects of the IR divergent part of the anomaly can be eliminated by a one-loop renormalization that shifts Tzz.
Comments: 12 pages
Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1701.00496 [hep-th]
(or arXiv:1701.00496v1 [hep-th] for this version)

Anamorphic Quasiperiodic Universes in Modified and Einstein Gravity with Loop Quantum Gravity Corrections
Marcelo M. Amaral, Raymond Aschheim, Laurenţiu Bubuianu, Klee Irwin, Sergiu I. Vacaru, Daniel Woolridge
(Submitted on 7 Nov 2016)
The goal of this work is to elaborate on new geometric methods of constructing exact and parametric quasiperiodic solutions for anamorphic cosmology models in modified gravity theories, MGTs, and general relativity, GR. There exist previously studied generic off-diagonal and diagonalizable cosmological metrics encoding gravitational and matter fields with quasicrystal like structures, QC, and holonomy corrections from loop quantum gravity, LQG. We apply the anholonomic frame deformation method, AFDM, in order to decouple the (modified) gravitational and matter field equations in general form. This allows us to find integral varieties of cosmological solutions determined by generating functions, effective sources, integration functions and constants. The coefficients of metrics and connections for such cosmological configurations depend, in general, on all spacetime coordinates and can be chosen to generate observable (quasi)-periodic/ aperiodic/ fractal / stochastic / (super) cluster / filament / polymer like (continuous, stochastic, fractal and/or discrete structures) in MGTs and/or GR. In this work, we study new classes of solutions for anamorphic cosmology with LQG holonomy corrections. Such solutions are characterized by nonlinear symmetries of generating functions for generic off--diagonal cosmological metrics and generalized connections, with possible nonholonomic constraints to Levi-Civita configurations and diagonalizable metrics depending only on a time like coordinate. We argue that anamorphic quasiperiodic cosmological models integrate the concept of quantum discrete spacetime, with certain gravitational QC-like vacuum and nonvacuum structures. And, that of a contracting universe that homogenizes, isotropizes and flattens without introducing initial conditions or multiverse problems.
Comments: latex2e, 11pt, 30 pages
Subjects: General Physics (physics.gen-ph)
Cite as: arXiv:1611.05295 [physics.gen-ph]

Moduli Structures, Separability of the Kinematic Hilbert Space and Frames in Loop Quantum Gravity
Bruno Carvalho, Daniel H. T. Franco
(Submitted on 28 Oct 2016)
We reassess the problem of separability of the kinematic Hilbert space in loop quantum gravity under a new mathematical point of view. We use the formalism of frames, a tool used in signal analysis, in order to remove the redundancy of the moduli structures in high valence graphs, without resorting to set extension of diffeomorphism group. For this, we introduce a local redundancy which encodes the concentration of frame vectors on the tangent spaces TpM around points of intersections p of smooth loops α in R3.
Subjects: General Relativity and Quantum Cosmology (gr-qc); Mathematical Physics (math-ph)
Cite as: arXiv:1610.09324 [gr-qc]
(or arXiv:1610.09324v1 [gr-qc] for this version)

Linking loop quantum gravity quantization ambiguities with phenomenology
Suddhasattwa Brahma, Michele Ronco, Giovanni Amelino-Camelia, Antonino Marciano
(Submitted on 25 Oct 2016)
Fundamental quantum gravity theories are known to be notoriously difficult to extract viable testable predictions out of. In this paper, we aim to incorporate putative quantum corrections coming from loop quantum gravity in deriving modified dispersion relations for particles on a deformed Minkowski spacetime. We show how different choices of the Immirzi parameter can, in some cases, serendipitously lead to different outcomes for such modifications, depending on the quantization scheme chosen. This allows one to differentiate between these quantization choices via testable phenomenological predictions.
Comments: 12 pages, 3 figures
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1610.07865 [gr-qc]

 

[kodama]

Cosmological singularity resolution from quantum gravity: the emergent-bouncing universe
Emanuele Alesci, Gioele Botta, Francesco Cianfrani, Stefano Liberati
(Submitted on 21 Dec 2016)
Alternative scenarios to the Big Bang singularity have been subject of intense research for several decades by now. Most popular in this sense have been frameworks were such singularity is replaced by a bounce around some minimal cosmological volume or by some early constant volume quantum phase. This latter scenario was devised a long time ago and referred as an "emergent universe" (in the sense that our universe emerged from a static quantum phase) but was never derived from a rigorous quantum gravitational calculation. We show here that within an improved framework of canonical quantum gravity (the so called Quantum Reduced Loop Gravity) the Friedmann equations for cosmology are modified in such a way to replace the big bang singularity with a short bounce preceded by a metastable phase in which the universe is static and characteristically quantum in nature (having a typical size of a few tenths of Planck volumes). We call this hybrid scenario an "emergent-bouncing universe". Perspective developments and possible tests of this scenario are discussed in the end.
Comments: 5 pages, 3 figures
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1612.07116 [gr-qc]
(or arXiv:1612.07116v1 [gr-qc] for this version)

Singularities and qualitative study in LQC
Llibert Aresté Saló, Jaume Amorós, Jaume de Haro
(Submitted on 16 Dec 2016 (v1), last revised 23 Dec 2016 (this version, v2))
This work contains a detailed analysis of singularities in General Relativity and in Loop Quantum Cosmology, yielding explicit analytical expressions for the energy density and the Hubble parameter for a given set of possible Equations of State. The case when the background is driven by a single scalar field is also considered, obtaining analytical expressions for the corresponding potential. And, in a given particular case, a qualitative study of the orbits in the associated phase space of the scalar field is performed.
Comments: 14 pages, 7 figures. New references added
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1612.05480 [gr-qc]

Viscous Chaplygin Gas Models as a Spherical Top-Hat Collapsing Fluids
Abdul Jawad, Ayesha Iqbal
(Submitted on 31 Oct 2016)
We study the spherical top-hat collapse in Einstein gravity and loop quantum cosmology by taking the non-linear evolution of viscous modified variable chaplygin gas and viscous generalized cosmic chaplygin gas. We calculate the equation of state parameter, square speed of sound, perturbed equation of state parameter, perturbed square speed of sound, density contrast and divergence of peculiar velocity in perturbed region and discussed their behavior. It is observed that both chaplygin gas models support the spherical collapse in Einstein as well as loop quantum cosmology because density contrast remains positive in both cases and the perturbed equation of state parameter remains positive at the present epoch as well as near future. It is remarked here that these parameters provide the consistence results for both chaplygin gas models in both gravities.
Comments: 19 pages; 12 figures. arXiv admin note: text overlap with arXiv:1110.6205, arXiv:1501.00486 by other authors
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Journal reference: Int. J. Mod. Phys. D 25(2016)1650074
DOI: 10.1142/S02182718165007

 

[kodama]

Towards apparent convergence in asymptotically safe quantum gravity
Tobias Denz, Jan M. Pawlowski, Manuel Reichert
(Submitted on 21 Dec 2016)
The asymptotic safety scenario in gravity is accessed within the systematic vertex expansion scheme for functional renormalisation group flows put forward in \cite{Christiansen:2012rx,Christiansen:2014raa}, and implemented in \cite{Christiansen:2015rva} for propagators and three-point functions. In the present work this expansion scheme is extended to the dynamical graviton four-point function. For the first time, this provides us with a closed flow equation for the graviton propagator: all vertices and propagators involved are computed from their own flows.
In terms of a covariant operator expansion the current approximation gives access to Λ, R, R2 as well as R2μν and higher derivative operators. We find a UV fixed point with three attractive and two repulsive directions, thus confirming previous studies on the relevance of the first three operators. In the infrared we find trajectories that correspond to classical general relativity and further show non-classical behaviour in some fluctuation couplings.
We also find signatures for the apparent convergence of the systematic vertex expansion. This opens a promising path towards establishing asymptotically safe gravity in terms of apparent convergence.
Comments: 24 pages, 6 figures
Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1612.07315 [hep-th]
(or arXiv:1612.07315v1 [hep-th] for this version)

 

[atyy]

https://arxiv.org/abs/1701.01383
Group Field theory and Tensor Networks: towards a Ryu-Takayanagi formula in full quantum gravity
Goffredo Chirco, Daniele Oriti, Mingyi Zhang
(Submitted on 5 Jan 2017)
We establish a dictionary between group field theory (thus, spin networks and random tensors) states and generalized random tensor networks. Then, we use this dictionary to compute the R\'{e}nyi entropy of such states and recover the Ryu-Takayanagi formula, in three different cases corresponding to three different truncations/approximations, suggested by the established correspondence.

 

[kodama]

Quantum gravity and Standard-Model-like fermions
Astrid Eichhorn, Stefan Lippoldt
(Submitted on 17 Nov 2016)
We discover that chiral symmetry does not act as an infrared attractor of the renormalization group flow under the impact of quantum gravity fluctuations. Thus, observationally viable quantum gravity models must respect chiral symmetry. In our truncation, asymptotically safe gravity does, as a chiral fixed point exists. A second non-chiral fixed point with massive fermions provides a template for models with dark matter. This fixed point disappears for more than 10 fermions, suggesting that an asymptotically safe ultraviolet completion for the standard model plus gravity enforces chiral symmetry.
Comments: 6 pages, 2 figures
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1611.05878 [gr-qc]
Quantum-gravity effects on a Higgs-Yukawa model
Astrid Eichhorn, Aaron Held, Jan M. Pawlowski
(Submitted on 7 Apr 2016 (v1), last revised 13 Sep 2016 (this version, v2))
A phenomenologically viable theory of quantum gravity must accommodate all observed matter degrees of freedom and their properties. Here, we explore whether a toy model of the Higgs-Yukawa sector of the Standard Model is compatible with asymptotically safe quantum gravity. We discuss the phenomenological implications of our result in the context of the Standard Model. We analyze the quantum scaling dimension of the system, and find an irrelevant Yukawa coupling at a joint gravity-matter fixed point. Further, we explore the impact of gravity-induced couplings between scalars and fermions, which are non-vanishing in asymptotically safe gravity.
Comments: 13 pages + appendix, 10 figures, simplified basis for induced couplings
Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph)
Journal reference: Phys. Rev. D 94, 104027 (2016)
DOI: 10.1103/PhysRevD.94.104027
Cite as: arXiv:1604.02041 [hep-th]
(or arXiv:1604.02041v2 [hep-th] for this version)

 

[kodama]

Quantum gravity on foliated spacetime - asymptotically safe and sound
Jorn Biemans, Alessia Platania, Frank Saueressig
(Submitted on 15 Sep 2016 (v1), last revised 17 Oct 2016 (this version, v2))
Asymptotic Safety provides a mechanism for constructing a consistent and predictive quantum theory of gravity valid on all length scales. Its key ingredient is a non-Gaussian fixed point of the gravitational renormalization group flow which controls the scaling of couplings and correlation functions at high energy. In this work we use a functional renormalization group equation adapted to the ADM-formalism for evaluating the gravitational renormalization group flow on a cosmological Friedmann-Robertson-Walker background. Besides possessing the UV-non-Gaussian fixed point characteristic for Asymptotic Safety the setting exhibits a second non-Gaussian fixed point with a positive Newton's constant and real critical exponents. The new fixed point alters the phase diagram in such a way that all renormalization group trajectories connected to classical general relativity are well-defined on all length scales. In particular a positive cosmological constant is dynamically driven to zero in the deep infrared. Moreover, the scaling dimensions associated with the universality classes emerging within the causal setting exhibit qualitative agreement with results found within the ϵ-expansion around two dimensions, Monte Carlo simulations based on Lattice Quantum Gravity, and the discretized Wheeler-deWitt equation.
Comments: 7 pages, 3 figures. Conclusions slightly modified
Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1609.04813 [hep-th]
(or arXiv:1609.04813v2 [hep-th] for this version)

 

[atyy]

https://arxiv.org/abs/1701.02282
Some Clarifications on the Duration of Inflation in Loop Quantum Cosmology
Boris Bolliet, Aurélien Barrau, Killian Martineau, Flora Moulin
(Submitted on 9 Jan 2017)
The prediction of a phase of inflation whose number of e-folds is constrained is an important feature of loop quantum cosmology. This work aims at giving some elementary clarifications on the role of the different hypotheses leading to this conclusion. We show that the duration of inflation does not depend significantly on the modified background dynamics in the quantum regime. [/PLAIN]

https://arxiv.org/abs/1701.02037
(3+1)-dimensional topological phases and self-dual quantum geometries encoded on Heegard surfaces
Bianca Dittrich
(Submitted on 8 Jan 2017)
We apply the recently suggested strategy to lift state spaces and operators for (2+1)-dimensional topological quantum field theories to state spaces and operators for a (3+1)-dimensional TQFT with defects. We start from the (2+1)-dimensional Turaev-Viro theory and obtain the state space for the Crane-Yetter model with line defects.
This work has important applications for quantum gravity as well as the theory of topological phases in (3+1) dimensions. It provides a self-dual quantum geometry realization based on a vacuum state peaked on a homogeneously curved geometry. The state spaces and operators we construct here provide also an improved version of the Walker-Wang model, and simplify its analysis considerably.
We in particular show that the fusion bases of the (2+1)-dimensional theory lead to a rich set of bases for the (3+1)-dimensional theory. This includes a quantum deformed spin network basis, which in a loop quantum gravity context diagonalizes spatial geometry operators. We also obtain a dual curvature basis, that diagonalizes the Walker-Wang Hamiltonian.
Furthermore, the construction presented here can be generalized to provide state spaces for the recently introduced dichromatic four--dimensional manifold invariants.

 

[atyy]

https://arxiv.org/abs/1701.02311
Hypercuboidal renormalization in spin foam quantum gravity
Benjamin Bahr, Sebastian Steinhaus
(Submitted on 9 Jan 2017)
In this article we apply background-independent renormalization group methods to spin foam quantum gravity. It is aimed at extending and elucidating the analysis of a companion letter, in which the existence of a fixed point in the truncated RG flow for the model was reported. Here we repeat the analysis with various modifications, and find that both qualitative and quantitative features of the fixed point are robust in this setting. We also go into details about the various approximation schemes employed in the analysis.

https://arxiv.org/abs/1701.02439
Discretization of 3d gravity in different polarizations
Maïté Dupuis, Laurent Freidel, Florian Girelli
(Submitted on 10 Jan 2017)
We study the discretization of 3d gravity with Λ=0 following the loop quantum gravity framework. In the process, we realize that different choices of polarization are possible. This allows to introduce a new discretization based on the triad as opposed to the connection as in the standard loop quantum gravity framework. We also identify the classical non-trivial symmetries of discrete gravity, namely the Drinfeld double, given in terms of momentum maps. Another choice of polarization is given by the Chern-Simons formulation of gravity. Our framework also provides a new discretization scheme of Chern-Simons, which keeps track of the link between the continuum variables and the discrete ones. We show how the Poisson bracket we recover between the Chern-Simons holonomies allows to recover the Goldman bracket. There is also a transparent link between the discrete Chern-Simons formulation and the discretization of gravity based on the connection (loop gravity) or triad variables (dual loop gravity).

 

[atyy]

https://arxiv.org/abs/1701.03029
Flowing to the continuum in discrete tensor models for quantum gravity
Astrid Eichhorn, Tim Koslowski
(Submitted on 11 Jan 2017)
Tensor models provide a way to access the path-integral for discretized quantum gravity in d dimensions. As in the case of matrix models for two-dimensional quantum gravity, the continuum limit can be related to a Renormalization Group fixed point in a setup where the tensor size N serves as the Renormalization Group scale. We develop functional Renormalization Group tools for tensor models with a main focus on a rank-3 model for three-dimensional quantum gravity. We rediscover the double-scaling limit and provide an estimate for the scaling exponent. Moreover, we identify two additional fixed points with a second relevant direction in a truncation of the Renormalization Group flow. The new relevant direction might hint at the presence of additional degrees of freedom in the corresponding continuum limit.

 

[kodama]

Naturalness of asymptotically safe Higgs
Giulio Maria Pelaggi, Francesco Sannino, Alessandro Strumia, Elena Vigiani
(Submitted on 5 Jan 2017)
We introduce a model that contains a Higgs-like scalar with gauge, Yukawa and quartic interactions that enter a perturbative asymptotically safe regime at energies above a scale Λ. The model serves as a concrete example to test whether scalars masses unavoidably receive quantum correction of order Λ. The answer is that scalars can be naturally lighter. Although we do not have an answer to whether the Standard Model hypercharge coupling growth towards a Landau pole around Λ∼1040 GeV can be tamed by non-perturbative asymptotic safety, our toy-SM shows that such a possibility is worth exploring. In fact, if successful, it might also offer an explanation for the unbearable lightness of the Higgs.
Comments: 16 pages
Subjects: High Energy Physics - Phenomenology (hep-ph)
Report number: CERN-PH-TH-2017-001, CP3-Origins-2017-001, IFUP-TH/2017
Cite as: arXiv:1701.01453 [hep-ph]
(or arXiv:1701.01453v1 [hep-ph] for this versio

 

[kodama]

Detailed investigation of the duration of inflation in loop quantum cosmology for a Bianchi-I universe with different inflaton potentials and initial conditions
Killian Martineau, Aurélien Barrau, Susanne Schander
(Submitted on 10 Jan 2017)
There is a wide consensus on the correct dynamics of the background in loop quantum cosmology. In this article we make a systematic investigation of the duration of inflation by varying what we think to be the most important "unknowns" of the model: the way to set initial conditions, the amount of shear at the bounce and the shape of the inflaton potential.
Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph)
Cite as: arXiv:1701.02703 [gr-qc]

 

[kodama]

Numerical simulations of loop quantum Bianchi-I spacetimes
Peter Diener, Anton Joe, Miguel Megevand, Parampreet Singh
(Submitted on 20 Jan 2017)
Due to the numerical complexities of studying evolution in an anisotropic quantum spacetime, in comparison to the isotropic models, the physics of loop quantized anisotropic models has remained largely unexplored. In particular, robustness of bounce and the validity of effective dynamics have so far not been established. Our analysis fills these gaps for the case of vacuum Bianchi-I spacetime. To efficiently solve the quantum Hamiltonian constraint we perform an implementation of the Cactus framework which is conventionally used for applications in numerical relativity. Using high performance computing, numerical simulations for a large number of initial states with a wide variety of fluctuations are performed. Big bang singularity is found to be replaced by anisotropic bounces for all the cases. We find that for initial states which are sharply peaked at the late times in the classical regime and bounce at a mean volume much greater than the Planck volume, effective dynamics is an excellent approximation to the underlying quantum dynamics. Departures of the effective dynamics from the quantum evolution appear for the states probing deep Planck volumes. A detailed analysis of the behavior of this departure reveals a non-monotonic and subtle dependence on fluctuations of the initial states. We find that effective dynamics in almost all of the cases underestimates the volume and hence overestimates the curvature at the bounce, a result in synergy with earlier findings in isotropic case. The expansion and shear scalars are found to be bounded throughout the evolution.
Comments: 27 pages, 22 figures
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1701.05824 [gr-qc]
(or arXiv:1701.05824v1 [gr-qc] for this version)
Submission history

 

[atyy]

https://arxiv.org/abs/1701.07420
Shape Dynamical Loop Gravity from a Conformal Immirzi Parameter
Patrick J. Wong
(Submitted on 25 Jan 2017)
The Immirzi parameter of loop quantum gravity is a one parameter ambiguity of the theory whose precise interpretation is not universally agreed upon. It is an inherent characteristic of the quantum theory as it appears in the spectra of geometric operators, despite being irrelevant at the classical lever. The parameter's appearance in the area and volume spectra to the same power as the Planck area suggest that it plays a role in determining the fundamental length scale of space. In fact, a consistent interpretation is that it represents a constant rescaling of the kinematical spatial geometry. An interesting realization is that promoting the Immirzi parameter to be a general conformal transformation leads to a system which can be identified as analogous to the linking theory of shape dynamics. A three-dimensional gravitational gauge connection is then constructed within the linking theory in a manner analogous to loop quantum gravity, thereby facilitating the application of the established procedure of loop quantization.

https://arxiv.org/abs/1701.07519
SO*(2N) coherent states for loop quantum gravity
Florian Girelli, Giuseppe Sellaroli
(Submitted on 25 Jan 2017)
A SU(2) intertwiner with N legs can be interpreted as the quantum state of a convex polyhedron with N faces (when working in 3d). We show that the intertwiner Hilbert space carries a representation of the non-compact group SO*(2N). This group can be viewed as the subgroup of the symplectic group Sp(4N,R) which preserves the SU(2) invariance. We construct the associated Perelomov coherent states and discuss the notion of semi-classical limit, which is more subtle that we could expect. Our work completes the work by Freidel and Livine which focused on the U(N) subgroup of SO*(2N).

 

[kodama]

The black hole quantum atmosphere
Ramit Dey, Stefano Liberati, Daniele Pranzetti
(Submitted on 22 Jan 2017)
Ever since the discovery of black hole evaporation, the region of origin of the radiated quanta has been a topic of debate. Recently it was argued by Giddings that the Hawking quanta originate from a region well outside the black hole horizon by calculating the effective radius of a radiating body via the Stefan--Boltzmann law. In this paper we try to further explore this issue and end up corroborating this claim, using both a heuristic argument and a detailed study of the stress energy tensor. We show that the Hawking quanta originate from what might be called a quantum atmosphere around the black hole with energy density and fluxes of particles peaked at about 4M, running contrary to the popular belief that these originate from the ultra high energy excitations very close to the horizon. This long distance origin of Hawking radiation could have a profound impact on our understanding of the information and transplanckian problems.
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1701.06161 [gr-qc]

 

[kodama]

research papers on the topic of MOND as a low energy 4D QG effect esp in Verlinde emergent gravity theory

Anisotropic Distribution of High Velocity Galaxies in the Local Group
Indranil Banik, Hongsheng Zhao
(Submitted on 21 Jan 2017)
We recently showed that several Local Group (LG) galaxies have much higher radial velocities (RVs) than in a 3D dynamical model of it based on ΛCDM, the standard cosmological paradigm (MNRAS, stx151). 5 out of these 6 galaxies are located very close to a plane with root mean square thickness of only 88.2 kpc despite a radial extent of almost 1 Mpc. This plane also passes within 140 kpc of both the Milky Way (MW) and M31 and just 6 kpc from their mid-point. The orientation of the plane is such that the MW-M31 line is only 20∘ from lying within it.
We develop a basic model in which a past MW-M31 flyby encounter forms tidal dwarf galaxies that later settle into the recently discovered planes of satellites around the MW and M31. The MW-M31 orbital plane required by this scenario is oriented similarly to that of the LG dwarfs with anomalously high RVs. The fast relative motion of the MW and M31 at one time would lead to LG dwarfs being flung out via gravitational slingshot encounters. These encounters would likely be most efficient for objects flung out close to the MW-M31 orbital plane. This suggests a possible dynamical reason for our findings, which are otherwise difficult to explain as a chance alignment of isotropically distributed galaxies (probability < 0.01).
Comments: 13 pages, 8 figures, 7 tables. Submitted to the Monthly Notices of the Royal Astronomical Society in this form
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:1701.06559 [astro-ph.GA]

Perspective on MOND emergence from Verlinde's "emergent gravity" and its recent test by weak lensing
Mordehai Milgrom (Weizmann Institute), Robert H. Sanders (Kapteyn Institute)
(Submitted on 30 Dec 2016 (v1), last revised 17 Jan 2017 (this version, v2))
We highlight phenomenological aspects of Verlinde's recent proposal to account for the mass anomalies in galactic systems without dark matter -- in particular in their relation to MOND. Welcome addition to the MOND lore as it is, this approach have reproduced, so far, only a small fraction of MOND phenomenology, and is still rather tentative, both in its theoretical foundations and in its phenomenology. What Verlinde has extracted from this approach, so far, is a formula -- of rather limited applicability, and with no road to generalization in sight -- for the effective gravitational field of a spherical, isolated, static baryonic system. This formula cannot be used to calculate the gravitational field of disk galaxies, with their rich MOND phenomenology. Notably, it cannot predict their rotation curves, except asymptotically. It does not apply to the few-, or many-body problem; so, it cannot give, e.g., the two-body force between two galaxies, or be used to conduct N-body calculations of galaxy formation, evolution, and interactions. The formula cannot be applied to the internal dynamics of a system embedded in an external field, where MOND predicts important consequences. etc. MOND is backed by full-fledged, Lagrangian theories that can be, and are, routinely applied to all the above phenomena, and more. Verlinde's formula, as it now stands, strongly conflicts with solar-system and possibly earth-surface constraints, and cannot fully account for the mass anomalies in the cores of galaxy clusters (a standing conundrum in MOND). The recent weak-lensing test of the formula is, in fact, testing a cornerstone prediction of MOND, one that the formula does reproduce, and which has been tested before in the very same way.
Comments: 6 pages; added discussion of earth-surface effects
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1612.09582 [astro-ph.GA]

A universal velocity dispersion profile for pressure supported systems: evidence for MONDian gravity across 7 orders of magnitude in mass
R. Durazo, X. Hernandez, B. Cervantes Sodi, S. F. Sanchez
(Submitted on 12 Jun 2015 (v1), last revised 2 Dec 2016 (this version, v2))
For any MONDian extended theory of gravity where the rotation curves of spiral galaxies are explained through a change in physics rather than the hypothesis of dark matter, a generic dynamical behavior is expected for pressure supported systems: an outer flattening of the velocity dispersion profile occurring at a characteristic radius, where both the amplitude of this flat velocity dispersion and the radius at which it appears are predicted to show distinct scalings with the total mass of the system. By carefully analyzing the dynamics of globular clusters and elliptical galaxies, we are able to significantly extend the astronomical diversity of objects in which MONDian gravity has been tested, from spiral galaxies, to the much larger mass range covered by pressure supported systems. We show that a universal projected velocity dispersion profile accurately describes various classes of pressure supported systems, and further, that the expectations of extended gravity are met, across seven orders of magnitude in mass. These observed scalings are not expected under dark matter cosmology, and would require particular explanations tuned at the scales of each distinct astrophysical system.
Comments: 12 pages, 15 figures 1 table
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:1506.04099 [astro-ph.GA]

dℓ(z) and BAO in the emergent gravity and the dark universe
Ding-fang Zeng
(Submitted on 3 Jan 2017)
We illustrate that ΛMOND cosmology following from E. Verlinde's emergent gravity idea which contains only constant dark energy and baryonic matters governed by linear inverse gravitation forces at and beyond galaxy scales fit with the luminosity distance v.s. redshift relationship, i.e. dℓ(z) of type Ia supernovae equally well as the standard ΛCDM cosmology does. But in a rather broad and reasonable parameter space, ΛMOND gives too strong baryon acoustic oscillation, i.e. BAO signals on the matter power spectrum contradicting with observations from various galaxy survey and counting experiments.
Comments: 4.2 two column style pages, 4 figures, version asking for comments
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1701.00690 [astro-ph.CO]
(or arXiv:1701.00690v1 [astro-ph.CO] for this version)

On the Davies-Unruh effect in a wide range of temperatures
Carlos E. Navia
(Submitted on 12 Jan 2017)
The Debye model of the specific heat of solid at low temperatures is incorporate in the Entropic Gravity Theory (EGT). Rather of a smooth surface, the holographic screen is considered as an oscillating elastic membrane, with a continuous range of frequencies, that cuts off at a maximum (Debye) temperature, TD. We show that at low temperatures T<TD, the conservation of the equivalence principle in EGT requires a modification of the Davies-Unruh effect. While the maintenance of Davies-Unruh effect requires a violation of the equivalence principle. These two possibilities are equivalents, because both can emulate the same quantity of dark matter. However, in both cases, the central mechanism is the Davies-Unruh effect, this seems to indicate that the modification of the Davies-Unruh effect emulates dark matter which in turn can be see as a violation of the equivalence principle. This scenario is promising to explain why MOND theory works at very low temperatures (accelerations) regime, i. e., the galaxies sector. We also show that in the intermediate region, for temperatures slightly lower or slightly higher than Debye temperature, EGT predicts the mass-temperature relation of hot X-ray galaxy clusters.
Comments: 4 pages, 4 figures
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:1701.03442 [astro-ph.GA]
(or arXiv:1701.03442v1 [astro-ph.GA] for this version)

 

[kodama]

general Relativity and Quantum Cosmology
Time evolution in deparametrized models of loop quantum gravity
Mehdi Assanioussi, Jerzy Lewandowski, Ilkka Mäkinen
(Submitted on 6 Feb 2017)
An important aspect in understanding the dynamics in the context of deparametrized models of LQG is to obtain a sufficient control on the quantum evolution generated by a given Hamiltonian operator. More specifically, we need to be able to compute the evolution of relevant physical states and observables with a relatively good precision. In this article, we introduce an approximation method to deal with the physical Hamiltonian operators in deparametrized LQG models, and apply it to models in which a free Klein-Gordon scalar field or a non-rotational dust field is taken as the physical time variable. This method is based on using standard time-independent perturbation theory of quantum mechanics to define a perturbative expansion of the Hamiltonian operator, the small perturbation parameter being determined by the Barbero-Immirzi parameter β. This method allows us to define an approximate spectral decomposition of the Hamiltonian operators and hence to compute the evolution over a certain time interval. As a specific example, we analyze the evolution of expectation values of the volume and curvature operators starting with certain physical initial states, using both the perturbative method and a straightforward expansion of the expectation value in powers of the time variable. This work represents a first step towards achieving the goal of understanding and controlling the new dynamics developed in [25, 26].
Comments: 23 pages, 18 figures
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1702.01688 [gr-qc]
(or arXiv:1702.01688v1 [gr-qc] for this version)

 

[kodama]

Loop-Quantum-Gravity Simplicity Constraint as Surface Defect in Complex Chern-Simons Theory
Muxin Han, Zichang Huang
(Submitted on 10 Feb 2017)
The simplicity constraint is studied in the context of 4d spinfoam models with cosmological constant. We find that the quantum simplicity constraint is realized as the 2d surface defect in SL(2,C) Chern-Simons theory in the construction of spinfoam amplitudes. By this realization of simplicity constraint in Chern-Simons theory, we are able to construct the new spinfoam amplitude with cosmological constant for arbitrary simplicial complex (with many 4-simplices). The semiclassical asymptotics of the amplitude is shown to reproduce correctly the 4-dimensional Einstein-Regge action with cosmological constant term.
Comments: 16 pages, 3 figures
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Mathematical Physics (math-ph)
Cite as: arXiv:1702.03285 [gr-qc]
(or arXiv:1702.03285v1 [gr-qc] for this version)

 

[kodama]

Hybrid loop quantum cosmology and predictions for the cosmic microwave background
Laura Castelló Gomar, Daniel Martín de Blas, Guillermo A. Mena Marugán, Javier Olmedo
(Submitted on 20 Feb 2017)
We investigate the consequences of the hybrid quantization approach for primordial perturbations in loop quantum cosmology, obtaining predictions for the cosmic microwave background and comparing them with data collected by the Planck mission. In this work, we complete previous studies about the scalar perturbations and incorporate tensor modes. We compute their power spectrum for a variety of vacuum states. We then analyze the tensor-to-scalar ratio and the consistency relation between this quantity and the spectral index of the tensor power spectrum. We also compute the temperature-temperature, electric-electric, temperature-electric, and magnetic-magnetic correlation functions. Finally, we discuss the effects of the quantum geometry in these correlation functions and confront them with observations.
Comments: 33 pages, 23 figures and 1 table
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Report number: IGC-17|2-1
Cite as: arXiv:1702.06036 [gr-qc]
(or arXiv:1702.06036v1 [gr-qc] for this version)

 

[kodama]

Gravity as an SU(1,1) gauge theory in four dimensions
Hongguang Liu, Karim Noui
(Submitted on 22 Feb 2017)
We start with the Hamiltonian formulation of the first order action of pure gravity with a full sl(2,C) internal gauge symmetry. We make a partial gauge-fixing which reduces sl(2,C) to its sub-algebra su(1,1). This case corresponds to a splitting of the space-time M=Σ×R where Σ inherits an arbitrary Lorentzian metric of signature (−,+,+). Then, we find a parametrization of the phase space in terms of an su(1,1) commutative connection and its associated conjugate electric field. Following the techniques of Loop Quantum Gravity, we start the quantization of the theory and we consider the kinematical Hilbert space on a given fixed graph Γ whose edges are colored with unitary representations of su(1,1). We compute the spectrum of area operators acting of the kinematical Hilbert space: we show that space-like areas have discrete spectra, in agreement with usual su(2) Loop Quantum Gravity, whereas time-like areas have continuous spectra. We conclude on the possibility to make use of this formulation of gravity to construct a holographic description of black holes in the framework of Loop Quantum Gravity.
Comments: 19 pages, 2 figures
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1702.06793 [gr-qc]
(or arXiv:1702.06793v1 [gr-qc] for this version)

 

[atyy]

https://arxiv.org/abs/1702.04137
Asymptotically safe cosmology - a status report
Alfio Bonanno, Frank Saueressig
(Submitted on 14 Feb 2017)
Asymptotic Safety, based on a non-Gaussian fixed point of the gravitational renormalization group flow, provides an elegant mechanism for completing the gravitational force at sub-Planckian scales. At high energies the fixed point controls the scaling of couplings such that unphysical divergences are absent while the emergence of classical low-energy physics is linked to a crossover between two renormalization group fixed points. These features make Asymptotic Safety an attractive framework for cosmological model building. The resulting scenarios may naturally give rise to a quantum gravity driven inflationary phase in the very early universe and an almost scale-free fluctuation spectrum. Moreover, effective descriptions arising from an renormalization group improvement permit a direct comparison to cosmological observations as, e.g. Planck data.

https://arxiv.org/abs/1702.04439
General Relativity and Quantum Cosmology
Black Hole Entropy from BMS Symmetry at the Horizon
S. Carlip
(Submitted on 15 Feb 2017 (v1), last revised 1 Mar 2017 (this version, v2))
Near the horizon, the obvious symmetries of a black hole spacetime---the horizon-preserving diffeomorphisms---are enhanced to a larger symmetry group with a BMS3 algebra. Using dimensional reduction and covariant phase space techniques, I investigate this augmented symmetry, and show that it is strong enough to determine the black hole entropy.

 

[atyy]

https://arxiv.org/abs/1703.01156
Quantum Surface and Intertwiner Dynamics in Loop Quantum Gravity
Alexandre Feller, Etera R. Livine
(Submitted on 3 Mar 2017)
We introduce simple generic models of surface dynamics in loop quantum gravity (LQG). A quantum surface is defined as a set of elementary patches of area glued together. We provide it with an extra structure of locality (nearest neighbors), thought of as induced by the whole spin network state defining the 3d bulk geometry around the quantum surface. Here, we focus on classical surface dynamics, using a spinorial description of surface degrees of freedom. We introduce two classes of dynamics, to be thought as templates for future investigation of LQG dynamics with in mind the dynamics of quantum black holes. The first defines global dynamics of the closure defect of the surface, with two basic toy-models, either a dissipative dynamics relaxing towards the closure constraint or a Hamiltonian dynamics precessing the closure defect. The second class of dynamics describes the isolated regime, when both area and closure defect are conserved throughout the evolution. The surface dynamics is implemented through U(N) transformations and generalizes to a Bose-Hubbard Hamiltonian with a local quadratic potential interaction. We briefly discuss the implications of modeling the quantum black hole dynamics by a surface Bose-Hubbard model.

 

[kodama]

A Covariant Version of Verlinde's Emergent Gravity
S. Hossenfelder
(Submitted on 4 Mar 2017)
A generally covariant version of Erik Verlinde's emergent gravity model is proposed. The Lagrangian constructed here allows an improved interpretation of the underlying mechanism. It suggests that de-Sitter space is filled with a vector-field that couples to baryonic matter and, by dragging on it, creates an effect similar to dark matter. We solve the covariant equation of motion in the background of a Schwarzschild space-time and obtain correction terms to the non-covariant expression. Furthermore, we demonstrate that the vector field can also mimic dark energy.
Comments: 14 pages, no figures
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1703.01415 [gr-q

 

[atyy]

https://arxiv.org/abs/1703.03710
The matter-ekpyrotic bounce scenario in Loop Quantum Cosmology
Jaume Haro, Jaume Amorós, Llibert Aresté Saló
(Submitted on 10 Mar 2017)
We will perform a detailed study of the matter-ekpyrotic bouncing scenario in Loop Quantum Cosmology using the methods of the dynamical systems theory. We will show that when the background is driven by a single scalar field, at very late times, in the contracting phase, all orbits depict a matter dominated Universe, which evolves to an ekpyrotic phase. After the bounce the Universe enters in the expanding phase, where the orbits leave the ekpyrotic regime going to a kination (also named deflationary) regime. Moreover, this scenario supports the production of heavy massive particles conformally coupled with gravity, which reheats the universe at temperatures compatible with the nucleosynthesis bounds and also the production of massless particles non-conformally coupled with gravity leading to very high reheating temperatures but ensuring the nucleosynthesis success. Dealing with cosmological perturbations, these background dynamics produce a nearly scale invariant power spectrum for the modes that leave the Hubble radius, in the contracting phase, when the Universe is quasi-matter dominated, whose spectral index and corresponding running is compatible with the recent experimental data obtained by PLANCK's team.

https://arxiv.org/abs/1703.03757
Canonical LQG operators and kinematical states for plane gravitational waves
F. Hinterleitner
(Submitted on 10 Mar 2017)
In a 1+1 dimensional model of plane gravitational waves the flux-holonomy algebra of loop quantum gravity is modified in such a way that the new basic operators satisfy canonical commutation relations. Thanks to this construction it is possible to find kinematical solutions for unidirectional plane gravitational waves with finite geometric expectation values and fluctuations, which was problematic in a more conventional approach in a foregoing paper by the author and coauthors.

 

[atyy]

https://arxiv.org/abs/1703.03443
Classical and quantum: a conflict of interest
T. P. Singh
(Submitted on 9 Mar 2017)
We highlight three conflicts between quantum theory and classical general relativity, which make it implausible that a quantum theory of gravity can be arrived at by quantising classical gravity. These conflicts are: quantum nonlocality and space-time structure; the problem of time in quantum theory; and the quantum measurement problem. We explain how these three aspects bear on each other, and how they point towards an underlying noncommutative geometry of space-time.

 

[kodama]

Repulsive gravity induced by a conformally coupled scalar field implies a bouncing radiation-dominated universe
Vicente Antunes, Mario Novello
(Submitted on 8 Mar 2017)
In the present work we revisit a model consisting of a scalar field with a quartic self-interaction potential non-minimally (conformally) coupled to gravity [1]. When the scalar field vacuum is in a broken symmetry state, an effective gravitational constant emerges which, in certain regimes, can lead to gravitational repulsive effects when only ordinary radiation is coupled to gravity. In this case, a bouncing universe is shown to be the only cosmological solution admissible by the field equations when the scalar field is in such a broken symmetry state.
Comments: 10 pages, 1 figure
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1703.03060 [gr-qc]

Poincaré gauge gravity: an emergent scenario
J.L. Chkareuli
(Submitted on 4 Mar 2017)
The Poincar\'e gauge gravity (PGG) with the underlying vector fields of tetrads and spin-connections is perhaps the best theory candidate for gravitation to be unified with the other three elementary forces of nature. There is a clear analogy between local frame in PGG and local internal symmetry space in the Standard Model. As a result, the spin-connection fields, gauging the local frame Lorentz symmetry group SO(1,3)_{LF}, appear in PGG much as photons and gluons appear in SM. We propose that such an analogy may follow from their common emergent nature allowing to derive PGG in the same way as conventional gauge theories. In essence, we start with an arbitrary theory of some vector and fermion fields which possesses only global spacetime symmetries, such as Lorentz and translational invariance, in flat Minkowski space. The two vector field multiplets involved are proposed to belong, respectively, to the adjoint (A_{{\mu}}^{ij}) and vector (e_{{\mu}}^{i}) representations of the starting global Lorentz symmetry. We show that if these prototype vector fields are covariantly constrained, A_{{\mu}}^{ij}A_{ij}^{{\mu}}=M_{A} and e_{{\mu}}^{i}e_{i}^{{\mu}}=M_{e}, thus causing a spontaneous violation of the accompanying global symmetries (M_{A,e} are their proposed violation scales), then the only possible theory compatible with these length-preserving constraints is turned out to be the gauge invariant PGG, while the corresponding massless (pseudo)Goldstone modes are naturally collected in the emergent gauge fields of tetrads and spin-connections. In a minimal theory case being linear in a curvature we unavoidably come to the Einstein-Cartan theory. The extending theories with propagating spin-connection and tetrad modes are also considered and their possible unification with the Standard Model is briefly discussed.
Comments: 33 pages, submitted to Physical Review D. arXiv admin note: text overlap with arXiv:1305.6898
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1703.01492 [gr-qc]
Gravastars in f(R,T) gravity
Amit Das, Shounak Ghosh, Swapan Das, B.K. Guha, Saibal Ray
(Submitted on 26 Feb 2017)
We propose a unique stellar model under the f(R,T) gravity by using the conjecture of Mazur-Mottola~\cite{Mazur2001,Mazur2004} which is known as gravastar and a viable alternative to the black hole as available in literature. This gravastar is described by the three different regions, viz., (I) Interior core region, (II) Intermediate thin shell, and (III) Exterior spherical region. The pressure within the interior region is equal to the constant negative matter density which provides a repulsive force over the thin spherical shell. This thin shell is assumed to be formed by a fluid of ultra relativistic plasma and the pressure, which is directly proportional to the matter-energy density according to Zel'dovich's conjecture of stiff fluid~\cite{zeldovich1972}, does counter balance the repulsive force exerted by the interior core region. The exterior spherical region is completely vacuum and assumed to be de Sitter spacetime which can be described by the Schwarzschild solution. Under this specification we find out a set of exact and singularity-free solution of the collapsing star which presents several other physically valid features within the framework of alternative gravity.
Comments: 7 pages, 4 figures
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1702.08873 [gr-qc]

 

[kodama]

Poincaré Gauge Gravity Cosmology
Hongchao Zhang, Lixin Xu
(Submitted on 2 Mar 2017)
In this work, we construct the logical framework of the Poincar\'e gauge gravity cosmology based on five postulations, and introduce the modified redshift relation within this framework. Then we solve a system with quadratic action and some other assumptions to get an analytic solution on background level. The evolution of the Universe on background can be reproduced from this solution without hypothesizing dark energy. Further, we use the type Ia supernova data set JLA to test the effect of the modified redshift relation under the constraints of system parameters. The results show that the constraint on some parameters are compact.
Comments: 5 pages, 3 figures, 2 tables
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1703.00639 [gr-qc]
(or arXiv:1703.00639v1 [gr-qc] for this version)

 

[kodama]

Analogue Gravity Models From Conformal Rescaling
Sabine Hossenfelder, Tobias Zingg
(Submitted on 13 Mar 2017)
Analogue gravity is based on a mathematical identity between quantum field theory in curved space-time and the propagation of perturbations in certain condensed matter systems. But not every curved space-time can be simulated in such a way, because one does not only need a condensed matter system that generates the desired metric tensor, but that system then also has to obey its own equations of motion. And specifying the metric tensor that one wishes to realize usually overdetermines the underlying condensed matter system, such that its equations of motion are in general not fulfilled, in which case the desired metric does not have an analogue.
Here, we show that the class of metrics that have an analogue is bigger than what a first cursory consideration might suggest. This is due to the analogue metric only being defined up to a choice of parametrization of the perturbation in the underlying condensed matter system. In this way, the class of analogue gravity models can be vastly expanded. In particular, we demonstrate how this freedom of choice can be used to insert an intermediary conformal factor. Then, as a corollary, we find that any metric conformal to a Painlev\'e--Gullstrand type line element can, potentially, result as an analogue of a perturbation propagating in a non-viscous, barotropic fluid.
Comments: 12 pages
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
MSC classes: 83-XX
Cite as: arXiv:1703.04462 [gr-qc]
(or arXiv:1703.04462v1 [gr-qc] for this version)

 

[kodama]

Gravitational echoes from macroscopic quantum gravity effects
Carlos Barceló, Raúl Carballo-Rubio, Luis J. Garay
(Submitted on 31 Jan 2017 (v1), last revised 1 Feb 2017 (this version, v2))
New theoretical approaches developed in the last years predict that macroscopic quantum gravity effects in black holes should lead to modifications of the gravitational wave signals expected in the framework of classical general relativity, with these modifications being characterized by the existence of dampened repetitions of the primary signal. Here we use the fact that non-perturbative corrections to the near-horizon external geometry of black holes are necessary for these modifications to exist, in order to classify different proposals and paradigms with respect to this criterion and study in a neat and systematic way their phenomenology. Proposals that lead naturally to the existence of echoes in the late-time ringdown of gravitational wave signals from black hole mergers must share the replacement of black holes by horizonless configurations with a physical surface showing reflective properties in the relevant range of frequencies. On the other hand, proposals or paradigms that restrict quantum gravity effects on the external geometry to be perturbative, such as black hole complementarity or the closely related firewall proposal, do not display echoes. For the sake of completeness we exploit the interplay between the timescales associated with the formation of firewalls and the mechanism behind the existence of echoes in order to conclude that even unconventional distortions of the firewall concept (such as naked firewalls) do not lead to this phenomenon.
Comments: V1: 21 pages + references, 1 figure. V2: updated references
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1701.09156 [gr-qc]
Flowing to the continuum in discrete tensor models for quantum gravity
Astrid Eichhorn, Tim Koslowski
(Submitted on 11 Jan 2017)
Tensor models provide a way to access the path-integral for discretized quantum gravity in d dimensions. As in the case of matrix models for two-dimensional quantum gravity, the continuum limit can be related to a Renormalization Group fixed point in a setup where the tensor size N serves as the Renormalization Group scale. We develop functional Renormalization Group tools for tensor models with a main focus on a rank-3 model for three-dimensional quantum gravity. We rediscover the double-scaling limit and provide an estimate for the scaling exponent. Moreover, we identify two additional fixed points with a second relevant direction in a truncation of the Renormalization Group flow. The new relevant direction might hint at the presence of additional degrees of freedom in the corresponding continuum limit.
Comments: 16 pages
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1701.03029 [gr-qc]
(or arXiv:1701.03029v1 [gr-qc] for this version)

 

[kodama]

Quantum Metric and Entanglement on Spin Networks
Fabio M. Mele
(Submitted on 19 Mar 2017)
Motivated by the idea that, in the background-independent framework of a Quantum Theory of Gravity, entanglement is expected to play a key role in the reconstruction of spacetime geometry, we investigate the possibility of using the formalism of Geometric Quantum Mechanics (GQM) to give a tensorial characterization of entanglement on spin network states. Our analysis focuses on the simple case of a single link graph (Wilson line state) for which we define a dictionary to construct a Riemannian metric tensor and a symplectic structure on the space of states. The manifold of (pure) quantum states is then stratified in terms of orbits of equally entangled states and the block-coefficient matrices of the corresponding pulled-back tensors fully encode the information about separability and entanglement. In particular, the off-diagonal blocks define an entanglement monotone interpreted as a distance with respect to the separable state. As such, it provides a measure of graph connectivity. Finally, in the maximally entangled gauge-invariant case, the entanglement monotone is proportional to a power of the area of the surface dual to the link. This suggests a connection between the GQM formalism and the (simplicial) geometric properties of spin network states through entanglement.
Comments: 162 pages, 11 figures, Master Thesis
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
Cite as: arXiv:1703.06415 [gr-qc]

Fisher Metric, Geometric Entanglement and Spin Networks
Goffredo Chirco, Fabio M. Mele, Daniele Oriti, Patrizia Vitale
(Submitted on 15 Mar 2017)
We introduce the geometric formulation of Quantum Mechanics in the quantum gravity context, and we use it to give a tensorial characterization of entanglement on spin network states. Starting from the simplest case of a single-link graph (Wilson line), we define a dictionary to construct a Riemannian metric tensor and a symplectic structure on the space of spin network states, showing how they fully encode the information about separability and entanglement, and, in particular, an entanglement monotone interpreted as a distance with respect to the separable state. In the maximally entangled gauge-invariant case, the entanglement monotone is proportional to a power of the area of the surface dual to the link thus supporting a connection between entanglement and the (simplicial) geometric properties of spin network states. We extend then such analysis to the study of non-local correlations between two non-adjacent regions of a generic spin network. In the end, our analysis shows that the same spin network graph can be understood as an information graph whose connectivity encodes, both at the local and non-local level, the quantum correlations among its parts. This gives a further connection between entanglement and geometry.
Comments: 51 pages, 3 figures
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
Cite as: arXiv:1703.05231 [gr-qc]

Holographic spin networks from tensor network states
Sukhwinder Singh, Nathan A. McMahon, Gavin K. Brennen
(Submitted on 1 Feb 2017)
In the holographic correspondence of quantum gravity, a global onsite symmetry at the boundary generally translates to a local gauge symmetry in the bulk. In this paper, we extend the tensor network based toy model for holography introduced in [arXiv:1701.04778] to incorporate this feature. We lift the multi-scale renormalization ansatz (MERA) representation of the ground state of a one dimensional (1D) local Hamiltonian, which has a global onsite symmetry, to a dual quantum state of a 2D lattice on which the symmetry appears gauged. We show how the 2D bulk state decomposes in terms of spin network states, which label a basis in the gauge-invariant sector of the bulk lattice. This decomposition is instrumental to obtain expectation values of gauge-invariant observables in the bulk, and also reveals that the bulk state is generally entangled between the gauge and the remaining bulk degrees of freedom that are not fixed by the symmetry. (In analogy with the holographic correspondence, we interpret the latter to possibly include "gravitational" degrees of freedom.) We illustrate these features for a particular subset of bulk states referred to as copy bulk states. We present numerical results for ground states of several 1D critical spin chains to illustrate that: (i) entanglement in the dual copy bulk states potentially depends on the central charge of the underlying conformal field theories, and (ii) the spectrum of a bulk density matrix, obtained by tracing out the gauge degrees of freedom, exhibits degeneracies, possibly suggesting an emergent symmetry in the non-gauge sector of the bulk. We also illustrate the possibility of emergent topological order in the bulk. More broadly, our holographic model translates the MERA, a tensor network state, to a superposition of spin network states, as they appear in lattice gauge theories in one higher dimension.
Comments: 21 pages, 18 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
Cite as: arXiv:1702.00392 [cond-mat.str-el]

 

[kodama]

On the fate of the Hoop Conjecture in quantum gravity
Fabio Anzà, Goffredo Chirco
(Submitted on 15 Mar 2017 (v1), last revised 21 Mar 2017 (this version, v2))
We consider a closed region R of 3d quantum space modeled by SU(2) spin-networks. Using the concentration of measure phenomenon we prove that, whenever the ratio between the boundary ∂R and the bulk edges of the graph overcomes a finite threshold, the state of the boundary is always thermal, with an entropy proportional to its area. The emergence of a thermal state of the boundary can be traced back to a large amount of entanglement between boundary and bulk degrees of freedom. Using the dual geometric interpretation provided by loop quantum gravity, we interprete such phenomenon as a pre-geometric analogue of Thorne's "Hoop conjecture", at the core of the formation of a horizon in General Relativity.
Comments: 7 pages, 2 figures, minor improvements
Subjects: General Relativity and Quantum Cosmology (gr-qc); Quantum Physics (quant-ph)
Cite as: arXiv:1703.05241 [gr-qc]
Naturalizing Gravity of the Quantum Fields, and the Hierarchy Problem
Durmus Demir
(Submitted on 10 Mar 2017)
It is shown that gravity can be incorporated into the Standard Model (SM) in a way solving the hierarchy problem. For this, the SM effective action in flat spacetime is adapted to curved spacetime via not only the general covariance but also the gauge invariance. For the latter, gauge field hard masses, induced by loops at the UV scale Λ, are dispelled by construing Λ as the constant value assigned to curvature. This gives way to an unprecedented mechanism for incorporating gravity into the SM in that the hierarchy problem is solved by transmutation of the Higgs boson Λ2--mass into the Higgs-curvature coupling, and the cosmological constant problem is alleviated by metamorphosis of the vacuum Λ4--energy into the Einstein-Hilbert term. Gravity emerges correctly if the SM is accompanied by a secluded dark sector sourcing non-interacting dark matter, dark energy and dark radiation. Physics beyond the SM, containing Higgs-phobic scalars that resolve the strong CP problem, flavor problem, baryogenesis and inflation, respects the hierarchy. Majorana neutrinos are naturally incorporated if Λ lies at the see-saw scale. This mechanism, in general, leaves no compelling reason to anticipate new particles at the LHC or higher-energy colliders.
Comments: 27 pages, 2 tables
Subjects: High Energy Physics - Phenomenology (hep-ph); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)

 

[atyy]

https://arxiv.org/abs/1703.07396
The Overview Chapter in Loop Quantum Gravity: The First 30 Years
Abhay Ashtekar, Jorge Pullin
(Submitted on 21 Mar 2017)
This is the introductory Chapter in the monograph Loop Quantum Gravity: The First 30 Years, edited by the authors, that was just published in the series "100 Years of General Relativity. The 8 invited Chapters that follow provide fresh perspectives on the current status of the field from some of the younger and most active leaders who are currently shaping its development. The purpose of this Chapter is to provide a global overview by bridging the material covered in subsequent Chapters. The goal and scope of the monograph is described in the Preface which can be read by following the Front Matter link at the website listed below.

https://arxiv.org/abs/1703.07556
Evolution of Universes in Causal Set Cosmology
Fay Dowker, Stav Zalel
(Submitted on 22 Mar 2017)
The causal set approach to the problem of quantum gravity is based on the hypothesis that spacetime is fundamentally discrete. Spacetime discreteness opens the door to novel types of dynamical law for cosmology and the Classical Sequential Growth (CSG) models of Rideout and Sorkin form an interesting class of such laws. It has been shown that a renormalisation of the dynamical parameters of a CSG model occurs whenever the universe undergoes a Big Crunch-Big Bang bounce. In this paper we propose a way to model the creation of a new universe after the singularity of a black hole. We show that renormalisation of dynamical parameters occurs in a CSG model after such a creation event. We speculate that this could realize aspects of Smolin's Cosmological Natural Selection proposal.

 

[kodama]

Fermions in Loop Quantum Cosmology
Beatriz Elizaga Navascués, Mercedes Martín-Benito, Guillermo A. Mena Marugán
(Submitted on 30 Mar 2017)
This work pioneers the quantization of primordial fermion perturbations in hybrid Loop Quantum Cosmology (LQC). We consider a Dirac field coupled to a spatially flat, homogeneous, and isotropic cosmology, sourced by a scalar inflaton, and treat the Dirac field as a perturbation. We describe the inhomogeneities of this field in terms of creation and annihilation variables, chosen to admit a unitary evolution if the Dirac fermion were treated as a test field. Considering instead the full system, we truncate its action at quadratic perturbative order and construct a canonical formulation. In particular this implies that, in the global Hamiltonian constraint of the model, the contribution of the homogeneous sector is corrected with a quadratic perturbative term. We then adopt the hybrid LQC approach to quantize the full model, combining the loop representation of the homogeneous geometry with the Fock quantization of the inhomogeneities. We assume a Born-Oppenheimer ansatz for physical states and show how to obtain a Schr\"odinger equation for the quantum evolution of the perturbations, where the role of time is played by the homogeneous inflaton. We prove that the resulting quantum evolution of the Dirac field is indeed unitary, despite the fact that the underlying homogeneous geometry has been quantized as well. Remarkably, in such evolution, the fermion field couples to an infinite sequence of quantum moments of the homogeneous geometry. Moreover, the evolved Fock vacuum of our fermion perturbations is shown to be an exact solution of the Schr\"odinger equation. Finally, we discuss in detail the quantum backreaction that the fermion field introduces in the global Hamiltonian constraint. For completeness, our quantum study includes since the beginning (gauge-invariant) scalar and tensor perturbations, that were studied in previous works.
Comments: 38 pages. Prepared to submit to JCAP
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1703.10391 [gr-qc]

Loop quantum cosmology and singularities
Ward Struyve
(Submitted on 30 Mar 2017)
Loop quantum gravity is believed to eliminate singularities such as the big bang and big crunch singularity. In order to base this belief on theoretical analysis, the notorious problems such as the problem of time and the problem of the actual meaning of singularities must be addressed and eventually overcome. In this paper, we address the problem of singularities in the context of the Bohmian formulation of loop quantum cosmology (which describes symmetry-reduced models of quantum gravity using the quantization techniques of loop quantum gravity). This formulation solves the mentioned conceptual problems. For example the notion of singularity is clear in this case, since there is an actual metric in addition to the wave function. As such, there is a singularity whenever this actual metric is singular. It is shown that in the loop quantum cosmology for a homogeneous and isotropic Friedmann-Lemaitre-Robertson-Walker space-time with arbitrary constant spatial curvature and possibly a cosmological constant, coupled to a massless homogeneous scalar field, a big bang or big crunch singularity is never obtained. This result is obtained without assuming any boundary conditions. This result should also be contrasted with the fact that in the Bohmian formulation of the Wheeler-DeWitt theory singularities may exist (depending on the wave function and the initial conditions for the metric and scalar field).
Comments: 17 pages, no figures, LaTeX
Subjects: General Relativity and Quantum Cosmology (gr-qc); Quantum Physics (quant-ph)
Cite as: arXiv:1703.10274 [gr-qc]
(or arXiv:1703.10274v1 [gr-qc] for this version)

Black Holes in Loop Quantum Gravity
Alejandro Perez
(Submitted on 27 Mar 2017)
This is a review of the results on black hole physics in the framework of loop quantum gravity. The key feature underlying the results is the discreteness of geometric quantities at the Planck scale predicted by this approach to quantum gravity. Quantum discreteness follows directly from the canonical quantization prescription when applied to the action of general relativity that is suitable for the coupling of gravity with gauge fields and specially with Fermions. Planckian discreteness and causal considerations provide the basic structure for the understanding of the thermal properties of black holes close to equilibrium. Discreteness also provides a fresh new look at more (at the moment) speculative issues such as those concerning the fate of information in black hole evaporation. The hypothesis of discreteness leads also to interesting phenomenology with possible observational consequences. The theory of loop quantum gravity is a developing program. This review reports its achievements and open questions in a pedagogical manner with an emphasis on quantum aspects of black hole physics.
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1703.09149 [gr-qc]
(or arXiv:1703.09149v1 [gr-qc] for this version)

Inflation and Bounce from Classical and Loop Quantum Cosmology Imperfect Fluids
V.K. Oikonomou
(Submitted on 27 Mar 2017)
We investigate how various inflationary and bouncing cosmologies can be realized by imperfect fluids with a generalized equation of state, in the context of both classical and loop quantum cosmology. With regards to the inflationary cosmologies, we study the intermediate inflation scenario, the R2 inflation scenario and two constant-roll inflation scenarios and with regards to the bouncing cosmologies we study the matter bounce scenario, the singular bounce and the super bounce scenario. Within the context of the classical cosmology, we calculate the spectral index of the power spectrum of primordial curvature perturbations, the scalar-to-tensor ratio and the running of the spectral index and we compare the resulting picture with the Planck data. As we demonstrate, partial compatibility with the observational data is achieved in the imperfect fluid description, however none of the above scenarios is in full agreement with data. This result shows that although it is possible to realize various cosmological scenarios using different theoretical frameworks, it is not guaranteed that all the theoretical descriptions are viable.
Comments: IJMPD Accepted
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1703.09009 [gr-qc]
(or arXiv:1703.09009v1 [gr-qc] for this version)

 

[kodama]

Resolving Cosmological Singularities
Ali H. Chamseddine, Viatcheslav Mukhanov
(Submitted on 18 Dec 2016)
We find a simple modification of the longitudinal mode in General Relativity which incorporates the idea of limiting curvature. In this case the singularities in contracting Friedmann and Kasner universes are avoided, and instead, the universe has a regular bounce which takes place during the time inversely proportional to the square root of the limiting curvature. Away from the bounce, corrections to General Relativity are negligible. In addition the non-singluar modification of General Relativity delivers for free a realistic candidate for Dark Matter.
Comments: 15 pages
Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Theory (hep-th)
DOI: 10.1088/1475-7516/2017/03/009
Cite as: arXiv:1612.05860 [gr-qc]
(or arXiv:1612.05860v1 [gr-qc] for this version)
Subjects:

General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1703.10670 [gr-qc]
(or arXiv:1703.10670v1 [gr-qc] for this version)
General Relativity and Quantum Cosmology
Effective loop quantum cosmology as a higher-derivative scalar-tensor theory
Hongguang Liu, Karim Noui, Edward Wilson-Ewing, David Langlois
(Submitted on 31 Mar 2017)
Recently, Chamseddine and Mukhanov introduced a higher-derivative scalar-tensor theory which leads to a modified Friedmann equation allowing for bouncing solutions. As we note in the present work, this Friedmann equation turns out to reproduce exactly the loop quantum cosmology effective dynamics for a flat isotropic and homogeneous space-time. We generalize this result to obtain a class of scalar-tensor theories, belonging to the family of mimetic gravity, which all reproduce the loop quantum cosmology effective dynamics for flat, closed and open isotropic and homogeneous space-times.
Comments: 19 pages
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Cite as: arXiv:1703.10812 [gr-qc]
On the canonical structure of general relativity with a limiting curvature and its relation to loop quantum gravity
Norbert Bodendorfer, Andreas Schäfer, John Schliemann
(Submitted on 30 Mar 2017)
Chamseddine and Mukhanov recently proposed a modified version of general relativity that implements the idea of a limiting curvature. In the spatially flat, homogeneous, and isotropic sector, their theory turns out to agree with the effective dynamics of the simplest version of loop quantum gravity if one identifies their limiting curvature with a multiple of the Planck curvature. At the same time, it extends to full general relativity without any symmetry assumptions and thus provides an ideal toy model for full loop quantum gravity in the form of a generally covariant effective action known to all orders. In this paper, we study the canonical structure of this theory and point out some interesting lessons for loop quantum gravity. We also highlight in detail how the two theories are connected in the spatially flat, homogeneous, and isotropic sector.
Comments: 12 pages
Subjects: General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:1703.10670 [gr-qc]
(or arXiv:1703.10670v1 [gr-qc] for this version)