First quarter 2014 MIP (most important QG paper) poll-great selection

[marcus]

Of the twenty candidates, indicate the ones you think will prove most significant for future QG research. Since the poll is multiple choice, it's possible to vote for several papers. Abstracts follow in the next post. I've included two non-QG-related Dark Matter papers at the end because of the special interest in that topic.

http://arxiv.org/abs/1403.7482
Deformed Spinor Networks for Loop Gravity: Towards Hyperbolic Twisted Geometries
Maité Dupuis, Florian Girelli, Etera R. Livine
(putting a non-zero cosmological constant into spinor-network terms)

http://arxiv.org/abs/1403.6396
Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
Jaume de Haro, Jaume Amorós
(self-explanatory)

http://arxiv.org/abs/1403.3190
A curvature operator for LQG
Emanuele Alesci, Mehdi Assanioussi, Jerzy Lewandowski
(a new LQG operator - 3D curvature, based on Regge, with good semi-classical limit.)

http://tel.archives-ouvertes.fr/docs/00/95/24/98/PDF/diss.pdf
The Chiral Structure of Loop Quantum Gravity
Wolfgang Wieland
(appropriate treatment of time and Hamiltonian in 4d spin foam, recovering EPRL amplitudes)

http://arxiv.org/abs/1402.6708
A one-dimensional action for simplicial gravity in three dimensions
Wolfgang M. Wieland
(consistency check of spinorial LQG in 3d, recovers Ponzano-Regge amplitudes.)

http://arxiv.org/abs/1402.6613
Numerical simulations of a loop quantum cosmos: robustness of the quantum bounce
Peter Diener, Brajesh Gupt, Parampreet Singh
(further numerical confirmation of the bounce and of Loop cosmology's effective equation model)

http://arxiv.org/abs/1402.4138
Near-Horizon Radiation and Self-Dual Loop Quantum Gravity
Marc Geiller, Karim Noui
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1402.3155
Quantum Reduced Loop Gravity: Semiclassical limit
Emanuele Alesci, Francesco Cianfrani
(semiclassical limit)

http://arxiv.org/abs/arXiv:1402.2084
Black Hole Entropy in Loop Quantum Gravity, Analytic Continuation, and Dual Holography
Muxin Han
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1402.1038
A note on entanglement entropy and quantum geometry
Norbert Bodendorfer
(entropy = entanglement entropy)

http://arxiv.org/abs/1401.7731
Quantization of systems with temporally varying discretization II: Local evolution moves
Philipp A Hoehn
(part of Dittrich team's reformulation)

http://arxiv.org/abs/1401.6562
Planck stars
Carlo Rovelli, Francesca Vidotto
(delayed bounce model solves the black hole problems)

http://arxiv.org/abs/arXiv:1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller
(reformulation by Dittrich group: refinement, coarse-graining, classical limit)

http://arxiv.org/abs/1401.6062
Quantization of systems with temporally varying discretization I: Evolving Hilbert spaces
Philipp A Hoehn
(part of Dittrich group's reformulation)

http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
(GR derived solely from LQG thermodynamics)

http://arxiv.org/abs/1401.5083
Non-Commutative Geometry, Non-Associative Geometry and the Standard Model of Particle Physics
Latham Boyle, Shane Farnsworth
(improves on Alain Connes NCG Standard Model)

http://arxiv.org/abs/1401.4452
Black holes within Asymptotic Safety
Benjamin Koch, Frank Saueressig
(asym. safe BH)

http://arxiv.org/abs/arXiv:1401.0931
Hamiltonian constraint in Euclidean LQG revisited: First hints of off-shell Closure
Alok Laddha
(problems with the Hamiltonian approach finally yielding)

Although not directly QG-related, two "dark matter particle" papers are included in the poll by popular request

http://arxiv.org/abs/1402.4119
An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster
Alexey Boyarsky, Oleg Ruchayskiy, Dmytro Iakubovskyi, Jeroen Franse
(possible dark matter particle?)

http://arxiv.org/abs/1402.2301
Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters
Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall
(hint of dark matter particle)

 

[marcus]

[EDIT: My informal comments follow the abstracts, in parentheses. I have shortened one of the abstracts for compactness]

http://arxiv.org/abs/1403.7482
Deformed Spinor Networks for Loop Gravity: Towards Hyperbolic Twisted Geometries
Maité Dupuis, Florian Girelli, Etera R. Livine
(Submitted on 28 Mar 2014)
In the context of a canonical quantization of general relativity, one can deform the loop gravity phase space on a graph by replacing the T*SU(2) phase space attached to each edge by SL(2,C) seen as a phase space. This deformation is supposed to encode the presence of a non-zero cosmological constant. Here we show how to parametrize this phase space in terms of spinor variables, thus obtaining deformed spinor networks for loop gravity, with a deformed action of the gauge group SU(2) at the vertices. These are to be formally interpreted as the generalization of loop gravity twisted geometries to a hyperbolic curvature.
15 pages, 4 figures
(putting a non-zero cosmological constant into spinor-network terms)

http://arxiv.org/abs/1403.6396
Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
Jaume de Haro, Jaume Amorós
(Submitted on 25 Mar 2014)
The CMB map provided by the Planck project constrains the value of the ratio of tensor-to-scalar perturbations, namely r, to be smaller than 0.11 (95% CL). This bound rules out the simplest models of inflation. However, recent data from BICEP2 is in strong tension with this constrain, as it finds a value r=0.20^+0.07_−0.05 with r=0 disfavored at 7.0σ, which allows these simplest inflationary models to survive. The remarkable fact is that, even though the BICEP2 experiment was conceived to search for evidence of inflation, its experimental data matches correctly theoretical results coming from the matter bounce scenario (the alternative model to the inflationary paradigm). More precisely, most bouncing cosmologies do not pass Planck's constrains due to the smallness of the value of the tensor/scalar ratio r≤0.11, but with new BICEP2 data some of them fit well with experimental data. This is the case with the matter bounce scenario in the teleparallel version of Loop Quantum Cosmology.
Comments: 1 figure.
(self-explanatory)

http://arxiv.org/abs/1403.3190
A curvature operator for LQG
Emanuele Alesci, Mehdi Assanioussi, Jerzy Lewandowski
(Submitted on 13 Mar 2014)
We introduce a new operator in Loop Quantum Gravity - the 3D curvature operator - related to the 3-dimensional scalar curvature. The construction is based on Regge Calculus. We define it starting from the classical expression of the Regge curvature, then we derive its properties and discuss some explicit checks of the semi-classical limit.
20 pages
(a new LQG operator - 3D curvature, based on Regge, with good semi-classical limit.)

http://tel.archives-ouvertes.fr/docs/00/95/24/98/PDF/diss.pdf
The Chiral Structure of Loop Quantum Gravity
Wolfgang Wieland
(posted on 3 March 2014)
Loop gravity ...comes in two versions. The canonical approach seeks to solve the Wheeler–DeWitt equation and find the physical states of the theory. Spinfoam gravity, on the other hand, takes a covariant path integral representation to define the transition amplitudes of the theory. Both approaches use the same Hilbert space, but we do not know whether they actually define the same theory.
In this thesis, I will present four results, all of which lie in between the two approaches…
We start with the classical theory…After the first introductory chapter, we will use the original self-dual connection to repeat the canonical analysis for the Holst action, while leaving the Barbero–Immirzi parameter untouched. The resulting constraint equations depend on this parameter, yet maintain a polynomial form. To guarantee that the metric is real, we have to introduce additional constraints. These reality conditions match the linear simplicity constraints of spinfoam gravity. They are preserved in time only if the spatial spin connection is torsionless, which appears as a secondary constraint in the canonical analysis. This is our first complex of results.

The next chapter is about the classical theory,... Here, we develop the generalisation to SL(2,C), that is we use twistors to parametrise the phase space of self-dual holonomy-flux variables. This is the second result.

We will then discuss the spinfoam dynamics in terms of these twistorial variables, and arrive at our third result: A new Hamiltonian formulation of discretised gravity. The Hamiltonian comes with a continuum action adapted to a fixed simplicial discretisation of spacetime. The action is a sum of the spinorial analogue of the topological “BF”- action and the reality conditions that guarantee the existence of a metric.

Chapter four studies the resulting quantum theory. Since the action is a polynomial in the spinors, canonical quantisation is straightforward. Transition amplitudes reproduce the EPRL (Engle–Pereira–Rovelli–Livine) spinfoam model. This is our final result. It shows that spinfoam gravity can be derived from a classical action, with spinors as the fundamental configuration variables.
160 pages, 9 figures
(appropriate treatment of time and Hamiltonian in 4d spin foam, recovering EPRL amplitudes)

http://arxiv.org/abs/1402.6708
A one-dimensional action for simplicial gravity in three dimensions
Wolfgang M. Wieland
(Submitted on 26 Feb 2014)
This article presents a derivation of the Ponzano--Regge model from a one-dimensional spinor action. The construction starts from the first-order Palatini formalism in three dimensions. We then introduce a simplicial decomposition of the three-dimensional manifold and study the discretised action in the spinorial representation of loop gravity. A one-dimensional refinement limit along the edges of the discretisation brings us back to a continuum formulation. The three-dimensional action turns into a line integral over the one-skeleton of the simplicial manifold. All fields are continuous but have support only along the one-dimensional edges. We define the path integral, and remove the redundant integrals over the local gauge orbits through the usual Faddeev--Popov procedure. The resulting state sum model reproduces the Ponzano--Regge amplitudes.
19 pages, 2 figures
(consistency check of spinorial LQG in 3d, recovers Ponzano-Regge amplitudes.)

http://arxiv.org/abs/1402.6613
Numerical simulations of a loop quantum cosmos: robustness of the quantum bounce and the validity of effective dynamics
Peter Diener, Brajesh Gupt, Parampreet Singh
(Submitted on 26 Feb 2014)
A key result of isotropic loop quantum cosmology is the existence of a quantum bounce which occurs when the energy density of the matter field approaches a universal maximum close to the Planck density. Though the bounce has been exhibited in various matter models, due to severe computational challenges some important questions have so far remained unaddressed. These include the demonstration of the bounce for widely spread states, its detailed properties for the states when matter field probes regions close to the Planck volume and the reliability of the continuum effective spacetime description in general. In this manuscript we rigorously answer these questions using the Chimera numerical scheme for the isotropic spatially flat model sourced with a massless scalar field. We show that as expected from an exactly solvable model, the quantum bounce is a generic feature of states even with a very wide spread, and for those which bounce much closer to the Planck volume. We perform a detailed analysis of the departures from the effective description and find some expected, and some surprising results. At a coarse level of description, the effective dynamics can be regarded as a good approximation to the underlying quantum dynamics unless the states correspond to small scalar field momenta, in which case they bounce closer to the Planck volume, or are very widely spread. Quantifying the amount of discrepancy between the quantum and the effective dynamics, we find that the departure between them depends in a subtle and non-monotonic way on the field momentum and different fluctuations. Interestingly, the departures are generically found to be such that the effective dynamics overestimates the spacetime curvature, and underestimates the volume at the bounce.
46 pages, 26 figures
(further numerical confirmation of the bounce and of Loop cosmology's effective equation model)

http://arxiv.org/abs/1402.4138
Near-Horizon Radiation and Self-Dual Loop Quantum Gravity
Marc Geiller, Karim Noui
(Submitted on 17 Feb 2014)
We compute the near-horizon radiation of quantum black holes in the context of self-dual loop quantum gravity. For this, we first use the unitary spinor basis of SL(2,ℂ) to decompose states of Lorentzian spin foam models into their self-dual and anti self-dual parts, and show that the reduced density matrix obtained by tracing over one chiral component describes a thermal state at Unruh temperature. Then, we show that the analytically-continued dimension of the SU(2) Chern-Simons Hilbert space, which reproduces the Bekenstein-Hawking entropy in the large spin limit in agreement with the large spin effective action, takes the form of a partition function for states thermalized at Unruh temperature, with discrete energy levels given by the near-horizon energy of Frodden-Gosh-Perez, and with a degenerate ground state which is holographic and responsible for the entropy.
6+2 pages
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1402.3155
Quantum Reduced Loop Gravity: Semiclassical limit
Emanuele Alesci, Francesco Cianfrani
(Submitted on 13 Feb 2014)
We discuss the semiclassical limit of Quantum Reduced Loop Gravity, a recently proposed model to address the quantum dynamics of the early Universe. We apply the techniques developed in full Loop Quantum Gravity to define the semiclassical states in the kinematical Hilbert space and evaluating the expectation value of the euclidean scalar constraint we demonstrate that it coincides with the classical expression, i.e. the one of a local Bianchi I dynamics. The result holds as a leading order expansion in the scale factors of the Universe and opens the way to study the subleading corrections to the semiclassical dynamics. We outline how by retaining a suitable finite coordinate length for holonomies our effective Hamiltonian at the leading order coincides with the one expected from LQC. This result is an important step in fixing the correspondence between LQG and LQC.
23 pages
(semiclassical limit)

http://arxiv.org/abs/arXiv:1402.2084
Black Hole Entropy in Loop Quantum Gravity, Analytic Continuation, and Dual Holography
Muxin Han
(Submitted on 10 Feb 2014)
A new approach to black hole thermodynamics is proposed in Loop Quantum Gravity (LQG), by defining a new black hole partition function, followed by analytic continuations of Barbero-Immirzi parameter to γ∈iℝ and Chern-Simons level to k∈iℝ. The analytic continued partition function has remarkable features: The black hole entropy S=A/4ℓ_P² is reproduced correctly for infinitely many γ=iη, at least for η∈ℤ∖{0}. The near-horizon Unruh temperature emerges as the pole of partition function. Interestingly, by analytic continuation the partition function can have a dual statistical interpretation corresponding to a dual quantum theory of γ∈iℤ. The dual quantum theory implies a semiclassical area spectrum for γ∈iℤ. It also implies that at a given near horizon (quantum) geometry, the number of quantum states inside horizon is bounded by a holographic degeneracy d=e^A/4ℓ_P2, which produces the Bekenstein bound from LQG.
On the other hand, the result in arXiv:1212.4060 receives a justification here.
5 pages
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1402.1038
A note on entanglement entropy and quantum geometry
Norbert Bodendorfer
(Submitted on 5 Feb 2014)
It has been argued that the entropy which one is computing in the isolated horizon framework of loop quantum gravity is closely related to the entanglement entropy of the gravitational field and that the calculation performed is not restricted to horizons. We recall existing work on this issue and explain how recent work on generalising these computations to arbitrary spacetime dimensions D+1>2 supports this point of view and makes the duality between entanglement entropy and the entropy computed from counting boundary states manifest. In a certain semiclassical regime in 3+1 dimensions, this entropy is given by the Bekenstein-Hawking formula.
14 pages
(entropy = entanglement entropy)

http://arxiv.org/abs/1401.7731
Quantization of systems with temporally varying discretization II: Local evolution moves
Philipp A Hoehn
(Submitted on 30 Jan 2014)
Several quantum gravity approaches and field theory on an evolving lattice involve a discretization changing dynamics generated by evolution moves. Local evolution moves in variational discrete systems (1) are a generalization of the Pachner evolution moves of simplicial gravity models, (2) update only a small subset of the dynamical data, (3) change the number of kinematical and physical degrees of freedom, and (4) generate a dynamical coarse graining or refining of the underlying discretization. To systematically explore such local moves and their implications in the quantum theory, this article suitably expands the quantum formalism for global evolution moves, constructed in a companion paper, by employing that global moves can be decomposed into sequences of local moves. This formalism is spelled out for systems with Euclidean configuration spaces. Various types of local moves, the different kinds of constraints generated by them, the constraint preservation and possible divergences in resulting state sums are discussed. It is shown that non-trivial local coarse graining moves entail a non-unitary projection of (physical) Hilbert spaces and `fine grained' Dirac observables defined on them. Identities for undoing a local evolution move with its (time reversed) inverse are derived. Finally, the implications of these results for a Pachner move generated dynamics in simplicial quantum gravity models are commented on.
36 pages, many figures
(part of Dittrich team's reformulation)

http://arxiv.org/abs/1401.6562
Planck stars
Carlo Rovelli, Francesca Vidotto
(Submitted on 25 Jan 2014)
A star that collapses gravitationally can reach a further stage of its life, where quantum-gravitational pressure counteracts weight. The duration of this stage is very short in the star proper time, yielding a bounce, but extremely long seen from the outside, because of the huge gravitational time dilation. Since the onset of quantum-gravitational effects is governed by energy density ---not by size--- the star can be much larger than Planckian in this phase. The object emerging at the end of the Hawking evaporation of a black hole can then be larger than Planckian by a factor (m/m_P)ⁿ, where m is the mass fallen into the hole, m_P is the Planck mass, and n is positive. We consider arguments for n=1/3 and for n=1. There is no causality violation or faster-than-light propagation. The existence of these objects alleviates the black-hole information paradox. More interestingly, these objects could have astrophysical and cosmological interest: they produce a detectable signal, of quantum gravitational origin, around the 10⁻¹⁴cm wavelength.
6 pages, 3 figures
(delayed bounce model solves the black hole problems)

http://arxiv.org/abs/arXiv:1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller
(Submitted on 24 Jan 2014)
We construct a new vacuum for loop quantum gravity, which is dual to the Ashtekar-Lewandowski vacuum. Because it is based on BF theory, this new vacuum is physical for (2+1)-dimensional gravity, and much closer to the spirit of spin foam quantization in general. To construct this new vacuum and the associated representation of quantum observables, we introduce a modified holonomy-flux algebra which is cylindrically consistent with respect to the notion of refinement by time evolution suggested in [1]. This supports the proposal for a construction of a physical vacuum made in [1,2], also for (3+1)-dimensional gravity. We expect that the vacuum introduced here will facilitate the extraction of large scale physics and cosmological predictions from loop quantum gravity.
10 pages, 5 figures
(reformulation by Dittrich group: refinement, coarse-graining, classical limit)

http://arxiv.org/abs/1401.6062
Quantization of systems with temporally varying discretization I: Evolving Hilbert spaces
Philipp A Hoehn
(Submitted on 23 Jan 2014)
A temporally varying discretization often features in discrete gravitational systems and appears in lattice field theory models subject to a coarse graining or refining dynamics. To better understand such discretization changing dynamics in the quantum theory, an according formalism for constrained variational discrete systems is constructed. While the present manuscript focuses on global evolution moves and, for simplicity, restricts to Euclidean configuration spaces, a companion article discusses local evolution moves. In order to link the covariant and canonical picture, the dynamics of the quantum states is generated by propagators which satisfy the canonical constraints and are constructed using the action and group averaging projectors. This projector formalism offers a systematic method for tracing and regularizing divergences in the resulting state sums. Non-trivial coarse graining evolution moves lead to non-unitary, and thus irreversible, projections of physical Hilbert spaces and Dirac observables such that these concepts become evolution move dependent on temporally varying discretizations. The formalism is illustrated in a toy model mimicking a 'creation from nothing'. Subtleties arising when applying such a formalism to quantum gravity models are discussed.
44 pages, 1 appendix, 6 figures
(part of Dittrich group's reformulation)

http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
(Submitted on 21 Jan 2014)
A celebrated result by Jacobson is the derivation of Einstein's equations from Unruh's temperature, the Bekenstein-Hawking entropy and the Clausius relation. This has been repeatedly taken as evidence for an interpretation of Einstein's equations as equations of state for unknown degrees of freedom underlying the metric. We show that a different interpretation of Jacobson result is possible, which does not imply the existence of additional degrees of freedom, and follows only from the quantum properties of gravity. We introduce the notion of quantum gravitational Hadamard states, which give rise to the full local thermodynamics of gravity.
12 pages, 1 figure
(GR derived solely from LQG thermodynamics)

http://arxiv.org/abs/1401.5083
Non-Commutative Geometry, Non-Associative Geometry and the Standard Model of Particle Physics
Latham Boyle, Shane Farnsworth
(Submitted on 20 Jan 2014)
Connes has developed a notion of non-commutative geometry (NCG) that generalizes Riemannian geometry, and provides a framework in which the standard model of particle physics, coupled to Einstein gravity, may be concisely and elegantly reformulated. We point out that his formalism may be recast in a way that generalizes immediately from non-commutative to non-associative geometry. In the process, several of the standard axioms and formulae are conceptually reinterpreted. This reformulation also suggests a new constraint on the finite NCG corresponding to the standard model of particle physics. Remarkably, this new condition resolves a long-standing puzzle about the NCG embedding of the standard model, by precisely eliminating from the action the collection of 7 unwanted terms that previously had to be removed by an extra (empirically-motivated) assumption.
5 pages
(improves on Alain Connes NCG Standard Model)

http://arxiv.org/abs/1401.4452
Black holes within Asymptotic Safety
Benjamin Koch, Frank Saueressig
(Submitted on 17 Jan 2014)
Black holes are probably among the most fascinating objects populating our universe. Their characteristic features found within general relativity, encompassing spacetime singularities, event horizons, and black hole thermodynamics, provide a rich testing ground for quantum gravity ideas. We review the status of black holes within a particular proposal for quantum gravity, Weinberg's asymptotic safety program. Starting from a brief survey of the effective average action and scale setting procedures, an improved quantum picture of the black hole is developed. The Schwarzschild black hole and its generalizations including angular momenta, higher-derivative corrections and the implications of extra dimensions are discussed in detail. In addition, the quantum singularity emerging for the inclusion of a cosmological constant is elucidated and linked to the phenomenon of a dynamical dimensional reduction of spacetime.
42 pages; Invited review for International Journal of Modern Physics A
(asym. safe BH)

http://arxiv.org/abs/arXiv:1401.0931
Hamiltonian constraint in Euclidean LQG revisited: First hints of off-shell Closure
Alok Laddha
(Submitted on 5 Jan 2014)
We initiate the hunt for a definition of Hamiltonian constraint in Euclidean Loop Quantum Gravity (LQG) which faithfully represents quantum Dirac algebra. Borrowing key ideas from previous works on Hamiltonian constraint in LQG and several toy models, we present some evidence that there exists such a continuum Hamiltonian constraint operator which is well defined on a suitable generalization of the Lewandowski-Marolf Habitat and is anomaly free off-shell.
68 pages, 6 figures
(problems with the Hamiltonian approach finally yielding)

Although not directly QG-related, two "dark matter particle" papers are included in the poll by popular request

http://arxiv.org/abs/1402.4119
An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster
Alexey Boyarsky, Oleg Ruchayskiy, Dmytro Iakubovskyi, Jeroen Franse
(possible dark matter particle?)

http://arxiv.org/abs/1402.2301
Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters
Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall
(hint of dark matter particle)

 

[Chronos]

An refreshing assortment of interesting papers this quarter. The DM papers, while not strictly BTSM, clearly hint at physics beyond the standard model.

 

[marcus]

An refreshing assortment of interesting papers this quarter. The DM papers, while not strictly BTSM, clearly hint at physics beyond the standard model.

They definitely do! A sterile neutrino of around 7 keV.

To quote the abstract of the paper by Bulbul et al:
"We detect a weak unidentified emission line at E=(3.55-3.57)+/-0.03 keV in a stacked XMM spectrum of 73 galaxy clusters spanning a redshift range 0.01-0.35."

and

"Assuming that all dark matter is in sterile neutrinos with m_s=2E=7.1 keV, our detection in the full sample corresponds to a neutrino decay mixing angle ...=7e-11, below the previous upper limits."

 

[marcus]

The first quarter of 2014 is barely over so clearly it's early for these papers to start being cited in subsequent research. But I was curious to see if any were, so I checked. Here are those that were, most recent listed first.

Three cites:
http://arxiv.org/abs/1401.6562
Planck stars
Carlo Rovelli, Francesca Vidotto

http://arxiv.org/abs/arXiv:1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller

http://arxiv.org/abs/1401.6062
Quantization of systems with temporally varying discretization I: Evolving Hilbert spaces
Philipp A Hoehn

Two cites:
http://arxiv.org/abs/1401.7731
Quantization of systems with temporally varying discretization II: Local evolution moves
Philipp A Hoehn

http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli

http://arxiv.org/abs/1401.4452
Black holes within Asymptotic Safety
Benjamin Koch, Frank Saueressig

One cite:
http://arxiv.org/abs/1403.6396
Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
Jaume de Haro, Jaume Amorós

http://arxiv.org/abs/1402.6613
Numerical simulations of a loop quantum cosmos: robustness of the quantum bounce
Peter Diener, Brajesh Gupt, Parampreet Singh

http://arxiv.org/abs/arXiv:1402.3155
Quantum Reduced Loop Gravity: Semiclassical limit
Emanuele Alesci, Francesco Cianfrani

http://arxiv.org/abs/arXiv:1402.2084
Black Hole Entropy in Loop Quantum Gravity, Analytic Continuation, and Dual Holography
Muxin Han

 

[marcus]

Thanks to Atyy, Chronos, and JulCab for getting the poll started! There was a wide range of opinion this time, and our votes got spread over FOURTEEN of the 20 candidates. So far only two papers stand out: Planck Stars (which defines a new model of the Black Hole as delayed gamma ray burst, no information loss, and avoiding paradoxes that plague other BH models. And secondly the Chirco et al paper which DERIVES GR solely from LQG thermodynamics. There was some agreement about the interest of those two papers.

Three votes:
http://arxiv.org/abs/1401.6562
Planck stars
Carlo Rovelli, Francesca Vidotto
(delayed bounce model solves the black hole problems)

Two votes:
http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
(GR derived solely from LQG thermodynamics)

One vote:
http://arxiv.org/abs/1403.6396
Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
Jaume de Haro, Jaume Amorós
(self-explanatory)

http://arxiv.org/abs/1403.3190
A curvature operator for LQG
Emanuele Alesci, Mehdi Assanioussi, Jerzy Lewandowski
(a new LQG operator - 3D curvature, based on Regge, with good semi-classical limit.)

http://tel.archives-ouvertes.fr/docs/00/95/24/98/PDF/diss.pdf
The Chiral Structure of Loop Quantum Gravity
Wolfgang Wieland
(appropriate treatment of time and Hamiltonian in 4d spin foam, recovering EPRL amplitudes)

http://arxiv.org/abs/1402.6613
Numerical simulations of a loop quantum cosmos: robustness of the quantum bounce
Peter Diener, Brajesh Gupt, Parampreet Singh
(further numerical confirmation of the bounce and of Loop cosmology's effective equation model)

http://arxiv.org/abs/arXiv:1402.3155
Quantum Reduced Loop Gravity: Semiclassical limit
Emanuele Alesci, Francesco Cianfrani
(semiclassical limit)

http://arxiv.org/abs/arXiv:1402.2084
Black Hole Entropy in Loop Quantum Gravity, Analytic Continuation, and Dual Holography
Muxin Han
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1402.1038
A note on entanglement entropy and quantum geometry
Norbert Bodendorfer
(entropy = entanglement entropy)

http://arxiv.org/abs/arXiv:1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller
(reformulation by Dittrich group: refinement, coarse-graining, classical limit)

http://arxiv.org/abs/1401.5083
Non-Commutative Geometry, Non-Associative Geometry and the Standard Model of Particle Physics
Latham Boyle, Shane Farnsworth
(improves on Alain Connes NCG Standard Model)

http://arxiv.org/abs/arXiv:1401.0931
Hamiltonian constraint in Euclidean LQG revisited: First hints of off-shell Closure
Alok Laddha
(problems with the Hamiltonian approach finally yielding)

http://arxiv.org/abs/1402.4119
An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster
Alexey Boyarsky, Oleg Ruchayskiy, Dmytro Iakubovskyi, Jeroen Franse
(possible dark matter particle?)

http://arxiv.org/abs/1402.2301
Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters
Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall
(hint of dark matter particle)

 

[marcus]

Hi Skydive! Hi Tom Stoer! Hi David Horgan! Glad to see you registering your forecasts of what's going to turn out to be important on the QG research scene! There's a wide range of interesting topics this time and our votes are spread fairly evenly over FOURTEEN of the 20 candidates. As of now, eight papers stand out: Planck Stars (which defines a new model of the Black Hole as delayed bounce gamma ray burst, with no information loss, that avoids paradoxes plaguing other BH models.) The de Haro et al paper working out LQG compatibility with the BICEP2 polarization swirls in the CMB. Also the Chirco et al paper which DERIVES GR solely from LQG thermodynamics.
Moreover there's a very interesting NON-ASSOCIATIVE GEOMETRY approach to unifying standard particle theory with spacetime. It is by Boyle et al. I suspect Latham Boyle is someone to watch.
I should also add mention of TWO by Alesci et al, as well as Wieland's thesis, which has had some discussion in a couple of other threads. Also a paper by Alok Laddha which revisits the old problem of closure of the constraint algebra in the LQG Hamiltonian approach, and makes some progress there.

Three votes:
http://arxiv.org/abs/1401.6562
Planck stars
Carlo Rovelli, Francesca Vidotto
(delayed bounce model solves the black hole problems)

http://arxiv.org/abs/1401.5083
Non-Commutative Geometry, Non-Associative Geometry and the Standard Model of Particle Physics
Latham Boyle, Shane Farnsworth
(improves on Alain Connes NCG Standard Model)

Two votes:
http://arxiv.org/abs/1403.6396
Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
Jaume de Haro, Jaume Amorós
(LQC matter bounce is OK with swirls in the ancient light)

http://arxiv.org/abs/1403.3190
A curvature operator for LQG
Emanuele Alesci, Mehdi Assanioussi, Jerzy Lewandowski
(a new LQG operator - 3D curvature, based on Regge, with good semi-classical limit.)

http://tel.archives-ouvertes.fr/docs/00/95/24/98/PDF/diss.pdf
The Chiral Structure of Loop Quantum Gravity
Wolfgang Wieland
(suitable treatment of time and Hamiltonian in 4d spin foam, recovering EPRL amplitudes)

http://arxiv.org/abs/arXiv:1402.3155
Quantum Reduced Loop Gravity: Semiclassical limit
Emanuele Alesci, Francesco Cianfrani
(semiclassical limit)

http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
(GR derived solely from LQG thermodynamics)

http://arxiv.org/abs/arXiv:1401.0931
Hamiltonian constraint in Euclidean LQG revisited: First hints of off-shell Closure
Alok Laddha
(problems with the Hamiltonian approach finally yielding)

One vote:
http://arxiv.org/abs/1402.6613
Numerical simulations of a loop quantum cosmos: robustness of the quantum bounce
Peter Diener, Brajesh Gupt, Parampreet Singh
(further numerical confirmation of the bounce and of Loop cosmology's effective equation model)

http://arxiv.org/abs/arXiv:1402.2084
Black Hole Entropy in Loop Quantum Gravity, Analytic Continuation, and Dual Holography
Muxin Han
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1402.1038
A note on entanglement entropy and quantum geometry
Norbert Bodendorfer
(entropy = entanglement entropy)

http://arxiv.org/abs/arXiv:1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller
(reformulation by Dittrich group: refinement, coarse-graining, classical limit)

http://arxiv.org/abs/1402.4119
An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster
Alexey Boyarsky, Oleg Ruchayskiy, Dmytro Iakubovskyi, Jeroen Franse
(possible dark matter particle?)

http://arxiv.org/abs/1402.2301
Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters
Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall
(hint of dark matter particle)

 

[marcus]

Now that eight of us have responded to the poll, I'll review how the votes stack up.

Four votes:
http://arxiv.org/abs/1401.6562
Planck stars
Carlo Rovelli, Francesca Vidotto
(delayed bounce model solves the black hole problems)

Three votes:
http://arxiv.org/abs/1401.5083
Non-Commutative Geometry, Non-Associative Geometry and the Standard Model of Particle Physics
Latham Boyle, Shane Farnsworth
(improves on Alain Connes NCG Standard Model)

Two votes:
http://arxiv.org/abs/1403.6396
Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
Jaume de Haro, Jaume Amorós
(LQC matter bounce is OK with swirls in the ancient light)

http://arxiv.org/abs/1403.3190
A curvature operator for LQG
Emanuele Alesci, Mehdi Assanioussi, Jerzy Lewandowski
(a new LQG operator - 3D curvature, based on Regge, with good semi-classical limit.)

http://tel.archives-ouvertes.fr/docs/00/95/24/98/PDF/diss.pdf
The Chiral Structure of Loop Quantum Gravity
Wolfgang Wieland
(suitable treatment of time and Hamiltonian in 4d spin foam, recovering EPRL amplitudes)

http://arxiv.org/abs/arXiv:1402.3155
Quantum Reduced Loop Gravity: Semiclassical limit
Emanuele Alesci, Francesco Cianfrani
(semiclassical limit)

http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
(GR derived solely from LQG thermodynamics)

http://arxiv.org/abs/arXiv:1401.0931
Hamiltonian constraint in Euclidean LQG revisited: First hints of off-shell Closure
Alok Laddha
(problems with the Hamiltonian approach finally yielding)

http://arxiv.org/abs/1402.4119
An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster
Alexey Boyarsky, Oleg Ruchayskiy, Dmytro Iakubovskyi, Jeroen Franse
(possible dark matter particle?)

http://arxiv.org/abs/1402.2301
Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters
Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall
(hint of dark matter particle)

One vote:
http://arxiv.org/abs/1402.6613
Numerical simulations of a loop quantum cosmos: robustness of the quantum bounce
Peter Diener, Brajesh Gupt, Parampreet Singh
(further numerical confirmation of the bounce and of Loop cosmology's effective equation model)

http://arxiv.org/abs/arXiv:1402.2084
Black Hole Entropy in Loop Quantum Gravity, Analytic Continuation, and Dual Holography
Muxin Han
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1402.1038
A note on entanglement entropy and quantum geometry
Norbert Bodendorfer
(entropy = entanglement entropy)

http://arxiv.org/abs/arXiv:1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller
(reformulation by Dittrich group: refinement, coarse-graining, classical limit)

Thanks to you all-- Atyy, Chronos, David Horgan, JulCab, MTd2, Skydive, and Tom Stoer --for making your views known about the importance of these paper to future research.
As of now, ten papers stand out: Planck Stars, with 4 votes, defines a new model of the Black Hole as delayed bounce gamma ray burst, with no information loss, that avoids paradoxes plaguing other BH models.
Then with 3 votes there's a very interesting NON-ASSOCIATIVE GEOMETRY approach to unifying standard particle theory with spacetime. It is by Boyle et al. I suspect Latham Boyle is someone to watch.
With 2 votes, we have all of eight papers: the de Haro et al paper working out LQG compatibility with the BICEP2 polarization swirls in the CMB, one by Lewandowski and friends which defines a Loop gravity CURVATURE operator, Wieland's thesis, which has already been discussed here in a couple of forum, the Chirco et al paper which DERIVES GR solely from LQG thermodynamics, the one by Alesci et al developing a quantum reduced LQG for application to COSMOLOGY, and a paper by Alok Laddha which constructively revisits the old problem of closure of the constraint algebra in the LQG Hamiltonian approach. Also with two votes are papers reporting the detection of a 3.5 keV X-ray line which COULD be pointing to a "dark matter particle" such as an inert or non-interactive type of neutrino.

==============
BTW I neglected to include this LQC paper on the first quarter poll:
http://arxiv.org/abs/1402.3009
Non-singular bounce scenarios in loop quantum cosmology and the effective field description
Yi-Fu Cai, Edward Wilson-Ewing
(Submitted on 13 Feb 2014 (v1), last revised 12 Mar 2014 (this version, v2))
A non-singular bouncing cosmology is generically obtained in loop quantum cosmology due to non-perturbative quantum gravity effects. A similar picture can be achieved in standard general relativity in the presence of a scalar field with a non-standard kinetic term such that at high energy densities the field evolves into a ghost condensate and causes a non-singular bounce. During the bouncing phase, the perturbations can be stabilized by introducing a Horndeski operator. Taking the matter content to be a dust field and an ekpyrotic scalar field, we compare the dynamics in loop quantum cosmology and in a non-singular bouncing effective field model with a non-standard kinetic term at both the background and perturbative levels. We find that these two settings share many important properties, including the result that they both generate scale-invariant scalar perturbations. This shows that some quantum gravity effects of the very early universe may be mimicked by effective field models.
12 pages, 5 figures

I now think it's potentially important. Maybe I can include it in the MIP poll for second quarter 2014.

 

[marcus]

Yay! Nonlinearity just weighed in! I'm always interested to take note of his picks---though I have zero direct knowledge, I get the sense that he is a physics pro with some interest in current QG research. We are 10 respondents at this point. Here's how the votes stack up. I always learn from this--seeing a group perspective emerge and trying to understand other people's points of view on the prospects of different lines of research.

Five votes:
http://arxiv.org/abs/1401.6562
Planck stars
Carlo Rovelli, Francesca Vidotto
(delayed bounce model solves the black hole problems)

Four votes:
http://arxiv.org/abs/1401.5083
Non-Commutative Geometry, Non-Associative Geometry and the Standard Model of Particle Physics
Latham Boyle, Shane Farnsworth
(improves on Alain Connes NCG Standard Model)

Three votes:
http://arxiv.org/abs/1403.6396
Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
Jaume de Haro, Jaume Amorós
(LQC matter bounce is OK with swirls in the ancient light)

http://arxiv.org/abs/1403.3190
A curvature operator for LQG
Emanuele Alesci, Mehdi Assanioussi, Jerzy Lewandowski
(a new LQG operator - 3D curvature, based on Regge, with good semi-classical limit.)

http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
(GR derived solely from LQG thermodynamics)

Two votes:
http://tel.archives-ouvertes.fr/docs/00/95/24/98/PDF/diss.pdf
The Chiral Structure of Loop Quantum Gravity
Wolfgang Wieland
(suitable treatment of time and Hamiltonian in 4d spin foam, recovering EPRL amplitudes)

http://arxiv.org/abs/arXiv:1402.3155
Quantum Reduced Loop Gravity: Semiclassical limit
Emanuele Alesci, Francesco Cianfrani
(semiclassical limit)

http://arxiv.org/abs/arXiv:1402.2084
Black Hole Entropy in Loop Quantum Gravity, Analytic Continuation, and Dual Holography
Muxin Han
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1402.1038
A note on entanglement entropy and quantum geometry
Norbert Bodendorfer
(entropy = entanglement entropy)

http://arxiv.org/abs/arXiv:1401.0931
Hamiltonian constraint in Euclidean LQG revisited: First hints of off-shell Closure
Alok Laddha
(problems with the Hamiltonian approach finally yielding)

http://arxiv.org/abs/1402.4119
An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster
Alexey Boyarsky, Oleg Ruchayskiy, Dmytro Iakubovskyi, Jeroen Franse
(possible dark matter particle?)

http://arxiv.org/abs/1402.2301
Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters
Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall
(hint of dark matter particle)

One vote:
http://arxiv.org/abs/1402.6613
Numerical simulations of a loop quantum cosmos: robustness of the quantum bounce
Peter Diener, Brajesh Gupt, Parampreet Singh
(further numerical confirmation of the bounce and of Loop cosmology's effective equation model)

http://arxiv.org/abs/1402.4138
Near-Horizon Radiation and Self-Dual Loop Quantum Gravity
Marc Geiller, Karim Noui
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller
(reformulation by Dittrich group: refinement, coarse-graining, classical limit)

http://arxiv.org/abs/1401.4452
Black holes within Asymptotic Safety
Benjamin Koch, Frank Saueressig
(asym. safe BH)Atyy, Chronos, David Horgan, JulCab, MTd2, Nonlinearity, Skydive, Tom Stoer, Twistor--thanks to all! I do think it was a really interesting bunch of research this quarter. A lot that looks potentially quite important. Appreciate your help making a tentative evaluation!

 

[Nonlinearity]

Hi Marcus,
I have a PhD in theoretical physics which might make me a physics pro. My interest in QG came about ten years ago, especially background independent theories. Thank you for providing these quarterly polls which I too find useful for comparing my opinions be with those are all others.

 

[marcus]

Hi Marcus,
I have a PhD in theoretical physics which might make me a physics pro. My interest in QG came about ten years ago, especially background independent theories. Thank you for providing these quarterly polls which I too find useful for comparing my opinions be with those are all others.

Thanks for the encouraging comment! Now that we are nearly a third of the way thru this quarter, I should start looking over the new crop of (esp. background independent) QG research. Let me know if any of the papers that have appeared so far stand out in your view, and you feel like suggesting them for the poll.

 

[marcus]

Here's a tentative list of some April research papers that could be included in this year's second quarter poll. Time permitting I'll try to add a brief one-line motivation for why each could have importance for future research.

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli
(Submitted on 23 Apr 2014)
It is possible that black holes hide a core of Planckian density, sustained by quantum-gravitational pressure. As a black hole evaporates, the core remembers the initial mass and the final explosion occurs at macroscopic scale. We investigate possible phenomenological consequences of this idea. Under several rough assumptions, we estimate that up to several short gamma-ray bursts per day, around 10 MeV, with isotropic distribution, can be expected coming from a region of a few hundred light years around us.
5 pages, 4 figures

http://arxiv.org/abs/1404.5635
Proof of a Quantum Bousso Bound
Raphael Bousso, Horacio Casini, Zachary Fisher, Juan Maldacena
(Submitted on 22 Apr 2014)
We prove the generalized Covariant Entropy Bound, ΔS≤(A−A′)/4Gℏ, for light-sheets with initial area A and final area A′. The entropy ΔS is defined as a difference of von Neumann entropies of an arbitrary state and the vacuum, with both states restricted to the light-sheet under consideration. The proof applies to free fields, in the limit where gravitational backreaction is small. We do not assume the null energy condition. In regions where it is violated, we find that the bound is protected by the defining property of light-sheets: that their null generators are nowhere expanding.
19 pages, 3 figures

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale
(Submitted on 16 Apr 2014)
I give a brief introduction to the relation between loop quantum gravity and twistor theory, and comment on some perspectives on the problem of time.
10 pages, invited lecture to the "2nd International Conference on New Frontiers in Physics 2013" (ICNFP 2013), to be published in EPJ Web of Conferences vol. 71

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing
(Submitted on 15 Apr 2014)
We study the loop quantum cosmology of a flat Friedmann-Lemaitre-Robertson-Walker space-time with a Maxwell field. We show that many of the qualitative properties derived for the case of a massless scalar field also hold for a Maxwell field. In particular, the big-bang singularity is replaced by a quantum bounce, and the operator corresponding to the matter energy density is bounded above by the same critical energy density. We also numerically study the evolution of wave functions that are sharply peaked in the low energy regime, and derive effective equations which very closely approximate the full quantum dynamics of sharply peaked states at all times, including the near-bounce epoch.
27 pages, 6 figures

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen
(Submitted on 10 Apr 2014)
Spatially homogeneous universes can be described in (loop) quantum gravity as condensates of elementary excitations of space. Their treatment is easiest in the second-quantised group field theory formalism which allows the adaptation of techniques from the description of Bose-Einstein condensates in condensed matter physics. Dynamical equations for the states can be derived directly from the underlying quantum gravity dynamics. The analogue of the Gross-Pitaevskii equation defines an anisotropic quantum cosmology model, in which the condensate wavefunction becomes a quantum cosmology wavefunction on minisuperspace. To illustrate this general formalism, we give a mapping of the gauge-invariant geometric data for a tetrahedron to a minisuperspace of homogeneous anisotropic 3-metrics. We then study an example for which we give the resulting quantum cosmology model in the general anisotropic case and derive the general analytical solution for isotropic universes. We discuss the interpretation of these solutions and comment on the validity of the WKB approximation used in previous studies.
20 pages, 2 figures

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen
(Submitted on 7 Apr 2014)
Following earlier insights by Livine and Terno, we develop a technique for describing quantum states of the gravitational field in terms of coarse grained spin networks. We show that the number of nodes and links and the values of the spin depend on the observables chosen for the description of the state. Hence the question in the title of this paper is ill posed, unless further information about what is being measured is given.
16 pages, 9 figures

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective
T. Padmanabhan, Hamsa Padmanabhan
(Submitted on 8 Apr 2014)
Observations indicate that our universe is characterized by a late-time accelerating phase, possibly driven by a cosmological constant Λ, with the dimensionless parameter ΛL_P² ≃ 10⁻¹²², where L_P=(Gℏ/c³)^1/2 is the Planck length. In this review, we describe how the emergent gravity paradigm provides a new insight and a possible solution to the cosmological constant problem. After reviewing the necessary background material, we identify the necessary and sufficient conditions for solving the cosmological constant problem. We show that these conditions are naturally satisfied in the emergent gravity paradigm in which (i) the field equations of gravity are invariant under the addition of a constant to the matter Lagrangian and (ii) the cosmological constant appears as an integration constant in the solution. The numerical value of this integration constant can be related to another dimensionless number (called CosMIn) that counts the number of modes inside a Hubble volume that cross the Hubble radius during the radiation and the matter dominated epochs of the universe. The emergent gravity paradigm suggests that CosMIn has the numerical value 4π, which, in turn, leads to the correct, observed value of the cosmological constant. Further, the emergent gravity paradigm provides an alternative perspective on cosmology and interprets the expansion of the universe itself as a quest towards holographic equipartition. We discuss the implications of this novel and alternate description of cosmology.
48 pages; 5 figures. Invited review to appear in Int. Jour. Mod. Phys. D

http://arxiv.org/abs/1404.0634
From B Modes to Quantum Gravity and Unification of Forces
Lawrence M. Krauss, Frank Wilczek
(Submitted on 2 Apr 2014)
It is commonly anticipated that gravity is subject to the standard principles of quantum mechanics. Yet some (including Einstein) have questioned that presumption, whose empirical basis is weak. Indeed, recently Freeman Dyson has emphasized that no conventional experiment is capable of detecting individual gravitons. However, as we describe, if inflation occurred, the Universe, by acting as an ideal graviton amplifier, affords such access. It produces a classical signal, in the form of macroscopic gravitational waves, in response to spontaneous (not induced) emission of gravitons. Thus recent BICEP2 observations of polarization in the cosmic microwave background will, if confirmed, provide empirical evidence for the quantization of gravity. Their details also support quantitative ideas concerning the unification of strong, electromagnetic, and weak forces, and of all these with gravity.
4 pages

 

[MTd2]

I think this is the best paper of the year so far:

http://arxiv.org/abs/1404.2284

 

[PAllen]

I think this is the best paper of the year so far:

http://arxiv.org/abs/1404.2284

Well, I've just looked at the abstract, but that does look really nice!

 

[marcus]

We are 12 respondents, so far. Here's how the votes stack up.

Five votes:
http://arxiv.org/abs/1401.6562
Planck stars
Carlo Rovelli, Francesca Vidotto
(delayed bounce model solves the black hole problems)

http://arxiv.org/abs/1401.5083
Non-Commutative Geometry, Non-Associative Geometry and the Standard Model of Particle Physics
Latham Boyle, Shane Farnsworth
(improves on Alain Connes NCG Standard Model)

Three votes:
http://arxiv.org/abs/1403.6396
Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
Jaume de Haro, Jaume Amorós
(LQC matter bounce is OK with swirls in the ancient light)

http://arxiv.org/abs/1403.3190
A curvature operator for LQG
Emanuele Alesci, Mehdi Assanioussi, Jerzy Lewandowski
(a new LQG operator - 3D curvature, based on Regge, with good semi-classical limit.)

http://arxiv.org/abs/arXiv:1402.2084
Black Hole Entropy in Loop Quantum Gravity, Analytic Continuation, and Dual Holography
Muxin Han
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
(GR derived solely from LQG thermodynamics)


Two votes:
http://tel.archives-ouvertes.fr/docs/00/95/24/98/PDF/diss.pdf
The Chiral Structure of Loop Quantum Gravity
Wolfgang Wieland
(suitable treatment of time and Hamiltonian in 4d spin foam, recovering EPRL amplitudes)

http://arxiv.org/abs/arXiv:1402.3155
Quantum Reduced Loop Gravity: Semiclassical limit
Emanuele Alesci, Francesco Cianfrani
(semiclassical limit)

http://arxiv.org/abs/arXiv:1402.1038
A note on entanglement entropy and quantum geometry
Norbert Bodendorfer
(entropy = entanglement entropy)

http://arxiv.org/abs/arXiv:1401.0931
Hamiltonian constraint in Euclidean LQG revisited: First hints of off-shell Closure
Alok Laddha
(problems with the Hamiltonian approach finally yielding)

http://arxiv.org/abs/1402.4119
An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster
Alexey Boyarsky, Oleg Ruchayskiy, Dmytro Iakubovskyi, Jeroen Franse
(possible dark matter particle?)

http://arxiv.org/abs/1402.2301
Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters
Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall
(hint of dark matter particle)

One vote:
http://arxiv.org/abs/1402.6613
Numerical simulations of a loop quantum cosmos: robustness of the quantum bounce
Peter Diener, Brajesh Gupt, Parampreet Singh
(further numerical confirmation of the bounce and of Loop cosmology's effective equation model)

http://arxiv.org/abs/1402.4138
Near-Horizon Radiation and Self-Dual Loop Quantum Gravity
Marc Geiller, Karim Noui
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller
(reformulation by Dittrich group: refinement, coarse-graining, classical limit)

http://arxiv.org/abs/1401.4452
Black holes within Asymptotic Safety
Benjamin Koch, Frank Saueressig
(asym. safe BH)

As always, thanks to all who have responded so far: Atyy, Chronos, David Horgan, Jason_0, JulCab, MTd2, Nonlinearity, Skydive, SMF, Tom Stoer, Twistor. I'm not the only one who appreciates your help making a collective evaluation. (Check out Nonlinearity's comment at #10, a few posts back.)

 

[marcus]

Grimx, Mordred, and Tfdiego weighed in with their assessment of the first quarter's QG research. We are 15 respondents, so far. Thanks all! Here's how the votes stack up.

Six votes:
http://arxiv.org/abs/1401.6562
Planck stars
Carlo Rovelli, Francesca Vidotto
(delayed bounce model solves the black hole problems)

http://arxiv.org/abs/1401.5083
Non-Commutative Geometry, Non-Associative Geometry and the Standard Model of Particle Physics
Latham Boyle, Shane Farnsworth
(improves on Alain Connes NCG Standard Model)

Four votes:
http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
(GR derived solely from LQG thermodynamics)

Three votes:
http://arxiv.org/abs/1403.6396
Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
Jaume de Haro, Jaume Amorós
(LQC matter bounce is OK with swirls in the ancient light)

http://arxiv.org/abs/1403.3190
A curvature operator for LQG
Emanuele Alesci, Mehdi Assanioussi, Jerzy Lewandowski
(a new LQG operator - 3D curvature, based on Regge, with good semi-classical limit.)

http://tel.archives-ouvertes.fr/docs/00/95/24/98/PDF/diss.pdf
The Chiral Structure of Loop Quantum Gravity
Wolfgang Wieland
(suitable treatment of time and Hamiltonian in 4d spin foam, recovering EPRL amplitudes)

http://arxiv.org/abs/arXiv:1402.2084
Black Hole Entropy in Loop Quantum Gravity, Analytic Continuation, and Dual Holography
Muxin Han
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1402.1038
A note on entanglement entropy and quantum geometry
Norbert Bodendorfer
(entropy = entanglement entropy)

Two votes:
http://arxiv.org/abs/arXiv:1402.3155
Quantum Reduced Loop Gravity: Semiclassical limit
Emanuele Alesci, Francesco Cianfrani
(semiclassical limit)

http://arxiv.org/abs/arXiv:1401.0931
Hamiltonian constraint in Euclidean LQG revisited: First hints of off-shell Closure
Alok Laddha
(problems with the Hamiltonian approach finally yielding)

http://arxiv.org/abs/1402.4119
An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster
Alexey Boyarsky, Oleg Ruchayskiy, Dmytro Iakubovskyi, Jeroen Franse
(possible dark matter particle?)

http://arxiv.org/abs/1402.2301
Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters
Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall
(hint of dark matter particle)

One vote:
http://arxiv.org/abs/1402.6613
Numerical simulations of a loop quantum cosmos: robustness of the quantum bounce
Peter Diener, Brajesh Gupt, Parampreet Singh
(further numerical confirmation of the bounce and of Loop cosmology's effective equation model)

http://arxiv.org/abs/1402.4138
Near-Horizon Radiation and Self-Dual Loop Quantum Gravity
Marc Geiller, Karim Noui
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller
(reformulation by Dittrich group: refinement, coarse-graining, classical limit)

http://arxiv.org/abs/1401.4452
Black holes within Asymptotic Safety
Benjamin Koch, Frank Saueressig
(asym. safe BH)

We're starting to have something like a collective evaluation! Thanks to Atyy, Chronos, David Horgan, Grimx, Jason_0, JulCab, Mordred, MTd2, Nonlinearity, Skydive, SMF, Tfdiego, Tom Stoer, Twistor, for making this happen. (Check out Nonlinearity's comment at post #10.)

 

[marcus]

We have a new respondent! HCSophia, thanks for joining the poll! Muxin Han's paper on Black Hole entropy moves up to the 4-vote category. Since we are now only a little over 3 weeks from the end of the quarter, I should review some possible candidates for second quarter 2014 MIP.

http://arxiv.org/abs/1406.1486
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
Peter Diener, Brajesh Gupt, Miguel Megevand, Parampreet Singh
(Submitted on 5 Jun 2014)
In recent years, numerical simulations with Gaussian initial states have demonstrated the existence of a quantum bounce in loop quantum cosmology in various models. A key issue pertaining to the robustness of the bounce and the associated physics is to understand the quantum evolution for more general initial states which may depart significantly from Gaussianity and may have no well defined peakedness properties. The analysis of such states, including squeezed and highly non-Gaussian states, has been computationally challenging until now. In this manuscript, we overcome these challenges by using the Chimera scheme for the spatially flat, homogeneous and isotropic model sourced with a massless scalar field. We demonstrate that the quantum bounce in loop quantum cosmology occurs even for states which are highly squeezed or are non-Gaussian with multiple peaks and with little resemblance to semi-classical states. The existence of the bounce is found to be robust, and does not depend on the properties of the states. The evolution of squeezed and non-Gaussian states turns out to be qualitatively similar to the Gaussian states and satisfies strong constraints on the growth of the relative fluctuations across the bounce. We also compare the results from the effective dynamics and find that, though it captures the qualitative aspects of the evolution for squeezed and highly non-Gaussian states, it always underestimates the bounce volume. We show that various properties of the evolution, such as the energy density at the bounce, are in excellent agreement with the predictions from an exactly solvable loop quantum cosmological model for arbitrary states.
26 pages, 16 figures.

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space
Miguel Campiglia, Madhavan Varadarajan
(Submitted on 3 Jun 2014)
The Koslowski-Sahlmann (KS) representation is a generalization of the representation underlying the discrete spatial geometry of Loop Quantum Gravity (LQG), to accommodate states labelled by smooth spatial geometries. As shown recently, the KS representation supports, in addition to the action of the holonomy and flux operators, the action of operators which are the quantum counterparts of certain connection dependent functions known as "background exponentials".
Here we show that the KS representation displays the following properties which are the exact counterparts of LQG ones:
(i) the abelian ∗ algebra of SU(2) holonomies and 'U(1)' background exponentials can be completed to a C* algebra
(ii) the space of semianalytic SU(2) connections is topologically dense in the spectrum of this algebra
(iii) there exists a measure on this spectrum for which the KS Hilbert space is realized as the space of square integrable functions on the spectrum
(iv) the spectrum admits a characterization as a projective limit of finite numbers of copies of SU(2) and U(1)
(v) the algebra underlying the KS representation is constructed from cylindrical functions and their derivations in exactly the same way as the LQG (holonomy-flux) algebra except that the KS cylindrical functions depend on the holonomies and the background exponentials, this extra dependence being responsible for the differences between the KS and LQG algebras.
While these results are obtained for compact spaces, they are expected to be of use for the construction of the KS representation in the asymptotically flat case.
33 pages.

http://arxiv.org/abs/1406.0369
Viability of the matter bounce scenario in Loop Quantum Cosmology for general potentials
Jaume Haro, Jaume Amorós
(Submitted on 2 Jun 2014)
We consider the matter bounce scenario in Loop Quantum Cosmology (LQC) for physical potentials that at early times provide a nearly matter dominated Universe in the contracting phase, having a reheating mechanism in the expanding phase, i.e., being able to release the energy of the scalar field creating particles that thermalize in order to match with the hot Friedmann Universe, and finally at late times leading to the current cosmic acceleration. For these models, numerically solving the dynamical equations we have seen that the teleparallel version of LQC leads to theoretical results that fit well with current observational data. More precisely, in teleparallel LQC there is a set of solutions which leads to theoretical results that match correctly with last BICEP2 data, and there is another set whose theoretical results fit well with Planck's experimental data. On the other hand, in holonomy corrected LQC the theoretical value of the tensor/scalar ratio is smaller than in teleparallel LQC, which means that there is always a set of solutions that matches with Planck's data, but for some potentials BICEP2 experimental results disfavours holonomy corrected LQC.
29 pages, 8 figures.

http://arxiv.org/abs/1405.7056
CFT/Gravity Correspondence on the Isolated Horizon
Amit Ghosh, Daniele Pranzetti
(Submitted on 27 May 2014)
A quantum isolated horizon can be modeled by an SU(2) Chern-Simons theory on a punctured 2-sphere. We show how a local 2-dimensional conformal symmetry arises at each puncture inducing an infinite set of new observables localized at the horizon which satisfy a Kac-Moody algebra. By means of the isolated horizon boundary conditions, we represent the gravitational fluxes degrees of freedom in terms of the zero modes of the Kac-Moody algebra defined on the boundary of a punctured disk. In this way, our construction encodes a precise notion of CFT/gravity correspondence. The higher modes in the algebra represent new nongeometric charges which can be represented in terms of free matter field degrees of freedom. When computing the CFT partition function of the system, these new states induce an extra degeneracy factor, representing the density of horizon states at a given energy level, which reproduces the Bekenstein's holographic bound for an imaginary Immirzi parameter. This allows us to recover the Bekenstein-Hawking entropy formula without the large quantum gravity corrections associated with the number of punctures.
18 pages, 2 figures.

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez
(Submitted on 28 May 2014)
We analyze the relationship between entanglement (or geometric) entropy with statistical mechanical entropy of horizon degrees of freedom when described in the framework of isolated horizons in loop quantum gravity. We show that, once the relevant degrees of freedom are identified, the two notions coincide. The key ingredient linking the two notions is the structure of quantum geometry at Planck scale implied by loop quantum gravity, where correlations between the inside and outside of the black hole are mediated by eigenstates of the horizon area operator.
11 pages, 3 figures.

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak
(Submitted on 16 May 2014)
We show within the usual two-dimensional approximation that unitarity and the restoration of Minkowski vacuum correlations at the end of black hole evaporation impose unexpected constraints on its mass loss rate: before disappearing the black hole emits one or more negative energy burst, leading to a temporary increase of its mass.
8 pages; nontechnical version of http://arxiv.org/abs/1404.0602 E. Bianchi, M.Smerlak
"Entanglement entropy and negative-energy fluxes in two-dimensional space times"

http://arxiv.org/abs/1405.4967
Grin of the Cheshire cat: Entropy density of spacetime as a relic from quantum gravity
Dawood Kothawala, T. Padmanabhan
(Submitted on 20 May 2014)
There is considerable evidence to suggest that the field equations of gravity have the same status as, say, the equations describing an emergent phenomenon like elasticity. In fact, it is possible to derive the field equations from a thermodynamic variational principle in which a set of normalized vector fields are varied rather than the metric. We show that this variational principle can arise as a low energy (L_P=(Gℏ/c³)^1/2→0) relic of a plausible nonperturbative effect of quantum gravity, viz. the existence of a zero-point-length in the spacetime. Our result is nonperturbative in the following sense: If we modify the geodesic distance in a spacetime by introducing a zero-point-length, to incorporate some effects of quantum gravity, and take the limit L_P→0 of the Ricci scalar of the modified metric, we end up getting a nontrivial, leading order (L_P - independent) term. This term is identical to the expression for entropy density of spacetime used previously in the emergent gravity approach. This reconfirms the idea that the microscopic degrees of freedom of the spacetime, when properly described in the full theory, could lead to an effective description of geometry in terms of a thermodynamic variational principle. This is conceptually similar to the emergence of thermodynamics from mechanics of, say, molecules. The approach also has important implications for cosmological constant which are briefly discussed.
21+9 pages.

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura
(Submitted on 19 May 2014)
We study the Einstein gravity equations projected on a timelike surface, which represents the time evolution of what we call a gravitational screen. We show that such a screen possesses a surface tension and an internal energy, and that the Einstein equations reduce to the thermodynamic equations of a viscous bubble. We also provide a complete dictionary between gravitational and thermodynamical variables. In the non-viscous cases there are three thermodynamic equations which characterise a bubble dynamics: These are the first law, the Marangoni flow equation and the Young-Laplace equation. In all three equations the surface tension plays a central role: In the first law it appears as a work term per unit area, in the Marangoni flow its gradient drives a force, and in the Young-Laplace equation it contributes to a pressure proportional to the surface curvature. The gravity equations appear as a natural generalization of these bubble equations when the bubble itself is viscous and dynamical. In particular, it shows that the mechanism of entropy production for the viscous bubble is mapped onto the production of gravitational waves. We also review the relationship between surface tension and temperature, and discuss the usual black-hole thermodynamics from this point of view.
27 pages, 3 figures.

http://arxiv.org/abs/1405.4585
Renormalization Group Flow in CDT
J. Ambjorn, A. Goerlich, J. Jurkiewicz, A. Kreienbuehl, R. Loll
(Submitted on 19 May 2014)
We perform a first investigation of the coupling constant flow of the nonperturbative lattice model of four-dimensional quantum gravity given in terms of Causal Dynamical Triangulations (CDT). After explaining how standard concepts of lattice field theory can be adapted to the case of this background-independent theory, we define a notion of "lines of constant physics" in coupling constant space in terms of certain semiclassical properties of the dynamically generated quantum universe. Determining flow lines with the help of Monte Carlo simulations, we find that the second-order phase transition line present in this theory can be interpreted as a UV phase transition line if we allow for an anisotropic scaling of space and time.
20 pages

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 7 May 2014)
This article addresses the issue of estimating the duration in inflation in bouncing cosmology when anisotropies, inevitably playing and important role, are taken into account. It is shown that in Bianchi-I loop quantum cosmology, the higher the shear, the shorter the period of inflation. For a wide range of parameters, the probability distribution function of the duration of inflation is however peaked at values compatible with data, but not much higher. This makes the whole bounce/inflationary scenario consistent and phenomenologically appealing as all the information from the bounce might then not have been fully washed out.
7pages, 5 figures.

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli
(Submitted on 23 Apr 2014)
It is possible that black holes hide a core of Planckian density, sustained by quantum-gravitational pressure. As a black hole evaporates, the core remembers the initial mass and the final explosion occurs at macroscopic scale. We investigate possible phenomenological consequences of this idea. Under several rough assumptions, we estimate that up to several short gamma-ray bursts per day, around 10 MeV, with isotropic distribution, can be expected coming from a region of a few hundred light years around us.
5 pages, 4 figures.

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale
(Submitted on 16 Apr 2014)
I give a brief introduction to the relation between loop quantum gravity and twistor theory, and comment on some perspectives on the problem of time.
10 pages, invited lecture to the "2nd International Conference on New Frontiers in Physics 2013" (ICNFP 2013), to be published in EPJ Web of Conferences vol. 71

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing
(Submitted on 15 Apr 2014)
We study the loop quantum cosmology of a flat Friedmann-Lemaitre-Robertson-Walker space-time with a Maxwell field. We show that many of the qualitative properties derived for the case of a massless scalar field also hold for a Maxwell field. In particular, the big-bang singularity is replaced by a quantum bounce, and the operator corresponding to the matter energy density is bounded above by the same critical energy density. We also numerically study the evolution of wave functions that are sharply peaked in the low energy regime, and derive effective equations which very closely approximate the full quantum dynamics of sharply peaked states at all times, including the near-bounce epoch.
27 pages, 6 figures.

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen
(Submitted on 10 Apr 2014)
Spatially homogeneous universes can be described in (loop) quantum gravity as condensates of elementary excitations of space. Their treatment is easiest in the second-quantised group field theory formalism which allows the adaptation of techniques from the description of Bose-Einstein condensates in condensed matter physics. Dynamical equations for the states can be derived directly from the underlying quantum gravity dynamics. The analogue of the Gross-Pitaevskii equation defines an anisotropic quantum cosmology model, in which the condensate wavefunction becomes a quantum cosmology wavefunction on minisuperspace. To illustrate this general formalism, we give a mapping of the gauge-invariant geometric data for a tetrahedron to a minisuperspace of homogeneous anisotropic 3-metrics. We then study an example for which we give the resulting quantum cosmology model in the general anisotropic case and derive the general analytical solution for isotropic universes. We discuss the interpretation of these solutions and comment on the validity of the WKB approximation used in previous studies.
20 pages, 2 figures.

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective
T. Padmanabhan, Hamsa Padmanabhan
(Submitted on 8 Apr 2014)
Observations indicate that our universe is characterized by a late-time accelerating phase, possibly driven by a cosmological constant Λ, with the dimensionless parameter ΛL_P² ≃ 10⁻¹²², where L_P=(Gℏ/c³)^1/2 is the Planck length. In this review, we describe how the emergent gravity paradigm provides a new insight and a possible solution to the cosmological constant problem. After reviewing the necessary background material, we identify the necessary and sufficient conditions for solving the cosmological constant problem. We show that these conditions are naturally satisfied in the emergent gravity paradigm in which
(i) the field equations of gravity are invariant under the addition of a constant to the matter Lagrangian and
(ii) the cosmological constant appears as an integration constant in the solution.
The numerical value of this integration constant can be related to another dimensionless number (called CosMIn) that counts the number of modes inside a Hubble volume that cross the Hubble radius during the radiation and the matter dominated epochs of the universe. The emergent gravity paradigm suggests that CosMIn has the numerical value 4π, which, in turn, leads to the correct, observed value of the cosmological constant. Further, the emergent gravity paradigm provides an alternative perspective on cosmology and interprets the expansion of the universe itself as a quest towards holographic equipartition. We discuss the implications of this novel and alternate description of cosmology.
48 pages; 5 figures. Invited review to appear in Int. Jour. Mod. Phys. D

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen
(Submitted on 7 Apr 2014)
Following earlier insights by Livine and Terno, we develop a technique for describing quantum states of the gravitational field in terms of coarse grained spin networks. We show that the number of nodes and links and the values of the spin depend on the observables chosen for the description of the state. Hence the question in the title of this paper is ill posed, unless further information about what is being measured is given.
16 pages, 9 figures.

 

[marcus]

Eighteen of us have voted in the first quarter 2014 poll so far. Thanks to Atyy, Blakkas, Chronos, David Horgan, Grimx, HCSophia, Jason_0, JulCab, Mordred, MTd2, Nonlinearity, Ovidiu69, Skydive, SMF, Tfdiego, Tom Stoer, Twistor, for making this happen! Here's how our votes stack up.

Six votes:
http://arxiv.org/abs/1401.6562
Planck stars
Carlo Rovelli, Francesca Vidotto
(delayed bounce model solves the black hole problems)

http://arxiv.org/abs/1401.5083
Non-Commutative Geometry, Non-Associative Geometry and the Standard Model of Particle Physics
Latham Boyle, Shane Farnsworth
(improves on Alain Connes NCG Standard Model)

Five votes:
http://arxiv.org/abs/arXiv:1402.2084
Black Hole Entropy in Loop Quantum Gravity, Analytic Continuation, and Dual Holography
Muxin Han
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
(GR derived solely from LQG thermodynamics)

Three votes:
http://arxiv.org/abs/1403.6396
Viability of the matter bounce scenario in Loop Quantum Cosmology from BICEP2 last data
Jaume de Haro, Jaume Amorós
(LQC matter bounce is OK with swirls in the ancient light)

http://arxiv.org/abs/1403.3190
A curvature operator for LQG
Emanuele Alesci, Mehdi Assanioussi, Jerzy Lewandowski
(a new LQG operator - 3D curvature, based on Regge, with good semi-classical limit.)

http://tel.archives-ouvertes.fr/docs/00/95/24/98/PDF/diss.pdf
The Chiral Structure of Loop Quantum Gravity
Wolfgang Wieland
(suitable treatment of time and Hamiltonian in 4d spin foam, recovering EPRL amplitudes)

http://arxiv.org/abs/arXiv:1402.1038
A note on entanglement entropy and quantum geometry
Norbert Bodendorfer
(entropy = entanglement entropy)

Two votes:
http://arxiv.org/abs/arXiv:1402.3155
Quantum Reduced Loop Gravity: Semiclassical limit
Emanuele Alesci, Francesco Cianfrani
(semiclassical limit)

http://arxiv.org/abs/arXiv:1401.0931
Hamiltonian constraint in Euclidean LQG revisited: First hints of off-shell Closure
Alok Laddha
(problems with the Hamiltonian approach finally yielding)

http://arxiv.org/abs/1402.4119
An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster
Alexey Boyarsky, Oleg Ruchayskiy, Dmytro Iakubovskyi, Jeroen Franse
(possible dark matter particle?)

http://arxiv.org/abs/1402.2301
Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters
Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall
(hint of dark matter particle)

One vote:
http://arxiv.org/abs/1402.6613
Numerical simulations of a loop quantum cosmos: robustness of the quantum bounce
Peter Diener, Brajesh Gupt, Parampreet Singh
(further numerical confirmation of the bounce and of Loop cosmology's effective equation model)

http://arxiv.org/abs/1402.4138
Near-Horizon Radiation and Self-Dual Loop Quantum Gravity
Marc Geiller, Karim Noui
(Ashtekar-type, self-dual variables making a comeback)

http://arxiv.org/abs/arXiv:1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller
(reformulation by Dittrich group: refinement, coarse-graining, classical limit)

http://arxiv.org/abs/1401.6062
Quantization of systems with temporally varying discretization I: Evolving Hilbert spaces
Philipp A Hoehn
(part of Dittrich group's reformulation)

http://arxiv.org/abs/1401.4452
Black holes within Asymptotic Safety
Benjamin Koch, Frank Saueressig
(asym. safe BH)

 

[marcus]

Now a bit over 2 weeks from the end of the quarter, I'll continue reviewing possible candidates for the next MIP poll.

http://arxiv.org/abs/1406.2611
Positive energy in quantum gravity
Lee Smolin
(Submitted on 10 Jun 2014)
This paper addresses the question of whether Witten's proof of positive ADM energy for classical general relativity can be extended to give a proof of positive energy for a non-perturbative quantization of general relativity. To address this question, a set of conditions is shown to be sufficient for showing the positivity of a Hamiltonian operator corresponding to the ADM energy. One of these conditions is a particular factor ordering for the constraints of general relativity, in a representation where the states are functionals of the Ashtekar connection, and the auxiliary, Witten spinor. 
These developments are partly based on results derived with Artem Starodubtsev (unpublished notes, 2004).
14 pages.

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin
(Submitted on 10 Jun 2014)
We consider a quantum field theory on a spherically symmetric quantum space time described by loop quantum gravity. The spin network description of space time in such a theory leads to equations for the quantum field that are discrete. We show that to avoid significant violations of Lorentz invariance one needs to consider specific non-local interactions in the quantum field theory similar to those that appear in string theory. This is the first sign that loop quantum gravity places restrictions on the type of matter considered, and points to a connection with string theory physics.
7 pages. Honorable mention Gravity Research Foundation 2014.

http://arxiv.org/abs/1406.1486
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
Peter Diener, Brajesh Gupt, Miguel Megevand, Parampreet Singh
(Submitted on 5 Jun 2014)
In recent years, numerical simulations with Gaussian initial states have demonstrated the existence of a quantum bounce in loop quantum cosmology in various models. A key issue pertaining to the robustness of the bounce and the associated physics is to understand the quantum evolution for more general initial states which may depart significantly from Gaussianity and may have no well defined peakedness properties. The analysis of such states, including squeezed and highly non-Gaussian states, has been computationally challenging until now. In this manuscript, we overcome these challenges by using the Chimera scheme for the spatially flat, homogeneous and isotropic model sourced with a massless scalar field. We demonstrate that the quantum bounce in loop quantum cosmology occurs even for states which are highly squeezed or are non-Gaussian with multiple peaks and with little resemblance to semi-classical states. The existence of the bounce is found to be robust, and does not depend on the properties of the states. The evolution of squeezed and non-Gaussian states turns out to be qualitatively similar to the Gaussian states and satisfies strong constraints on the growth of the relative fluctuations across the bounce. We also compare the results from the effective dynamics and find that, though it captures the qualitative aspects of the evolution for squeezed and highly non-Gaussian states, it always underestimates the bounce volume. We show that various properties of the evolution, such as the energy density at the bounce, are in excellent agreement with the predictions from an exactly solvable loop quantum cosmological model for arbitrary states.
26 pages, 16 figures.

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space
Miguel Campiglia, Madhavan Varadarajan
(Submitted on 3 Jun 2014)
The Koslowski-Sahlmann (KS) representation is a generalization of the representation underlying the discrete spatial geometry of Loop Quantum Gravity (LQG), to accommodate states labelled by smooth spatial geometries. As shown recently, the KS representation supports, in addition to the action of the holonomy and flux operators, the action of operators which are the quantum counterparts of certain connection dependent functions known as "background exponentials".
Here we show that the KS representation displays the following properties which are the exact counterparts of LQG ones:
(i) the abelian ∗ algebra of SU(2) holonomies and 'U(1)' background exponentials can be completed to a C* algebra
(ii) the space of semianalytic SU(2) connections is topologically dense in the spectrum of this algebra
(iii) there exists a measure on this spectrum for which the KS Hilbert space is realized as the space of square integrable functions on the spectrum
(iv) the spectrum admits a characterization as a projective limit of finite numbers of copies of SU(2) and U(1)
(v) the algebra underlying the KS representation is constructed from cylindrical functions and their derivations in exactly the same way as the LQG (holonomy-flux) algebra except that the KS cylindrical functions depend on the holonomies and the background exponentials, this extra dependence being responsible for the differences between the KS and LQG algebras.
While these results are obtained for compact spaces, they are expected to be of use for the construction of the KS representation in the asymptotically flat case.
33 pages.

http://arxiv.org/abs/1406.0369
Viability of the matter bounce scenario in Loop Quantum Cosmology for general potentials
Jaume Haro, Jaume Amorós
(Submitted on 2 Jun 2014)
We consider the matter bounce scenario in Loop Quantum Cosmology (LQC) for physical potentials that at early times provide a nearly matter dominated Universe in the contracting phase, having a reheating mechanism in the expanding phase, i.e., being able to release the energy of the scalar field creating particles that thermalize in order to match with the hot Friedmann Universe, and finally at late times leading to the current cosmic acceleration. For these models, numerically solving the dynamical equations we have seen that the teleparallel version of LQC leads to theoretical results that fit well with current observational data. More precisely, in teleparallel LQC there is a set of solutions which leads to theoretical results that match correctly with last BICEP2 data, and there is another set whose theoretical results fit well with Planck's experimental data. On the other hand, in holonomy corrected LQC the theoretical value of the tensor/scalar ratio is smaller than in teleparallel LQC, which means that there is always a set of solutions that matches with Planck's data, but for some potentials BICEP2 experimental results disfavours holonomy corrected LQC.
29 pages, 8 figures.

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez
(Submitted on 28 May 2014)
We analyze the relationship between entanglement (or geometric) entropy with statistical mechanical entropy of horizon degrees of freedom when described in the framework of isolated horizons in loop quantum gravity. We show that, once the relevant degrees of freedom are identified, the two notions coincide. The key ingredient linking the two notions is the structure of quantum geometry at Planck scale implied by loop quantum gravity, where correlations between the inside and outside of the black hole are mediated by eigenstates of the horizon area operator.
11 pages, 3 figures.

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak
(Submitted on 16 May 2014)
We show within the usual two-dimensional approximation that unitarity and the restoration of Minkowski vacuum correlations at the end of black hole evaporation impose unexpected constraints on its mass loss rate: before disappearing the black hole emits one or more negative energy burst, leading to a temporary increase of its mass.
8 pages; nontechnical version of http://arxiv.org/abs/1404.0602 E. Bianchi, M.Smerlak
"Entanglement entropy and negative-energy fluxes in two-dimensional space times"

http://arxiv.org/abs/1405.4967
Grin of the Cheshire cat: Entropy density of spacetime as a relic from quantum gravity
Dawood Kothawala, T. Padmanabhan
(Submitted on 20 May 2014)
There is considerable evidence to suggest that the field equations of gravity have the same status as, say, the equations describing an emergent phenomenon like elasticity. In fact, it is possible to derive the field equations from a thermodynamic variational principle in which a set of normalized vector fields are varied rather than the metric. We show that this variational principle can arise as a low energy (L_P=(Gℏ/c³)^1/2→0) relic of a plausible nonperturbative effect of quantum gravity, viz. the existence of a zero-point-length in the spacetime. Our result is nonperturbative in the following sense: If we modify the geodesic distance in a spacetime by introducing a zero-point-length, to incorporate some effects of quantum gravity, and take the limit L_P→0 of the Ricci scalar of the modified metric, we end up getting a nontrivial, leading order (L_P - independent) term. This term is identical to the expression for entropy density of spacetime used previously in the emergent gravity approach. This reconfirms the idea that the microscopic degrees of freedom of the spacetime, when properly described in the full theory, could lead to an effective description of geometry in terms of a thermodynamic variational principle. This is conceptually similar to the emergence of thermodynamics from mechanics of, say, molecules. The approach also has important implications for cosmological constant which are briefly discussed.
21+9 pages.

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura
(Submitted on 19 May 2014)
We study the Einstein gravity equations projected on a timelike surface, which represents the time evolution of what we call a gravitational screen. We show that such a screen possesses a surface tension and an internal energy, and that the Einstein equations reduce to the thermodynamic equations of a viscous bubble. We also provide a complete dictionary between gravitational and thermodynamical variables. In the non-viscous cases there are three thermodynamic equations which characterise a bubble dynamics: These are the first law, the Marangoni flow equation and the Young-Laplace equation. In all three equations the surface tension plays a central role: In the first law it appears as a work term per unit area, in the Marangoni flow its gradient drives a force, and in the Young-Laplace equation it contributes to a pressure proportional to the surface curvature. The gravity equations appear as a natural generalization of these bubble equations when the bubble itself is viscous and dynamical. In particular, it shows that the mechanism of entropy production for the viscous bubble is mapped onto the production of gravitational waves. We also review the relationship between surface tension and temperature, and discuss the usual black-hole thermodynamics from this point of view.
27 pages, 3 figures.

http://arxiv.org/abs/1405.4585
Renormalization Group Flow in CDT
J. Ambjorn, A. Goerlich, J. Jurkiewicz, A. Kreienbuehl, R. Loll
(Submitted on 19 May 2014)
We perform a first investigation of the coupling constant flow of the nonperturbative lattice model of four-dimensional quantum gravity given in terms of Causal Dynamical Triangulations (CDT). After explaining how standard concepts of lattice field theory can be adapted to the case of this background-independent theory, we define a notion of "lines of constant physics" in coupling constant space in terms of certain semiclassical properties of the dynamically generated quantum universe. Determining flow lines with the help of Monte Carlo simulations, we find that the second-order phase transition line present in this theory can be interpreted as a UV phase transition line if we allow for an anisotropic scaling of space and time.
20 pages

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 7 May 2014)
This article addresses the issue of estimating the duration in inflation in bouncing cosmology when anisotropies, inevitably playing and important role, are taken into account. It is shown that in Bianchi-I loop quantum cosmology, the higher the shear, the shorter the period of inflation. For a wide range of parameters, the probability distribution function of the duration of inflation is however peaked at values compatible with data, but not much higher. This makes the whole bounce/inflationary scenario consistent and phenomenologically appealing as all the information from the bounce might then not have been fully washed out.
7pages, 5 figures.

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli
(Submitted on 23 Apr 2014)
It is possible that black holes hide a core of Planckian density, sustained by quantum-gravitational pressure. As a black hole evaporates, the core remembers the initial mass and the final explosion occurs at macroscopic scale. We investigate possible phenomenological consequences of this idea. Under several rough assumptions, we estimate that up to several short gamma-ray bursts per day, around 10 MeV, with isotropic distribution, can be expected coming from a region of a few hundred light years around us.
5 pages, 4 figures.

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale
(Submitted on 16 Apr 2014)
I give a brief introduction to the relation between loop quantum gravity and twistor theory, and comment on some perspectives on the problem of time.
10 pages, invited lecture to the "2nd International Conference on New Frontiers in Physics 2013" (ICNFP 2013), to be published in EPJ Web of Conferences vol. 71

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing
(Submitted on 15 Apr 2014)
We study the loop quantum cosmology of a flat Friedmann-Lemaitre-Robertson-Walker space-time with a Maxwell field. We show that many of the qualitative properties derived for the case of a massless scalar field also hold for a Maxwell field. In particular, the big-bang singularity is replaced by a quantum bounce, and the operator corresponding to the matter energy density is bounded above by the same critical energy density. We also numerically study the evolution of wave functions that are sharply peaked in the low energy regime, and derive effective equations which very closely approximate the full quantum dynamics of sharply peaked states at all times, including the near-bounce epoch.
27 pages, 6 figures.

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen
(Submitted on 10 Apr 2014)
Spatially homogeneous universes can be described in (loop) quantum gravity as condensates of elementary excitations of space. Their treatment is easiest in the second-quantised group field theory formalism which allows the adaptation of techniques from the description of Bose-Einstein condensates in condensed matter physics. Dynamical equations for the states can be derived directly from the underlying quantum gravity dynamics. The analogue of the Gross-Pitaevskii equation defines an anisotropic quantum cosmology model, in which the condensate wavefunction becomes a quantum cosmology wavefunction on minisuperspace. To illustrate this general formalism, we give a mapping of the gauge-invariant geometric data for a tetrahedron to a minisuperspace of homogeneous anisotropic 3-metrics. We then study an example for which we give the resulting quantum cosmology model in the general anisotropic case and derive the general analytical solution for isotropic universes. We discuss the interpretation of these solutions and comment on the validity of the WKB approximation used in previous studies.
20 pages, 2 figures.

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective
T. Padmanabhan, Hamsa Padmanabhan
(Submitted on 8 Apr 2014)
Observations indicate that our universe is characterized by a late-time accelerating phase, possibly driven by a cosmological constant Λ, with the dimensionless parameter ΛL_P² ≃ 10⁻¹²², where L_P=(Gℏ/c³)^1/2 is the Planck length. In this review, we describe how the emergent gravity paradigm provides a new insight and a possible solution to the cosmological constant problem. After reviewing the necessary background material, we identify the necessary and sufficient conditions for solving the cosmological constant problem. We show that these conditions are naturally satisfied in the emergent gravity paradigm in which
(i) the field equations of gravity are invariant under the addition of a constant to the matter Lagrangian and
(ii) the cosmological constant appears as an integration constant in the solution.
The numerical value of this integration constant can be related to another dimensionless number (called CosMIn) that counts the number of modes inside a Hubble volume that cross the Hubble radius during the radiation and the matter dominated epochs of the universe. The emergent gravity paradigm suggests that CosMIn has the numerical value 4π, which, in turn, leads to the correct, observed value of the cosmological constant. Further, the emergent gravity paradigm provides an alternative perspective on cosmology and interprets the expansion of the universe itself as a quest towards holographic equipartition. We discuss the implications of this novel and alternate description of cosmology.
48 pages; 5 figures. Invited review to appear in Int. Jour. Mod. Phys. D

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen
(Submitted on 7 Apr 2014)
Following earlier insights by Livine and Terno, we develop a technique for describing quantum states of the gravitational field in terms of coarse grained spin networks. We show that the number of nodes and links and the values of the spin depend on the observables chosen for the description of the state. Hence the question in the title of this paper is ill posed, unless further information about what is being measured is given.
16 pages, 9 figures.

http://arxiv.org/abs/1403.6457
Purity is not eternal at the Planck scale
Michele Arzano
(Submitted on 25 Mar 2014)
Theories with Planck-scale deformed symmetries exhibit quantum time evolution in which purity of the density matrix is not preserved. In particular we show that the non-trivial structure of momentum space of these models is reflected in a deformed action of translation generators on operators. Such action in the case of time translation generators leads to a Lindblad-like evolution equation for density matrices when expanded at leading order in the Planckian deformation parameter. This evolution equation is covariant under the deformed realization of Lorentz symmetries characterizing these models.
6 pages.

 

[julian]

I'm interested in "The Conditional Probability Interpretation of the Hamiltonian Constraint" by
Carl E Dolby.

 

[marcus]

I'm interested in "The Conditional Probability Interpretation of the Hamiltonian Constraint" by
Carl E Dolby.

I didn't know of this paper. you could start (or somebody could) a thread about it. It is from 10 years ago, I see:

http://arxiv.org/abs/gr-qc/0406034
The Conditional Probability Interpretation of the Hamiltonian Constraint
Carl E Dolby
(Submitted on 9 Jun 2004)
The Conditional Probability Interpretation (CPI), first introduced by Page and Wootters, is reviewed and refined. It is argued that in it's refined form the CPI is capable of answering various past criticisms. In particular, questions involving more than one clock time are described in detail, resolving the problems raised in Kuchar's "reduction ad absurdum''. In the case of Parametrized Particle Dynamics, conventional quantum mechanics is recovered in the ideal clock limit. When E=0 is among the continuous spectrum of the Hamiltonian, the induced inner product is used to construct the physical Hilbert space H_{\rm ph} from the generalized eigenvectors in (the topological dual of) H_{\rm aux}. This allows the CPI to be applied to these 'continuous-spectrum' cases in a more rigorous fashion than that described previously. The discrete spectrum case is also treated.
19 pages

I see that Dolby's paper has been cited 7 times:
http://inspirehep.net/search?ln=en&p=refersto:recid:652072
The eariest cite was by the 2004 paper of Gambini Porto Pullin about Realistic Clocks (and the fading of unitarity) about which we have a thread. Maybe the Dolby reference could be added to that.
The most recent cite to Dolby was the 2011 paper of Rovelli "On the structure of a background independent quantum theory: Hamilton function, transition amplitudes, classical limit and continuous limit"

 

[julian]

Hi Marcus - oops I thought that was 2014, no it is 2004 - he doesn't seem to have put anything on the pre-print for 10 years. Dolby, Rovelli, and Gambini Porto Pullin all deal with conditional prob interpretations I think. Not sure how they are all related. Bit busy at the moment to review them - but would like to get into it again at some point.

Going off at a bit of a tangent, I have been making notes on a the paper "On (Cosmological) Singularity Avoidance in Loop Quantum Gravity" by Johannes Brunnemann and Thomas Thiemann http://uk.arxiv.org/pdf/gr-qc/0505032 ... it is rather negative about the physical role of LQC (at least the earlier versions that don't try to incorporate inhomogeneous quantum excitations) and that singularity avoidance in the full theory is by a more subtle mechanism.

Thought it might be an idea to review this as there seems to be some activity in trying to bridge LQC and the full theory. And ideas of LQC being used to make physical predictions (e.g. Planck stars seem to be using bounce ideas from LQC). Something you might be interested in?

 

[julian]

I wrote something on the cond prob ideas on the wiki page for LQG in a section on the problem of time - but seems like this section has been transposed (well part of) to a new page "Problem of time". What I had written was incomplete - like I said something I meant to get back into it.

Problem of time is rather important part of any version of quantum gravity and takes different forms in different approaches so not entirely sure it should have got deleted/moved.

I hadn't even got onto the problem of quantum cosmology and that the physical states that solve the constraints of LQG represent quantum states of the entire universe and as such exclude an outside observer...

 

[marcus]

... an idea to review this as there seems to be some activity in trying to bridge LQC and the full theory. And ideas of LQC being used to make physical predictions (e.g. Planck stars seem to be using bounce ideas from LQC). Something you might be interested in?

... the problem of quantum cosmology and that the physical states that solve the constraints of LQG represent quantum states of the entire universe and as such exclude an outside observer...

I hope you do start a thread that brings up some of your thoughts on these topics and some of the papers you have been reading, particularly papers after, say, 2010. There have been developments on several fronts since then. You are right about efforts to bridge from cosmology to the full LQG theory (and also efforts at including anisotropy, perturbations, matter: anisotropy among other ways using Bianchi models, matter among other ways using "hybrid" models adding a Fock space.)

My advice would be don't wait until you have read everything to start a thread. If you are lucky in your choice of topics the thread will get a little momentum and you can gain some understanding as you go, read some more papers etc.

You are right about "the problem of time" being important. It seems to have its roots already in classical GR. And not only in GR but probably also in quantum foundations.

About whether quantum cosmology needs to be formulated free of an observer (because what does "outside" mean?) you might be interested in Rovelli's view presented in the first 10 or 15 minutes of a talk he gave at Oxford in 2013. It's on YouTube.
Google "rovelli cosmology relational".

The talk was at a conference on "Cosmology and Quantum Foundations". The title of Rovelli's talk was something like "Cosmology and quantum mechanics: a relational view". If you try googling "rovelli cosmology relational" and it doesn't come up, like me know and I'll get the link.

And yes! Planck stars! Physical predictions! You touch on quite a few topics that I find very interesting, in your posts. I can't respond articulately to them all in one post

 

[julian]

Hello Marcus

I have to admit I haven't been keeping up with recent events in LQG but still find that the ideas you like are ideas I like (at glancing). I think I have to take some credit for introducing the non-existence of time idea which you have taken up so admirably...in 2012 I started a thread with:

"Actually, I myself have had the thought for a very long time that the `now' is ALL there is to reality and the past and the becoming are illusions - a thought I have found depressing, for example what if the `moment' is when you have just got knocked out of the world cup on penalties? When I found out about the timeless nature of GR this thought reoccurred to me. It would seem, maybe I'm reading too much into the Plato reference, that this is what Julian Barbour is talking about, but in a much deeper way than me."

Well the world cup is here, let's test the theory!

 

[marcus]

Possible candidates for the next MIP poll. For compactness I've dropped the abstracts except for the one new addition to the list:

http://arxiv.org/abs/1406.3706
Our Universe from the cosmological constant
Aurelien Barrau, Linda Linsefors
(Submitted on 14 Jun 2014)
In this article, we consider a bouncing Universe, as described for example by Loop Quantum Cosmology. If the current acceleration is due to a true cosmological constant, this constant is naturally conserved through the bounce and the Universe should also be in a (contracting) de Sitter phase in the remote past. We investigate here the possibility that the de Sitter temperature in the contracting branch fills the Universe with radiation and causes the bounce and the subsequent inflation and reheating. We also consider the possibility that this gives rise to a cyclic model of the Universe and suggest some possible tests.
5 pages

http://arxiv.org/abs/1406.2611
Positive energy in quantum gravity
Lee Smolin

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1406.1486
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
Peter Diener, Brajesh Gupt, Miguel Megevand, Parampreet Singh

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space
Miguel Campiglia, Madhavan Varadarajan

http://arxiv.org/abs/1406.0369
Viability of the matter bounce scenario in Loop Quantum Cosmology for general potentials
Jaume Haro, Jaume Amorós

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak

http://arxiv.org/abs/1405.4967
Grin of the Cheshire cat: Entropy density of spacetime as a relic from quantum gravity
Dawood Kothawala, T. Padmanabhan

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura

http://arxiv.org/abs/1405.4585
Renormalization Group Flow in CDT
J. Ambjorn, A. Goerlich, J. Jurkiewicz, A. Kreienbuehl, R. Loll

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective
T. Padmanabhan, Hamsa Padmanabhan

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen

http://arxiv.org/abs/1403.6457
Purity is not eternal at the Planck scale
Michele Arzano

We now are 19 respondents to the first quarter MIP poll. Thanks Bobie! Wolfgang Wieland's remarkable thesis The Chiral Structure of Loop Quantum Gravity moves up from three votes to four.

 

[marcus]

Readying the list for the 2nd quarter poll.

http://arxiv.org/abs/1406.7304
Entanglement entropy and nonabelian gauge symmetry
William Donnelly

http://arxiv.org/abs/1406.6021
Analytic Continuation of Black Hole Entropy in Loop Quantum Gravity
Jibril Ben Achour, Amaury Mouchet, Karim Noui

http://arxiv.org/abs/1406.3706
Our Universe from the cosmological constant
Aurelien Barrau, Linda Linsefors

http://arxiv.org/abs/1406.2611
Positive energy in quantum gravity
Lee Smolin

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1406.1486
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
Peter Diener, Brajesh Gupt, Miguel Megevand, Parampreet Singh

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space
Miguel Campiglia, Madhavan Varadarajan

http://arxiv.org/abs/1406.0369
Viability of the matter bounce scenario in Loop Quantum Cosmology for general potentials
Jaume Haro, Jaume Amorós

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura

http://arxiv.org/abs/1405.4585
Renormalization Group Flow in CDT
J. Ambjorn, A. Goerlich, J. Jurkiewicz, A. Kreienbuehl, R. Loll

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective
T. Padmanabhan, Hamsa Padmanabhan

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen

http://arxiv.org/abs/1403.6457
Purity is not eternal at the Planck scale
Michele Arzano

===

Entanglement entropy and nonabelian gauge symmetry
Analytic Continuation of Black Hole Entropy in Loop Quantum Gravity
Our Universe from the cosmological constant
Positive energy in quantum gravity
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
The Koslowski-Sahlmann representation: Quantum Configuration Space
Viability of the matter bounce scenario in Loop Quantum Cosmology for general potentials
Statistical and entanglement entropy for black holes in quantum geometry
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Non-equilibrium thermodynamics of gravitational screens
Renormalization Group Flow in CDT
Exhaustive investigation of the duration of inflation in effective anisotropic LQC
Planck star phenomenology
Loop quantum gravity, twistors, and some perspectives on the problem of time
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Quantum cosmology of (loop) quantum gravity condensates: An example
Cosmological Constant from the Emergent Gravity Perspective
How many quanta are there in a quantum spacetime?
Purity is not eternal at the Planck scale