Update on Loops as of July 2013

[marcus]

A while back, Tom asked me for a report on where the Loop program stands and I didn't give a clear answer. In the meanwhile several other people have expressed interest in what the current status of LQG is. It has become a little clearer recently because abstracts of the talks being given at the Warsaw GR20 conference have been posted. I'll give a sampling of them (one can get an idea from them of which problems are being addressed and what progress has recently been made.)

The triennial GR conference is being held this year in Warsaw 7-13 July.
http://gr20-amaldi10.edu.pl/index.php?id=18
Somewhat over 600 participants, presenting talks in a number of different parallel sessions. The parallel sessions in which I notice Loop researchers presenting papers are:
A3 modified gravity
A4 complex and conformal methods
D1 LQG and spin foams
D3 caussets, cdt, ngc, and other
D4 qft on curved, semiclassical, qg pheno, analog gravity
Joint Session (D1+D2+D4) on quantum mechanics of BH evaporation

There are 36 abstracts shown in session D1, and other LQG talks not confined to D1 are scattered elsewhere, in the other sessions. I can only give a small sample here which I'll try to make representative of interesting recent developments.

From that sample (and from abstracts of talks to be given at Loops 2013) we should be able to form some impression of the current status and progress in the field.

 

[marcus]

Page numbers refer to the GR20 book of abstracts

Loop quantum gravity, twisted geometries and twistors
Speziale S
Loop quantum gravity is a background-independent approach to the quantization of general relativity. While the theory is continuous at the fundamental level, it is often useful to consider a truncation thereof, defined on the lattice dual to a graph. This truncation captures a finite number of degrees of freedom, which have been shown to describe a certain generalization of Regge geometries, called twisted geometries. In this talk, I will give a brief overview of the theory and its geometric interpretation. Then, I will describe how these discrete geometries can be described in terms of a collection of twistors associated to the lattice, thus providing a new sense in which twistors can be seen as non-linear gravitons. Finally, I will briefly discuss how dynamical transition amplitudes can be represented as integrals in twistor space.(p.55)

Complete quantization of vacuum spherically symmetric gravity
Pullin J
We find a rescaling of the Hamiltonian constraint for vacuum spherically symmetric gravity that makes the constraint algebra a true Lie algebra. We can implement the Dirac quantization procedure finding in closed form the space of physical states. New observables without classical counterpart arise. The metric can be understood as an evolving constant of the motion defined as a quantum operator on the space of physical states. For it to be self adjoint its range needs to be restricted, which in turn implies that the singularity is eliminated. One is left with a region of high curvature that tunnels into another portion of space-time. The results may have implications for the current discussion of ”firewalls” in black hole evaporation.(p.220)

On the quasilocal first law for isolated horizon and its uses in the euclidean partition function
Frodden E, Pérez A
In this talk I will discuss a new quasilocal black hole energy proposal, it is based on near horizon observers and is suitable for loop quantum gravity statistical computations. I will also present a simple application of this energy notion in the context of Euclidean partition function.(p.220)

Black hole entropy and entanglement in spinfoam gravity
Bianchi E
I report recent progress on the study of entanglement entropy in spinfoam quantum gravity, and its rela- tion with the Bekenstein-Hawking area law. Based on arXiv:1212.5183 in collaboration with R.Myers.(p.220)

Quantum isotropy and dynamical quantum symmetry reduction
Engle J
We give a diffeomorphism and gauge covariant condition equivalent to homogeneity and isotropy which can be quantized, yielding a definition of a diffeomorphism-invariant, homogeneous isotropic sector of loop quantum gravity without fixing a graph. We then specialize this condition to Bianchi I cosmologies, in which case it becomes a condition for isotropy. We show how, by quantizing and imposing this condition in Bianchi I loop quantum cosmology, one exactly recovers isotropic loop quantum cosmology, including the usual ‘improved dynamics.’ We will also discuss how this reduction sheds light on which operator ordering to use when defining operators corresponding to directional Hubble rates, expansion, and shear––quantities relevant for discussing the resolution of the initial singularity.(p.223)

Loop quantum cosmology: fundamentals and phenomenology
Ashtekar A
We will review recent advances in our understanding of conceptual as well as observational issues related to the very early universe that have come from loop quantum cosmology. We will emphasize the interplay between the theory and observations has has the potential to enrich both areas.(p.224)

Duration of inflation as a prediction of effective lqc
Linsefors L, Barrau A
Loop quantum cosmology, together with a massive scalar field, has been shown to predict a high probability of sufficiently long enough inflation to fit observations. However these predictions were derived from setting initial conditions at the bounce. In this study, we take seriously the direction of causality from past to future, and therefore set initial conditions before the bounce. The phase of the scalar field is assumed to be a random variable with a flat probability distribution. A key point of this distribution is that it is not linked to a specific point in time. Our result is independent of how long before the bounce we set the initial conditions, given reasonable assumptions. In this framework, we can show that the number of e-folds of slow-roll inflation is peaked around N=145. This is one of the first clear theoretical prediction for the duration of inflation and it is also in agreement with observations. In addition, the fraction of potential energy at the bounce, usually taken as a free unknown parameter, driving many observable effects, can also be shown to be sharply peaked. Finally, we use those results to derive an original upper limit on the Barbero-Immirzi parameter : gamma < 11, which is two orders of magnitude better than the previous limit coming from cosmology.(p.224)

Quantum reduced loop gravity
Alesci E
We present a new framework to study symmetric sectors of loop quantum gravity: mimicking the spinfoam quantization procedure the reduction is imposed weakly on the full kinematical Hilbert space of the canonical theory. As a first application we study the inhomogeneous Bianchi I model and discuss the semiclassical limit of the theory.(p.225)

Radiative corrections in covariant loop quantum gravity
Rovelli C
I explain the problem of the radiative corrections is covariant quantum gravity and summarize the present status of the research. The control of these radiative corrections is the main open issue in the theory.(p.227)

Curvature constraints in spin foam models
Hellman F, Kaminski W
I will describe surprising constraints on the internal holonomies in the asymptotic limit of current spin foam models like EPRL. Our result concerns euclidean models but indicates that similar phenomena may occur also in their physical lorentzian counterpart.(p.227)

Hamiltonian spinfoam gravity
Wieland W
The talk presents a new Hamiltonian formulation of discretised gravity, based upon the twistorial frame-work of loop quantum gravity. Within this framework, I am able to derive a continuum action adapted to a simplicial decomposition of space-time. 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. The equations of motion admit a Hamiltonian formulation, that allows to perform the constraint analysis. I do not find any secondary constraints, but only get restrictions on the Lagrange multipliers enforcing the reality conditions. With the action polynomial in the spinors, canonical quantisation is straightforward. Transition amplitudes reproduce the EPRL (Engle–Pereira–Rovelli–Livine) spinfoam model.(p.227)

The continuum limit of spin foams and spin nets
Dittrich B
Spin foam models are candidate models for quantum gravity, constructed via a quantum mechanical (not Wick rotated) path integral for discrete gravity. We aim to extract the behaviour of these models on scales large compared to the discretization scale.
To this end we employ recently introduced coarse graining techniques, known as tensor network renor- malization methods, that allow us to obtain a renormalization flow of these models. Fixed points of this flow correspond to the infinite refinement, that is continuum, limit.
These techniques are applied to dimensionally reduced models, which we coined spin net models. However important general mechanisms can be already studied for these reduced models and we will comment on these as well as on general strategies for renormalization in background independent systems.(p.240)

What happens when a freely-falling observer crosses an event horizon–semiclassically?
Smerlak M
What happens when a freely-falling observer crosses a black hole horizon? In spite of recent challenges by Almheiri, Marolf, Polchinski and Scully—the ”firewall” hypothesis—, the consensual answer to this question tends to remain ”nothing special”. In this talk, I will show that something rather special happens near the horizon, already at the semiclassical level: particle detectors record ingoing Hawking radiation at a temperature inversely proportional to their velocity relative to the horizon. To establish this result, I will (i) introduce an adiabatic expansion for Unruh-DeWitt response functions along non-stationary trajectories, and apply it to radial Schwarzschild geodesics, and (ii) compute the flux perceived by such infalling observers. I will close with a few comments on the role of spacetime curvature in this surprising effect.(p.249)

Death and resurrection of the zeroth principle of thermodynamics
Haggard H , Rovelli C
The zeroth principle of thermodynamics in the form ”temperature is uniform at equilibrium” is notoriously violated in relativistic gravity. Temperature uniformity is often derived from the maximization of the total number of microstates of two interacting systems under energy exchanges. Here we discuss a generalized version of this derivation, based on informational notions, which remains valid in the general context. The result is based on the observation that the time taken by any system to move to a distinguishable (nearly orthogonal) quantum state is a universal quantity that depends solely on the temperature. At equilibrium the net information flow between two systems must vanish, and this happens when two systems transit the same number of distinguishable states in the course of their interaction.(p.250)

Quantum space-times and unitarity of bh evaporation
Ashtekar A
There is growing evidence that, because of the singularity resolution, quantum space-times can be vastly larger than what classical general relativity would lead us to believe. We review arguments that, thanks to this enlargement, unitarity is restored in the evaporation of black holes. In contrast to ADS/CFT, these arguments deal with the evaporation process directly in the physical space-time.(p.218)


Many interesting abstracts were necessarily omitted from this small sample. To see more:
http://gr20-amaldi10.edu.pl/userfiles/book_05_07_2013.pdf

 

[qinglong.1397]

Haven't pay much attention to the development of LQG. Looks it's made some progress. I'm so glad.

 

[tom.stoer]

marcus, thanks for the update; seems that spinors/twistors become important at the fundamental level

 

[marcus]

I agree, Tom, and also something else you mentioned as a significant theme, in one of your posts a while back, is not represented in the sample here: there were talks listed by V. Husain and by K. Giesel about getting a regular Hamiltonian version of LQG by adding dust or some other reference material.
Giesel's talk was titled "Scalar material reference systems and loop quantum gravity."

I was thinking of you when I decided to start this thread, and wanting to get some help interpreting and forming an overall picture. So your comment just now was exactly the kind of thing I was hoping for.

Qinglong, glad you saw the thread!

 

[marcus]

Tom, do you have any reaction to Jorge Pullin's talk? It is obviously based on http://arxiv.org/abs/1302.5265. In the acknowledgments Gambini and Pullin thank Don Marolf and Jerzy Landowski.

==quote conclusions Gambini and Pullin February 2013==
We have performed a loop quantization of the vacuum spherically symmetric space-times. Apart from using variables adapted to spherical symmetry, we did not perform any additional gauge fixing. Through a rescaling of the Hamiltonian constraint, the constraint algebra was turned into a Lie algebra. We were able to exactly solve the constraints and find the space of physical states. We encounter that in addition to the ADM mass and its canonically conjugate momentum other Dirac observables arise in the quantum theory associated with the bulk of the space-time. The metric of the space-time can be analyzed as an operator in the physical space of state viewing it as an evolving constant of the motion, written in terms of the Dirac observables and free parameters that represent the coordinate freedom. One sees that the singularity that arises in the classical theory is eliminated and is replaced by a region of high curvature through which the space-time could be extended, yielding a global structure similar to that of the Reissner–Nordstrom space-time but without singularities, as had been anticipated in a previous treatment using the effective semi-classical theory [7].
The existence of the new quantum observables, and the associated degrees of freedom, may have some relevance for the recent discussion of “firewalls” in black hole evaporation. Almheiri et al. [12] (and earlier Braunstein et al. [13]) showed that in order to preserve the unitarity of the S matrix during black hole evaporation drastic changes in the usual picture were needed, like surrounding the black hole with a firewall. This follows from fundamental hypotheses, like the existence of a unitary S matrix that describes the evolution of the incoming pure state that forms the black hole and the outgoing Hawking radiation. From the perspective of our analysis, this hypothesis is not obvious since in principle there could be part of the information lost when falling into the black hole interior tunneling into another region or into the new local degrees of freedom we discussed. Our analysis is at the moment limited to the vacuum case. However, from the form of the Hamiltonian constraint coupled to matter one can see that the bulk observables persist in that case, suggesting that the analysis of the information issue made could be carried out explicitly in the case of an evaporating black hole.
==endquote==

 

[marcus]

Also, I think you have some interest in following the BH entropy (and immirzi parameter) issue. Here is the word as of 29 May from D. Pranzetti, one of the researchers involved. The main papers he cites are Ghosh Perez[10] and one that Frodden co-authored with them [11].

== http://arxiv.org/abs/1305.6714 , second paragraph of introduction==
Indeed, this feature of the LQG calculation has received quite some attention during the years and some proposals for its removal have appeared (see, e.g., [8, 9]). However, only recently a key observation was made: in all of the state counting techniques developed in the literature the notion of temperature was never explicitly used. This observation led to the local stationary observer description of the horizon properties introduced in [10, 11], which allowed to single out a physical notion of local horizon energy. Such an ingredient, together with the introduction by hand of the Unruh temperature for the thermal atmosphere around the horizon, provides a leading term for the state counting independent on the Barbero-Immirzi parameter and in agreement with the Bekenstein-Hawking entropy [10]
==endquote==

So the statement that in LQG you either have state-counting and BI dependence, or else if your method is BI independent then it is not state-counting...that statement could actually be false.
But I don't know for sure about that. I'm waiting for things to sort themselves out. You may have more definite information.

Reference [10] is Ghosh Perez Physical Review Letters 107 (2011)
http://arxiv.org/abs/1107.1320 (I see the paper has 32 cites so far).
errata:http://prl.aps.org/pdf/PRL/v108/i16/e169901

 

[marcus]

In post #2 I gave the abstracts of a sample of the QG talks being given in Warsaw this week at theGR20 conference. One of the most intriguing was this by Matteo Smerlak, I thought.

Page numbers refer to the GR20 book of abstracts
...
What happens when a freely-falling observer crosses an event horizon–semiclassically?
Smerlak M
What happens when a freely-falling observer crosses a black hole horizon? In spite of recent challenges by Almheiri, Marolf, Polchinski and Scully—the ”firewall” hypothesis—, the consensual answer to this question tends to remain ”nothing special”. In this talk, I will show that something rather special happens near the horizon, already at the semiclassical level: particle detectors record ingoing Hawking radiation at a temperature inversely proportional to their velocity relative to the horizon. To establish this result, I will (i) introduce an adiabatic expansion for Unruh-DeWitt response functions along non-stationary trajectories, and apply it to radial Schwarzschild geodesics, and (ii) compute the flux perceived by such infalling observers. I will close with a few comments on the role of spacetime curvature in this surprising effect.(p.249)
...
Many interesting abstracts were necessarily omitted from this small sample. To see more:
http://gr20-amaldi10.edu.pl/userfiles/book_05_07_2013.pdf

Today the paper this talk was based on appeared on arxiv.
http://arxiv.org/abs/1307.2227
The two faces of Hawking radiation
Matteo Smerlak
(Submitted on 5 Jul 2013)
What happens when Alice falls into a black hole? In spite of recent challenges by Almheiri et al. -- the ""firewall" hypothesis -- the consensus on this question tends to remain "nothing special". Here I argue that something rather special can happen near the horizon, already at the semiclassical level: besides the standard Hawking outgoing modes, Alice can record a quasi-thermal spectrum of ingoing modes, whose temperature and intensity diverges as Alice's Killing energy E goes to zero. I suggest that this effect can be thought of in terms a "horizon-infinity duality", which relates the perception of near-horizon and asymptotic geodesic observers -- the two faces of Hawking radiation.
7 pages Honorable Mention in the Gravity Research Foundation 2013 Essay Competition

 

[marcus]

In post#1 I underestimated the attendance of GR20 ("somewhat over 600") based on past experience. Actually this time the official tally of participants was 845.
http://gr20-amaldi10.edu.pl/index.php?id=11
The conference is now in progress and concludes in two days, Saturday 13 July.

It looks like part of what's happening at this year's GR conference is a challenge to the BH "firewall" idea. A special joint session (D1+D2+D4, i.e. "loops+strings+phenomenology") was organized to give critical examination of that proposal .The abstracts for that joint session on "The Quantum Mechanics of Black Hole Evaporation" start on page 218 of the abstracts PDF: http://gr20-amaldi10.edu.pl/userfiles/book_05_07_2013.pdf . They are brief, so I may as well copy them here to save others who might be interested the trouble of downloading the whole document. I also include the abstract of Jorge Pullin's talk from the regular D1 session, which bears on the "firewall" question.
==quote==
Complete quantization of vacuum spherically symmetric gravity
Pullin J
We find a rescaling of the Hamiltonian constraint for vacuum spherically symmetric gravity that makes the constraint algebra a true Lie algebra. We can implement the Dirac quantization procedure finding in closed form the space of physical states. New observables without classical counterpart arise. The metric can be understood as an evolving constant of the motion defined as a quantum operator on the space of physical states. For it to be self adjoint its range needs to be restricted, which in turn implies that the singularity is eliminated. One is left with a region of high curvature that tunnels into another portion of space-time. The results may have implications for the current discussion of ”firewalls” in black hole evaporation.

Information loss
Wald R
We review the arguments in favor of loss of information in the process of black hole formation and evaporation.

Why is the generalized second law true?
Wall A
The entropy outside of an event horizon can never decrease if one includes a term proportional to the horizon area. For a long time, this astonishing result had only been shown for quantum fields that are in an approximately steady state. I will describe a new proof of the generalized second law for arbitrary slices of semiclassical, rapidly-changing horizons. I will start with the simplest case, Rindler horizons, and then describe how the proof can be adapted to other cases (black holes, de Sitter, etc.) by restricting the field algebra to the horizon. The generalized second law holds because the horizon is invariant under a larger symmetry group than the rest of the spacetime.

Quantum space-times and unitarity of bh evaporation
Ashtekar A
There is growing evidence that, because of the singularity resolution, quantum space-times can be vastly larger than what classical general relativity would lead us to believe. We review arguments that, thanks to this enlargement, unitarity is restored in the evaporation of black holes. In contrast to ADS/CFT, these arguments deal with the evaporation process directly in the physical space-time.

Dynamical evaporation of quantum horizons
Pranzetti D
We describe the black hole evaporation process driven by the dynamical evolution of the quantum gravitational degrees of freedom resident at the horizon, as identified by the loop quantum gravity kinematics. Using a parallel with the Brownian motion, we interpret the first law of quantum dynamical horizon in terms of a fluctuation-dissipation relation applied to this fundamental discrete structure. In this way, the horizon evolution is described in terms of relaxation to an equilibrium state balanced by the excitation of Planck scale constituents of the horizon. We investigate the final stage of the evaporation process and show how the dynamics leads to the formation of a massive remnant. Implications for the information paradox are discussed.

Black hole information from the viewpoint of string theory
Horowitz G
We review the contributions that string theory has made to understanding black hole information. This includes the remarkable gauge/gravity duality and the counting of microstates of certain black holes. We also comment on more speculative ideas including fuzzballs and final state boundary conditions.

Ads/cft, unitary black hole evaporation, and firewalls
Marolf D
We review arguments that black hole evaporation is unitary in AdS/CFT. As a result, the physics expe- rienced by infalling observers at the horizon of at least sufficiently old black holes described by AdS/CFT must be dramatically different from that described by familiar field theory in a smooth spacetime.

Falling into a black hole and the information paradox in ads/cft
Papadodimas K
I will describe how the interior of a black hole can be reconstructed from the point of view of the dual gauge theory in the framework of the AdS/CFT correspondence. I will argue that the infalling observer does not notice anything special when crossing the horizon and that it is possible to resolve the information paradox without dramatic violations of effective field theory, in contrast to predictions by the recent fuzzball and firewall proposals.
==endquote==
For more information here's the list of parallel sessions:
http://gr20-amaldi10.edu.pl/index.php?id=18

 

[marcus]

... The abstracts for that joint session on "The Quantum Mechanics of Black Hole Evaporation" start on page 218 of the abstracts PDF: http://gr20-amaldi10.edu.pl/userfiles/book_05_07_2013.pdf ... I also include the abstract of Jorge Pullin's talk from the regular D1 session, [which bears on preserving unitarity---removing BH singularity without loss of information]
==quote==
Complete quantization of vacuum spherically symmetric gravity
Pullin J
...which in turn implies that the singularity is eliminated. One is left with a region of high curvature that tunnels into another portion of space-time. The results may have implications for the current discussion of ”firewalls” in black hole evaporation.
...
...
Quantum space-times and unitarity of bh evaporation
Ashtekar A
There is growing evidence that, because of the singularity resolution, quantum space-times can be vastly larger than what classical general relativity would lead us to believe. We review arguments that, thanks to this enlargement, unitarity is restored in the evaporation of black holes. In contrast to ADS/CFT, these arguments deal with the evaporation process directly in the physical space-time.
...
==endquote==
For more information here's the list of parallel sessions:
http://gr20-amaldi10.edu.pl/index.php?id=18

It does seem that the Loop gravity story of BH evaporation in quite a bit easier to swallow than the bizarre "firewall" alternative.

In the picture presented in Ashtekar's talk, the information does not have to come out during evaporation, encoded somehow in thermal Hawking radiation. Instead, the information that fell into the BH is simply shunted into a new portion of space-time---as has to happen because the singularity is resolved.

So you kill two birds with one stone: resolve the singularity, restore unitarity.