Leading lines of development in LQG, main issues, Loops 15

[marcus]

The Loops conference held every two years, or so, gives us a good window on what the active lines of development are, what the organizers consider the main issues to be addressed and who they see as leaders in the various LQG research areas.

LQG is a research community rather than a single "official" formulation of a single agreed-on theory (that would be kind of ridiculous ) and there are in the community a number of active productive lines of development. It's the business of conference organizers to identify these and get them in focus in their list of PLENARY SPEAKERS and in their schedule of PANEL DISCUSSIONS.

Plenary speakers are the ones who usually speak in the morning sessions and present their talks to the whole conference (which might be 300 people in the main hall). Then in the afternoon the organizers let people divide up into 3, possibly more, "parallel sessions" run in smaller rooms. So there is time for many more parallel talks.

So you can see how this year's Loops 2015 might provide a kind of snapshot of who the people are and what the promising developments are in the LQG community.

Fortunately the list of plenary speakers has been posted, and also the list of 4 panel discussions (one on each of four evenings M T Th F.)

It's worth studying, if you are interested in learning about LQG.

 

[marcus]

July 6-10, Erlangen Germany
http://www.gravity.physik.fau.de/events/loops15/home.shtml

The invited plenary speakers are:

• Emanuele Alesci (Warsaw)
• Benjamin Bahr (Hamburg)
• Aristide Baratin (University of Waterloo)
• Norbert Bodendorfer (University of Warsaw)
• Astrid Eichhorn (Imperial College)
• Jonathan Engle (FAU Boca Raton)
• Hal Haggard (Bard College)
• Muxin Han (FAU Erlangen)
• Anna Ijjas (Princeton University)
• Wojciech Kaminski (Warsaw)
• Alok Laddha (Chennai)
• Stefano Liberati (Trieste)
• Mercedes Martin-Benito (Radboud University Nijmegen)
• Karim Noui (LMPT Tours)
• Javier Olmedo (LSU)
• Parampreet Singh (LSU)
• Matteo Smerlak (Perimeter Institute)
• Wolfgang Wieland (IGC Penn State)
• Edward Wilson-Ewing (Albert Einstein Institute)

The invited panel speakers are:

• Abhay Ashtekar (PSU)
• Aurelien Barrau (Grenoble)
• John Barrett (Nottingham)
• Eugenio Bianchi (Penn State)
• Steve Carlip (UC Davis)
• Bianca Dittrich (PI)
• Laurent Freidel (PI)
• Stefan Hofmann (Munich)
• Jerzy Lewandowski (Warsaw)
• Tomasz Pawlowski (Warsaw/Santiago de Chile)
• Roberto Percacci (Trieste)
• Alejandro Perez (Marseille)
• Jorge Pullin (LSU)
• Carlo Rovelli (Aix-Marseille University)
• Lee Smolin (PI)
• Madhavan Varadarajan (Raman Research Institute)

Here's the list of evening panel discussion TOPICS:

Monday: The issue of the continuum limit in QG
Tuesday: Status and prospects in (Loop) Quantum Cosmology
Thursday: Quantum Black Holes
Friday: Relation between canonical and covariant LQG (Dynamics)

Here is a timetable that has additional information embedded in it.
http://www.gravity.physik.fau.de/events/loops15/program.shtml
For instance, in future when plenary speakerswill have turned in their talk titles and abstracts you will be able to hover over an icon in any of the morning sessions and have a "pop-up" appear giving speaker's names and short descriptions of the planned talks.
But for now the "pop-ups" only give the names of the speakers and say "TBA"

 

[marcus]

Looking at the line-up some things immediately come to mind. In the "invited plenary speakers" list it is remarkable that these three are all there:
Haggard, Han, Kaminski
They were all three co-authors (with Riello) of a paper that put forward a new way to incorporate the cosmological curvature constant Lambda in simplicial LQG (i.e. spinfoam path integral dynamics). The idea is that instead of FLAT simplices one can use ones that all have this small curvature Lambda built into them.
This is at once a simple idea and one with far-reaching consequences.

It has the effect of making the phase-space of GEOMETRY compact and giving a kind of quantum jump discreteness to time and transitions from one state of geometry to the next. It reinforces the idea that the cosmological curvature constant has a deep connection to time. In cosmology I am finding it actually defines a natural scale of time and distance which greatly simplifies the equations.

So I have strong associations with this work of Haggard, Han, Kaminski, Riello (the socalled "HHKR" paper) and am alerted by three of the authors being invited to give morning talks to the whole conference. What the talks will be about, I do not know. I feel that at least ONE of them must be about how to include the cosmological constant in LQG.
HHKR paper:
http://arxiv.org/abs/1412.7546
Work by CL in several ways parallel to HHKR
http://arxiv.org/abs/1501.00855
which cites it as reference [22] and points out the parallels at a number of points (twice on page 3, twice on page 17, twice more in the conclusions)
An important extension, building on HHKR, see Vidotto's Tux3 slides and the paper they are based on:
https://www.gravity.physik.fau.de/events/tux3/vidotto.pdf
http://arxiv.org/abs/1502.00278

 

[marcus]

One way to tell the Loops 15 topics from the list of speakers is to check out the slides of the talks given at the February 2015 Tux3 conference:
https://www.gravity.physik.fau.de/events/tux3/tux3.shtml

• Mehdi Assanioussi, https://www.gravity.physik.fau.de/events/tux3/assanioussi.pdf
• Norbert Bodendorfer, https://www.gravity.physik.fau.de/events/tux3/bodendorfer.pdf
• Goffredo Chirco, https://www.gravity.physik.fau.de/events/tux3/chirco.pdf
• Andrea Dapor, https://www.gravity.physik.fau.de/events/tux3/dapor.pdf
• Beatriz Elizaga de Navascues, https://www.gravity.physik.fau.de/events/tux3/elizaga.pdf
• Muxin Han, https://www.gravity.physik.fau.de/events/tux3/han.pdf
• Maximilian Hanusch, https://www.gravity.physik.fau.de/events/tux3/hanusch.pdf
• Marcin Kisielowski, https://www.gravity.physik.fau.de/events/tux3/kisielowski.pdf
• Ilkka Mäkinen, https://www.gravity.physik.fau.de/events/tux3/maekinen.pdf
• Mercedes Martin-Benito, https://www.gravity.physik.fau.de/events/tux3/martin.pdf
• Guillermo Mena Marugan, https://www.gravity.physik.fau.de/events/tux3/mena.pdf
• Tomasz Pawlowski, https://www.gravity.physik.fau.de/events/tux3/pawlowski.pdf
• Jorge Pullin, https://www.gravity.physik.fau.de/events/tux3/pullin.pdf
• Saeed Rastgoo, https://www.gravity.physik.fau.de/events/tux3/rastgoo.pdf
• Carlo Rovelli, https://www.gravity.physik.fau.de/events/tux3/rovelli.pdf
• Giuseppe Sellaroli, https://www.gravity.physik.fau.de/events/tux3/sellaroli.pdf
• Jedrzej Swiezewski, https://www.gravity.physik.fau.de/events/tux3/swiezewski.pdf
• Francesca Vidotto, https://www.gravity.physik.fau.de/events/tux3/vidotto.pdf
• Wolfgang Wieland, https://www.gravity.physik.fau.de/events/tux3/wieland.pdf
• Edward Wilson-Ewing, https://www.gravity.physik.fau.de/events/tux3/wilson.pdf
• Xiangdong Zhang, https://www.gravity.physik.fau.de/events/tux3/zhang.pdf

You can see immediately that Wolfgang Wieland and Edward Wilson-Ewing, who are both invited plenary speakers at the July Loops 15, gave talks at Tux3, and the links are to their slides. So if you click on them you stand a good chance to get a foretaste of their talks at Erlangen in July.
Wieland's work is (to the extent it succeeds) transforming LQG, and Wilson-Ewing's incorporates the Loop cosmology bounce in the standard cosmic model obtaining observable predictions
Both are important developments in LQG.
You see several other Loops 15 speakers in the Tux3 lineup: Muxin Han, Martin-Benito, Bodendorfer...
their slides are there to check out as well.

 

[julian]

But no talks by Thiemann et al? Some recent progress made by Thiemann et al encourages me that a completion of the programme of canonical quantization of LQG may not be out of reach. (If anybody could do this it would be Thiemann et al).

One of the things that caught my eye was in "Coherent states, quantum gravity and the Born-Oppenheimer approximation, III: Applications to loop quantum gravity" was: "some preliminary results on the semi-classical limit of graph-changing operators might be obtained through the use of the proposed Weyl quantisation". This was a major block to establishing that the canonical theory had the correct semi-classical limit.

 

[marcus]

Julian, good comment! Thiemann has posted major work just in the last year, over a half-dozen long papers, with Suzanne Lanery and with Alex Stottmeister. His collaborators are sure to be talking in the Parallel Sessions, and to be getting a lot of attention.
A possible consideration here is that Thomas Thiemann is the host of the conference, and there is an old custom (as I understand it I may be wrong) that the host does not showcase his own work.

 

[marcus]

I want to see what examining the structure of the conference can teach me
http://www.gravity.physik.fau.de/events/loops15/program.shtml
On Monday the four invited speakers in the morning are
Astrid Eichhorn
Alok Laddha
Wolfgang Wieland
Benjamin Bahr
Then there will be other speakers in the afternoon parallel sessions (that happens every day except Wednesday. Then in the evening there is panel discussion on continuum limit (panelists: Dittrich, Percacci, Rovelli, Varadarajan)
The recent work of Eichhorn, Laddha, Wieland, Bahr (all differing in their approach, IIRC no two are co-authors) must contain pointers to how the continuum limit issue is being addressed. I associate both Eichhorn and Percacci with Asymptotic Safety QG research. Laddha and Varadarajan share an interest in variations of LQG achieving closure of Hamiltonian constraint algebra. Bahr's work is closely allied to that of Dittrich, who has a longstanding interest in deriving the continuum limit. I'm having difficulty integrating all this and getting a focused picture. I see Wieland's recent work as original and potentially transformative.

On Tuesday morning the four are
Anna Ijjas
Parampreet Singh
Mercedes Martin-Benito
Edward Wilson-Ewing
And the panel topic is Loop quantum cosmology. Martin-Benito has been writing about the different channels of information we have flowing from the early universe, and the specific relevance to quantum cosmology. It's interesting that the recent works both of Ijjas and Wilson-Ewing involve a serious challenge to the supposition of inflation. The inflation paradigm has recently become more vulnerable as people discover other ways that known features of the early universe may have arisen. See video talks on this, and panel discussion, from the December 2014 Paris conference "Primordial Universe after Planck" http://webcast.in2p3.fr/events-primordial_universe_after_planck
Param Singh has been a central figure in LQC for at least the past 10 years, with a special interest in connecting the LQC bounce model to observations.

On Thursday morning the four speakers are
Hal Haggard
Aristide Baratin
Matteo Smerlak
Javier Olmedo
all of whom have recent work on black hole issues, different ways to resolve the information loss problem, to replace the singularity, to outline the progress of evaporation. Quantum Black Holes is the topic of the evening panel discussion ( Eugenio Bianchi, Steve Carlip, Alejandro Perez, Jorge Pullin)

On Friday the morning speakers are
Wojciech Kaminski
Muxin Han
Emanuele Alesci
with a conference summary presented by Ashtekar, Rovelli, Lewandowski
In this case I don't immediately see a connection with the evening panel topic, which is the relation between canonical and covariant LQG (Dynamics)
Emanuele Alesci has been hard at work showing the connection between Loop cosmology and the main LQG theory. I believe he has also contributed to unifying canonical LQG with the spin foam path integral version, collaborating with Lewandowski on this, so that would fit the panel theme. But I think of Han and Kaminski as currently involved in the exciting development of a way to build the cosmological curvature constant into simplicial QG by using uniformly curved simplexes rather than flat ones.

It's harder to identify a single main theme to the Wednesday morning talks. and there is no evening panel discussion that day ( a free afternoon and the conference dinner is scheduled for Wednesday). The speakers are:
Stefano Liberati
Norbert Bodendorfer
Karim Noui
Jonathan Engle.

I'll look up the recent papers and in some cases recent talks by some of these morning speakers and see if anything more comes to mind. I'd be interested in other people's reactions. What are especially interesting recent papers by any of the 19 plenary speakers, which could give us clues as to what might come up and be discussed at the conference?

 

[marcus]

Of the Monday speakers I particularly want to have a couple of Wieland abstracts for easy reference:
http://arxiv.org/abs/1301.5859
Hamiltonian spinfoam gravity
Wolfgang M. Wieland
(Submitted on 24 Jan 2013)
This paper presents a Hamiltonian formulation of spinfoam-gravity, which leads to a straight-forward canonical quantisation. To begin with, we derive a continuum action adapted to the simplicial decomposition. The equations of motion admit a Hamiltonian formulation, allowing us to perform the constraint analysis. We do not find any secondary constraints, but only get restrictions on the Lagrange multipliers enforcing the reality conditions. This comes as a surprise. In the continuum theory, the reality conditions are preserved in time, only if the torsionless condition (a secondary constraint) holds true. Studying an additional conservation law for each spinfoam vertex, we discuss the issue of torsion and argue that spinfoam gravity may indeed miss an additional constraint. Next, we canonically quantise. Transition amplitudes match the EPRL (Engle--Pereira--Rovelli--Livine) model, the only difference being the additional torsional constraint affecting the vertex amplitude.
28 pages, 2 figures

http://arxiv.org/abs/1407.0025
New action for simplicial gravity in four dimensions
Wolfgang M. Wieland
(Submitted on 30 Jun 2014)
We develop a proposal for a theory of simplicial gravity with spinors as the fundamental configuration variables. The underlying action describes a mechanical system with finitely many degrees of freedom, the system has a Hamiltonian and local gauge symmetries. We will close with some comments on the resulting quantum theory, and explain the relation to loop quantum gravity and twisted geometries. The paper appears in parallel with an article by Cortês and Smolin, who study the relevance of the model for energetic causal sets and various other approaches to quantum gravity.
26 pages, 2 figures

Also on the Monday topic, one can argue that the continuum limit issue has been already resolved and some people just didn't notice What do you want the limit to be? Well the GR equation. But the GR equation is not something to quantize, it is the thermodynamic equation of state of some microscopic geometry degrees of freedom. And it was already shown that the Loop geometric degrees of freedom have the correct behavior and collectively obey that equation of state. Air has sound waves, water has water waves, but neither the air molecules or water molecules have those things. One does not try to quantize such large-scale collective behavior, the goal is not to quantize an equation of state but to understand what underlies it.

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

So general covariant statistical mechanics is where progress has to be made. We don't yet have a diff-invariant (reparametrization invariant) stat mech or thermodynamics. The classical theories involve a pre-ordained fixed time parameter (unknown in nature and a no-no in GR).

http://arxiv.org/abs/1503.08725
Statistical mechanics of reparametrization invariant systems. Takes Three to Tango
Thibaut Josset, Goffredo Chirco, Carlo Rovelli
(Submitted on 30 Mar 2015)
It is notoriously difficult to apply statistical mechanics to generally covariant systems, because the notions of time, energy and equilibrium are seriously modified in this context. We discuss the conditions under which weaker versions of these notions can be defined, sufficient for statistical mechanics. We focus on reparametrization invariant systems without additional gauges. The key idea is to reconstruct statistical mechanics from the ergodic theorem. We find that a suitable split of the system into two non-interacting components is sufficient for generalizing statistical mechanics. While equilibrium acquires sense only when the system admits a suitable split into three weakly interacting components ---roughly: a clock and two systems among which a generalization of energy is equi-partitioned. The key property that allows the application of statistical mechanics and thermodynamics is an additivity condition of such generalized energy.
9 pages, 2 figures

So I guess that one of the speakers on Monday afternoon will be Goffredo Chirco, because the work he is involved with is related, in a somewhat surprising way, to the issue of the "continuum limit". I cannot check that because the schedule for the parallel sessions is not yet posted.

 

[atyy]

Reading too fast. I first read Wieland's title as "New simplicial action for dummies".

 

[wabbit]

one can argue that the continuum limit issue has been already resolved and some people just didn't notice What do you want the limit to be? Well the GR equation. But the GR equation is not something to quantize, it is the thermodynamic equation of state of some microscopic geometry degrees of freedom. And it was already shown that the Loop geometric degrees of freedom have the correct behavior and collectively obey that equation of state. Air has sound waves, water has water waves, but neither the air molecules or water molecules have those things. One does not try to quantize such large-scale collective behavior, the goal is not to quantize an equation of state but to understand what underlies it.

This sounds like a very strong argument to me. I must admit I have been confused for a while about this point. What would be a good reference about the emergence of GR from LQG in the thermodynamic perspective ?

Also this is naive and comes from my ignorance of how things are done usually in QM, but how else could GR possibly emerge other than as collective behaviour ? (If there is a simple non technical answer that is, anything else would be beyond my grasp)

 

[marcus]

I think a good reference (not specifically from LQG perspective) would be Jacobson's 1995 paper "The Einstein Equation of State".

 

[wabbit]

Right, I read this one (though perhaps not closely enough), the part about which I'm unclear is the proof that LQG gives the correct equation of state when going from quantum to classical limit.

 

[marcus]

I don't think that part is all wrapped up yet. It might be, it should be discussed at Loops 2015 in July. The next step in the argument is the paper I mentioned by Chirco, Haggard, Riello, Rovelli. We can look at that and see if there are any missing steps or big "ifs".

The last Loops conference was in 2013, before the CHRR paper.

 

[atyy]

Also on the Monday topic, one can argue that the continuum limit issue has been already resolved and some people just didn't notice What do you want the limit to be? Well the GR equation.

I think there are two continuum limits
(1) the "low energy" limit, which should be GR
(2) the "high energy" limit which should be a continuum theory in order to link it with the Hamiltonian formulation.

I don't think that part is all wrapped up yet. It might be, it should be discussed at Loops 2015 in July. The next step in the argument is the paper I mentioned by Chirco, Haggard, Riello, Rovelli. We can look at that and see if there are any missing steps or big "ifs".

Yes, are there missing steps and big "ifs"? Here's the CHRR paper: http://arxiv.org/abs/1401.5262.

 

[wabbit]

I don't think that part is all wrapped up yet. It might be, it should be discussed at Loops 2015 in July. The next step in the argument is the paper I mentioned by Chirco, Haggard, Riello, Rovelli. We can look at that and see if there are any missing steps or big "ifs"

Ah yes missed that, thanks !

 

[wabbit]

I think there are two continuum limits
(1) the "low energy" limit, which should be GR
(2) the "high energy" limit which should be a continuum theory in order to link it with the Hamiltonian formulation

But at high energy you wouldn't expect a continuum, more something like a disconnected phase, no ? Perhaps I am misunderstanding what you say here.[/size]

 

[marcus]

It may help to read the qualifications in the CHRR paper. In the introduction they seem to stress that Jacobson's is a local result, valid in Minkowski space, that would be in a local neighborhood of GR spacetime. I'm not sure that implies that GR is recovered globally. It may. I'm just not sure.

For whatever reason in subsequent work Chirco and Rovelli seem focused on finding how to formulate statistical mechanics and thermodynamics in a general covariant way. Maybe this is needed to complete the recovery of GR from QG?

What they show in CHRR is that QG degrees of freedom can serve Jacobson's turn. No need to postulate some unknown molecules of geometry, we have them. But is Jacobson's result fully adequate? It uses thermodynamics but thermodynamics cannot as yet be formulated in a global general covariant way.

 

[atyy]

But at high energy you wouldn't expect a continuum, more something like a disconnected phase, no ? Perhaps I am misunderstanding what you say here.[/size]

In a QG theory in general, there need not be a fundamental continuum. In LQG, if spin foams are to be connected to canonical or Hamiltonian LQG, which was their original point, then there must be a fundamental continuum because canonical or Hamiltonian LQG is formulated with a fundamental continuum.

This is, I believe, why Kaminiski, Wieland's work is interesting.

But there are people who think spin foams point towards a QG theory that is fine without connecting to canonical LQG, eg. some of the GFT people like Rivasseau and Oriti. I think Freidel, Livine, Smolin, Krasnov are all also sympathetic to this possibility.

 

[atyy]

I'm not sure this question makes sense, because I'm always a bit confused about space versus spacetime, but here it is anyway. For CHRR to work, doesn't it have to be shown that the LQG dynamics takes Hadamard states to Hadamard states?

 

[marcus]

Likewise. I'm not sure your question makes sense either, for somewhat the same reason. I don't see any place where their argument requires a "hadamard" state defined at a particular time to evolve into another "hadamard" state at a later time.
However it could depend on that and I simply fail to find the logical step.

I think it is fascinating that the CHRR authors have defined a Hadamard-like property in the QG context. they use the term "Hadamard-like" and make it clear they are exploring something that is not the hadamard property of Quantum Field Theory on curved spacetime, but something new in QG which they see as analogous. And they use " " quotes to make sure we understand it is an analog.

There is a 1995 paper by Robert Wald which is in part an instructive survey of QFT on curved spacetime, rich in intuition. I think it's not a bad introduction to the hadamard property in the previous context. I'd like to understand the analogy better. Formally there is no visible connection.
http://arxiv.org/abs/gr-qc/9509057
Quantum Field Theory in Curved Spacetime
Robert M. Wald
(Submitted on 29 Sep 1995)
We review the mathematically rigorous formulation of the quantum theory of a linear field propagating in a globally hyperbolic spacetime. This formulation is accomplished via the algebraic approach, which, in essence, simultaneously admits all states in all possible (unitarily inequivalent) Hilbert space constructions. The physically nonsingular states are restricted by the requirement that their two-point function satisfy the Hadamard condition, which insures that the ultra-violet behavior of the state be similar to that of the vacuum state in Minkowski spacetime, and that the expected stress-energy tensor in the state be finite. We briefly review the Unruh and Hawking effects from the perspective of the theoretical framework adopted here. A brief discussion also is given of several open issues and questions in quantum field theory in curved spacetime regarding the treatment of ``back-reaction", the validity of some version of the ``averaged null energy condition'', and the formulation and properties of quantum field theory in causality violating spacetimes.
18 pages. Write-up of plenary talk given at GR14

Here's a sample excerpt to suggest the flavor, it is on the whole somewhat C* and observable *-algebra A, and distributions:
==Wald page 12==
The main results of the analysis of < T_ab > are the following (see [4] for more details): (i) < T_ab >ω can be defined only for states, ω, that satisfy the Hadamard condition, which, in essence, states that the “ultra-violet” behavior of the state – as measured by the short distance behavior of the two point distribution < φ(x)φ(x′) >ω – is similar in nature to the short distance behavior of the two-point distribution for the vacuum state in Minkowski spacetime. (A precise definition of the “global Hadamard condition” can be found in [8]; its equivalence to a “local Hadamard condition” was proven in [9].) States which fail to satisfy the Hadamard condition are to be viewed as “physically singular”, in that their stress-energy is infinite (or otherwise ill defined). The Hadamard condition thus provides an important additional restriction on the class of states which otherwise would be admissible when only the fundamental observables are considered...
==endquote==

For comparison, here's an excerpt from CHRR:
==CHRR page 7==
Let us now consider a particular family of states |Φ₀⟩ such that the associated reduced density matrix takes the the form
ρ_f = e^−2πK_f .
where K_f = K⃗_f⋅ñ is the boost generator in the direction normal to the facet (the notation ρ_f = ρ_S = ρ_T indicates the symmetry of the reduced density matrix). We call these states “Hadamard states” for a reason that will be clear below.
...
To the ensemble of single facet states given by (54), the observer can effectively associate an absolute tem- perature, via the general definition
T = δE/δS = aħ/2π
because if it interacts with a large number of these, this is the temperature determining the transition probabilities between its eigenstates. This is the Unruh temperature.1 Therefore all the ingredients for Jacobson’s derivation follow (see also [27]).
==endquote==

 

[marcus]

BTW I noticed that Suzanne Lanéry, who did that interesting series of papers with Thomas Thiemann, is one those who have already registered--so one can count on presentation of some of the recent Thiemann et al work in the parallel sessions.
There are already over 130 participants listed. Gofreddo Chirco (of the CHRR) is another of those listed, who will most likely be presenting results in parallel session. Francesca Vidotto as well. I'm interested in that "compact phase space" result which comes out of incorporating the cosmological constant in simplicial QG.

 

[wabbit]

Just in passing, thanks for the pointer to that paper about recasting Jacobson's result, I can't say I fully understand the details but I found it very beautiful : ) the two-step analysis of Jacobson's result and the identification of the entropy involved with entanglement entropy seem to clarify and simplify the situation in a powerful way.

 

[marcus]

I'm still waiting to see what will be the main themes of new research reported at Loops 15.
The plenary speakers list is interesting and indicates some directions the field is going. A few abstracts have now been posted.
Plenary Session
Wednesday, 09:00 - 10:45
Pathways in Quantum Gravity Phenomenology
09:00 - 09:45, Stefano Liberati (SISSA, Trieste, Italy)
In this talk I will offer a panoramic view on the lessons and the achievements gathered so far in the quest for probing the fabric of spacetime. In particular, I will discuss possible scenarios for the mesoscopic physics between our classical world and full quantum gravity regimes. In doing so I shall focus on the perspectives to test them so to gain some guidance in the development of quantum gravity.

Higher dimensional connection dynamics and applications
10:00 - 10:45, Norbert Bodendorfer (University of Warsaw, Poland)
A reformulation of general relativity in terms of connection variables constitutes the classical basis of loop quantum gravity. In this talk, we will review the construction of a set of connection variables which generalise those of Ashtekar and Barbero, and allow to apply the quantisation techniques of loop quantum gravity also to higher-dimensional gravitational theories. Recent developments such as the computation of black hole entropy in higher dimensions and the treatment of generalised gravity theories will be discussed. Future prospects and promising lines of research will be outlined.

Plenary Session
Thursday, 11:15 - 13:00
TBA
11:15 - 12:00, Matteo Smerlak (Perimeter Institute, Canada)

Loop quantization of vacuum spacetimes with spherical symmetry
12:15 - 13:00, Javier Olmedo (Louisiana State University, USA)
In this talk I will briefly review the quantization of spherically symmetric vacuum spacetimes. Concretely, I will discuss in more detail the quantization of this midisuperspace setting within loop quantum gravity. I will adopt a redefinition of the classical constraints characterized by an algebra free of structure functions. I will then adopt the Dirac quantization approach together with a representation mimicking the one of loop quantum gravity. I will provide the solutions to the constraints, a suitable inner product, and the quantum observables of the model (some of them without classical counterpart). I will eventually discuss about the applications and extensions to other midisuperspace models.

 

[marcus]

I'll bring forward the list of plenary speakers to have it handy:
July 6-10, Erlangen Germany
http://www.gravity.physik.fau.de/events/loops15/home.shtml

The invited plenary speakers are:

• Emanuele Alesci (Warsaw)
• Benjamin Bahr (Hamburg)
• Aristide Baratin (University of Waterloo)
• Norbert Bodendorfer (University of Warsaw)
• Astrid Eichhorn (Imperial College)
• Jonathan Engle (FAU Boca Raton)
• Hal Haggard (Bard College)
• Muxin Han (FAU Erlangen)
• Anna Ijjas (Princeton University)
• Wojciech Kaminski (Warsaw)
• Alok Laddha (Chennai)
• Stefano Liberati (Trieste)
• Mercedes Martin-Benito (Radboud University Nijmegen)
• Karim Noui (LMPT Tours)
• Javier Olmedo (LSU)
• Parampreet Singh (LSU)
• Matteo Smerlak (Perimeter Institute)
• Wolfgang Wieland (IGC Penn State)
• Edward Wilson-Ewing (Albert Einstein Institute)

At the end of the day (on M, T, Th, and F) after the plenary and parallel session talks, the organizers have arranged for panel discussions on selected topics. Here are the panelists.

Monday: The issue of the continuum limit in QG
Bianca Dittrich (PI)
Roberto Percacci (Trieste)
Carlo Rovelli (Aix-Marseille University)
Madhavan Varadarajan (Raman Research Institute)

Tuesday: Status and prospects in (Loop) Quantum Cosmology
Abhay Ashtekar (PSU)
Aurelien Barrau (Grenoble)
Stefan Hofmann (Munich)
Tomasz Pawlowski (Warsaw/Santiago de Chile)

Thursday: Quantum Black Holes
Eugenio Bianchi (Penn State)
Steve Carlip (UC Davis)
Alejandro Perez (Marseille)
Jorge Pullin (LSU)

Friday: Relation between canonical and covariant LQG (Dynamics)
Jorge Pullin (LSU)
Laurent Freidel (PI)
Jerzy Lewandowski (Warsaw)
Lee Smolin (PI)

http://www.gravity.physik.fau.de/events/loops15/program.shtml

 

[julian]

You're are getting us all excited marcus, but we'll have to wait until July then? How many of the talks will be availibe on line based on what happened two years ago? I'm interested in mesoscopics not just because it might be bridge between quantum gravity and observation, but because mesoscopics is something I did in my PhD.

 

[atyy]

What is mesoscopics?

 

[marcus]

I think mesoscopic is what comes halfway between macroscopic and microscopic.

Based on Loops 2013 which put video of all the talks (plenary and parallel) online but the parallel talks were run together so less easily accessible, I don't know but I would guess Erlangen will have video online of at least the plenary talks. Maybe Erlangen will have parallel session talks online in some form or other too. And the four panel discussions. That would be excellent! The panel discussions could be really instructive.

This is where the abstracts will show up, when more are posted.
http://www.gravity.physik.fau.de/events/loops15/program.shtml
Scroll down to the bottom of each page to see the plenary talk abstracts, if they are posted.
Mon
http://www.gravity.physik.fau.de/events/loops15/program.shtml#markersid2
http://www.gravity.physik.fau.de/events/loops15/program.shtml#markersid12
Tue
http://www.gravity.physik.fau.de/events/loops15/program.shtml#markersid3
http://www.gravity.physik.fau.de/events/loops15/program.shtml#markersid13
Wed
http://www.gravity.physik.fau.de/events/loops15/program.shtml#markersid4
http://www.gravity.physik.fau.de/events/loops15/program.shtml#markersid14
Thur
http://www.gravity.physik.fau.de/events/loops15/program.shtml#markersid5
http://www.gravity.physik.fau.de/events/loops15/program.shtml#markersid15
Fri
http://www.gravity.physik.fau.de/events/loops15/program.shtml#markersid6
http://www.gravity.physik.fau.de/events/loops15/program.shtml#markersid16

 

[julian]

Roughly speaking it is half way between microscopis and macroscopis. Condensed matter theory was where it was first recognized. It was thought that systems with disorder the result of the measurement of any physical quantity of a single sample could be calculated by taking a disorder ensemble average. This approach gives a good description of the system if the system is sufficiently large; if it is macroscopic. The reason for this is if the system is sufficiently large it may be considered to be composed of a large number of sub-systems, each of which may be considered to be a realization of the system with a particular choice of disorder. These systems are said to be self-averaging. The typical sample to sample difference delta x in any physical property becomes smaller than the ensemble average value <X> as the size of the system increases. We split our sample into N smaller pieces which give additive contributions to X, and are statistically independent from each other - central limit theorem type situation.

It has been found that this "macroscopic" description can break down on surprisingly large length scales in disordered systems. This breakdown was recognized by the development of a new field of mesoscopics. This new approach gives a systematic statistical description of sample specific properties.

With a mesoscopic system self-averaging cannot be assumed. There exists a mesoscopic length if the system is of the same size as this we cannot assume any statistically independent realizations of the disorder. So there is an unpredictability of the result of a measurement of physical quantities; the physical quantities are sensitive to changes in the microscopic details such as the configuration of impurities.

You can relate this statistical description to experimental study of a particular sample. There are experiments in which it is possible by tuning external parameters, the sample can be taken through most members of the ensemble. It is possible to then produce strong fluctuations which looked random but are reproducible and sample specific.

This is what I know from condensed matter.

 

[atyy]

But what has mesoscopics got to do with QG (where you said mesoscopics might bridge QG and observation)?

 

[marcus]

But what has mesoscopics got to do with QG (where you said mesoscopics might bridge QG and observation)?

This may not be quite how Julian understands the term but seems to me that Harvey Nicolic's recent paper is all about mesoscopic QG. He uses the analogy with a crystal (which is a quantum system analogous to a molecule but order of magnitudes larger scale.
He argues that ordinary GR is about geometry when geometry is in a fluid phase. But he says that geometry can form crystals, can condense into a solid phase, where GR equation no longer describes how geometry evolves.
http://arxiv.org/abs/1505.04088
Gravitational crystal inside the black hole
H. Nikolic
(Submitted on 15 May 2015)
Crystals, as quantum objects typically much larger than their lattice spacing, are a counterexample to a frequent prejudice that quantum effects should not be pronounced at macroscopic distances. We propose that the Einstein theory of gravity only describes a fluid phase and that a phase transition of crystallization can occur under extreme conditions such as those inside the black hole. Such a crystal phase with lattice spacing of the order of the Planck length offers a natural mechanism for pronounced quantum-gravity effects at distances much larger than the Planck length. A resolution of the black-hole information paradox is proposed, according to which all information is stored in a crystal-phase remnant with size and mass much above the Planck scale.
6 pages

"We propose that the Einstein theory of gravity only describes a fluid phase and that a phase transition of crystallization can occur under extreme conditions such as those inside the black hole. Such a crystal phase with lattice spacing of the order of the Planck length offers a natural mechanism for pronounced quantum-gravity effects at distances much larger than the Planck length."

As I understand it (Julian may disagree) Harvey's paper is about nothing else than mesoscopic QG.
And he cites a half dozen other papers that also explore QG effects at significantly larger than Planck scale.
These are often papers where the authors are struggling to resolve black hole paradoxes and come to the point of conjecturing some kind of observable QG effect (e.g. Rovelli-Vidotto Planck star). Something that reveals QG but at a scale that we can observe---this could be at what we call macroscopic scale, but it could also, I think, be at some intermediate range that we don't quite like to call macro, and so call meso.
Maybe it is not so workable to try to make a strict dichotomy with nothing intermediate. Where do you draw the line?

 

[julian]

That is what I would like to find out atty. I need to look into it more, thanks marcus. They may be using a more "vague" definition of mesoscopics to what I am used to and means a regime where spacetime can be approximated classically but with small corrections of quantum gravitational origin, somehow amplified so as to be observable in this regime.

 

[marcus]

http://www.gravity.physik.fau.de/events/loops15/program.shtml
Nearly all the plenary session abstracts have now been posted, most recently at 15.28 hours central on 23 June.

Plenary Session
Monday, 08:45 - 10:45
Opening
08:45 - 09:00, Joachim Hornegger (President FAU Erlangen, Germany)

Spacetime and matter in asymptotic safety
09:00 - 09:45, Astrid Eichhorn (Imperial College, United Kingdom)
I will explain the main conceptual ideas underlying the asymptotic safety scenario in quantum gravity. I will then summarize the status of this approach, and highlight some recent developments. In particular, I will discuss why matter matters in this model of quantum gravity and how this could potentially open a window into experimental tests of quantum gravity.

Aspects of Hamiltonian constraint in Loop Quantum Gravity
10:00 - 10:45, Alok Laddha (Chennai Mathematical Institute, India)
We review some of the recent progress that has been made in constructing the dynamical equations of canonical Loop Quantum Gravity in the Euclidean sector. We start be recalling some of the well known problems which plague the dynamical aspects of canonical theory and set out a strategy aimed at resolving them. After reviewing the lessons which have been obtained in various toy models, we define a continuum quantum Hamiltonian constraint of the theory and show some intriguing evidence that this constraint generates an anomaly free Quantum constraint algebra off-shell.

Plenary Session
Monday, 11:15 - 13:00
Covariant Loop Quantum Gravity: Models and Developments
11:15 - 12:00, Wolfgang Wieland (IGC Penn State, USA)
I start with a pedagogical introduction to covariant loop quantum gravity. I select a number of recent results and discuss the open problems in the field. I present, in particular, a program to derive the spinfoam amplitudes from a classical action for discretized gravity. This action, which is a one-dimensional integral over a system of auxiliary worldlines, has the same field content as loop quantum gravity, which explains its relevance for the theory.

Continuum limit and renormalization in spin foam models
12:15 - 13:00, Benjamin Bahr (University of Hamburg, Deutschland)
Spin foam models provide a tentative proposal for the path integral of quantum gravity. In recent years, there have been very successful candidates for such models, and while their properties on a microscopic level are very promising, little is known about the contuum limit of many building blocks.
In this talk I will give an overview of background-independent renormalization techniques, which can not only provide a framework for investigating the continuum limit, but also as a method to construct the analogue of low-energy effective actions for spin foam models. I will also discuss the connection with diffeomorphism-invariance.

Plenary Session
Tuesday, 09:00 - 10:45
Early-Universe Cosmology: Issues and Opportunities
09:00 - 09:45, Anna Ijjas (Princeton University, USA)
Inflationary cosmology is commonly considered as the “standard model" of the early-universe, though, it has several open issues. Some of the problems have been known since the introduction of inflationary theory in the early 80s but some of the problems have been realized first later as we learned to better understand the theory and gained more experimental data. I will present the main problems with inflation, discuss alternative approaches and point to the opportunities the current situation gives us.

Numerical loop quantum cosmology: overview and recent results
10:00 - 10:45, Parampreet Singh (Louisiana State Universitty, USA)
In this talk, we will discuss some of the recent developments in the investigations on singularity resolution using numerical simulations in loop quantum cosmology. Thanks to the introduction of new techniques, the bounce can now be established for a wide variety of states and the reliability of the effective dynamics can be checked rigorously. These results will be discussed in isotropic and anisotropic models.

Plenary Session
Tuesday, 11:15 - 13:00
Hybrid Loop Quantum Cosmology
11:15 - 12:00, Mercedes Martin-Benito (Radboud University Nijmegen, Netherlands)
I will present an overview of the hybrid approach to quantize inhomogeneous cosmologies in the framework of loop quantum cosmology. Though I will keep the discussion as general as possible, for definiteness I will focus on two particular examples largely studied so far: linearly polarized Gowdy cosmologies with spatial three-torus topology, which is the simplest inhomogeneous cosmological model, and the flat homogeneous and isotropic model minimally coupled to a scalar field and with cosmological perturbations. After explaining the hybrid quantization of these models, I will discuss some aspects of the resulting quantum dynamics, and also comment on approximations that one can introduce to extract physical results.

Loop quantum cosmology and alternatives to inflation
12:15 - 13:00, Edward Wilson-Ewing (Albert Einstein Institute, Germany)
High precision observations of the cosmic microwave background provide strong constraints on the dynamics of the early universe and raise the hope that it may be possible to detect quantum gravity effects. In this talk, I will focus on realizations of the ekpyrotic and matter bounce scenarios in loop quantum cosmology. These are alternatives to inflation where scale-invariant perturbations are generated in a contracting background Friedmann space-time which later bounces due to loop quantum cosmology effects. I will show how it is possible to explicitly calculate the evolution of the perturbations through the non-singular bounce and explain under what conditions scale-invariance is preserved. I will also discuss how loop quantum cosmology can affect observational quantities; one such effect is a damping of the amplitude of tensor modes.

Plenary Session
Wednesday, 09:00 - 10:45
Pathways in Quantum Gravity Phenomenology
09:00 - 09:45, Stefano Liberati (SISSA, Trieste, Italy)
In this talk I will offer a panoramic view on the lessons and the achievements gathered so far in the quest for probing the fabric of spacetime. In particular, I will discuss possible scenarios for the mesoscopic physics between our classical world and full quantum gravity regimes. In doing so I shall focus on the perspectives to test them so to gain some guidance in the development of quantum gravity.

Higher dimensional connection dynamics and applications
10:00 - 10:45, Norbert Bodendorfer (University of Warsaw, Poland)
A reformulation of general relativity in terms of connection variables constitutes the classical basis of loop quantum gravity. In this talk, we will review the construction of a set of connection variables which generalise those of Ashtekar and Barbero, and allow to apply the quantisation techniques of loop quantum gravity also to higher-dimensional gravitational theories. Recent developments such as the computation of black hole entropy in higher dimensions and the treatment of generalised gravity theories will be discussed. Future prospects and promising lines of research will be outlined.

Plenary Session
Wednesday, 11:15 - 13:00
Getting rid of the Barbero-Immirzi parameter?
12:15 - 13:00, Karim Noui (LMPT Tours, France)
The Barbero-Immirzi parameter plays a rather intriguing role in loop quantum gravity. At the classical level, it has no physical relevance whereas it plays a crucial role in the quantum theory (quantum geometry, black hole entropy, spin foams, etc). In a recent series of articles, we have shown that the right value of this parameter could be the complex number +/- i. We will review how this result appears in the contexts of black holes and three dimensional gravity. We will also discuss some consequences of this observation in the full theory.

Isolated and Dynamical Horizons: Overview and recent results
12:15 - 13:00, Jonathan Engle (Florida Atlantic University, USA)
The notion of isolated horizon encodes just that `minimum' of the properties of a stationary black hole necessary to ensure that the laws of black hole mechanics hold. It is quasi-local, and makes possible a well-defined canonical framework in which energy and angular momentum of a black hole can be defined. Dynamical horizons generalize this notion to the non-stationary case. I review the basic ideas of this framework, and discuss its applications, from the statistical mechanics of quantum black holes to numerical relativity. Both classic and more recent results and developments will be reviewed.

Plenary Session
Thursday, 09:00 - 10:45
A New Decay Mode for Black Holes
09:00 - 09:45, Hal Haggard (Bard College, USA)
The recent discovery of a metric satisfying the Einstein equations outside a finite spacetime region where matter collapses into a black hole and then emerges from a white hole has ignited interest in an alternative mode for black hole decay: through quantum tunneling of its geometry a black hole might turn into a white hole. I will discuss the current status, weaknesses and strengths, of this proposal and review potential phenomenological possibilities for the observation of decaying primordial black holes.

Group field theory for (loop) quantum gravity
10:00 - 10:45, Aristide Baratin (University of Waterloo, Canada)
Group field theory is a higher dimensional analogue of matrix models, which gives a prescription for summing over triangulations in spin foam models. I will give an overview of the framework and discuss some of the (numerous) recent developments in the field.

Plenary Session
Thursday, 11:15 - 13:00
On black hole design
11:15 - 12:00, Matteo Smerlak (Perimeter Institute, Canada)
Black hole design is the art of constructing black hole spacetimes from scratch, i.e. without solving the Einstein equations. This endeavor is motivated by Penrose's realization that classical black hole generically have singularities, and by the belief that these singularities must be resolved by quantum effects. I will share my perspective on the black hole design process, its upshots, and its pitfalls.

Loop quantization of vacuum spacetimes with spherical symmetry
12:15 - 13:00, Javier Olmedo (Louisiana State University, USA)
In this talk I will briefly review the quantization of spherically symmetric vacuum spacetimes. Concretely, I will discuss in more detail the quantization of this midisuperspace setting within loop quantum gravity. I will adopt a redefinition of the classical constraints characterized by an algebra free of structure functions. I will then adopt the Dirac quantization approach together with a representation mimicking the one of loop quantum gravity. I will provide the solutions to the constraints, a suitable inner product, and the quantum observables of the model (some of them without classical counterpart). I will eventually discuss about the applications and extensions to other midisuperspace models.

Plenary Session
Friday, 09:00 - 10:45
Gravity in the radial gauge
09:00 - 09:45, Wojciech Kaminski (University of Warsaw, Poland)
A prominent feature of General Relativity is the presence of diffeomorphism group as gauge transformations. As a result, evolution of the metric tensor in a given space point is not a well defined notion unless coordinate system is uniquely specified. The physically motivated choices are certain versions of normal or Fermi coordinates, which are determined by a single observer moving in space-time. The particularly simple form of the metric in these radial coordinate systems allows to eliminate spurious degrees of freedom. This geometrical meaning attracted a lot of attention. I will give a sketch of recent works (of N. Bodendorfer, P. Duch, J. Lewandowski, J. Swiezewski) on the hamiltonian formulation in terms of the radial gauge. The aim of the construction is to define relational observables.

TBA
10:00 - 10:45, Muxin Han (FAU Erlangen, Germany)

Plenary Session
Friday, 11:15 - 13:00
Quantum Reduced Loop Gravity
11:15 - 12:00, Emanuele Alesci (Warsaw University, Poland)
We present Quantum Reduced Loop Gravity a gauge fixed version of LQG. We show how QRLG provides a promising framework for a consistent characterization of the early Universe.
The effective semiclassical dynamics, the differences with Loop Quantum Cosmology and the inclusion of a scalar field will be discussed.

Summary
12:00 - 13:00, Abhay Ashtekar, Carlo Rovelli, Jerzy Lewandowski

 

[marcus]

http://www.gravity.physik.fau.de/events/loops15/program.shtml
Of the Loops 15 abstracts posted so far the most interesting, as i see it, are the Monday, Tuesday and Thursday morning talks by Wieland, Wilson-Ewing, and Haggard. I'm also looking forward to seeing the abstract of the Friday talk by Muxin Han.

Here's the abstract for Wieland's talk:
Covariant Loop Quantum Gravity: Models and Developments
Monday 11:15 - 12:00, Wolfgang Wieland (IGC Penn State, USA)
I start with a pedagogical introduction to covariant loop quantum gravity. I select a number of recent results and discuss the open problems in the field. I present, in particular, a program to derive the spinfoam amplitudes from a classical action for discretized gravity. This action, which is a one-dimensional integral over a system of auxiliary worldlines, has the same field content as loop quantum gravity, which explains its relevance for the theory.

Here are some Wieland papers that can help give us clues about his talk:
http://arxiv.org/abs/1402.6708
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. http://inspirehep.net/record/1282887?ln=en

I think he has been working for part of the past year on generalizing this from 3d to 4d. This could be what the talk is about.

http://arxiv.org/abs/1407.0025
A new action for simplicial gravity in four dimensions
Wolfgang M. Wieland
(Submitted on 30 Jun 2014)
We develop a proposal for a theory of simplicial gravity with spinors as the fundamental configuration variables. The underlying action describes a mechanical system with finitely many degrees of freedom, the system has a Hamiltonian and local gauge symmetries. We will close with some comments on the resulting quantum theory, and explain the relation to loop quantum gravity and twisted geometries. The paper appears in parallel with an article by Cortês and Smolin, who study the relevance of the model for energetic causal sets and various other approaches to quantum gravity.
26 pages, 2 figures. http://inspirehep.net/record/1304277?ln=en

Here's the abstract for the Tuesday talk by Wilson-Ewing:
Loop quantum cosmology and alternatives to inflation
Tuesday 12:15 - 13:00, Edward Wilson-Ewing (Albert Einstein Institute, Germany)
High precision observations of the cosmic microwave background provide strong constraints on the dynamics of the early universe and raise the hope that it may be possible to detect quantum gravity effects. In this talk, I will focus on realizations of the ekpyrotic and matter bounce scenarios in loop quantum cosmology. These are alternatives to inflation where scale-invariant perturbations are generated in a contracting background Friedmann space-time which later bounces due to loop quantum cosmology effects. I will show how it is possible to explicitly calculate the evolution of the perturbations through the non-singular bounce and explain under what conditions scale-invariance is preserved. I will also discuss how loop quantum cosmology can affect observational quantities; one such effect is a damping of the amplitude of tensor modes.

Wilson-Ewing's talk is especially interesting because inflationology has come under increasing criticism for its ability to "predict" contradictory observations. It's nice that we're seeing alternative ways of explaining the conditions that arose in the early universe so that inflation no longer need be considered "only game in town". (see also http://www.nature.com/news/scientific-method-defend-the-integrity-of-physics-1.16535 and quotes from recent Princeton conference in http://www.math.columbia.edu/~woit/wordpress/?p=7812 ) Here's a paper that relates to the topic of his talk:
http://arxiv.org/abs/1412.2914
A ΛCDM bounce scenario
Yi-Fu Cai, Edward Wilson-Ewing
(Submitted on 9 Dec 2014)
We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Importantly, as this scenario predicts a positive running of the scalar index, observations can potentially differentiate between it and inflationary models. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.
14 pages, 3 figures. http://inspirehep.net/record/1333367?ln=en

Here are the abstracts for Haggard's talk and a related paper:
A New Decay Mode for Black Holes
Thursday 09:00 - 09:45, Hal Haggard (Bard College, USA)
The recent discovery of a metric satisfying the Einstein equations outside a finite spacetime region where matter collapses into a black hole and then emerges from a white hole has ignited interest in an alternative mode for black hole decay: through quantum tunneling of its geometry a black hole might turn into a white hole. I will discuss the current status, weaknesses and strengths, of this proposal and review potential phenomenological possibilities for the observation of decaying primordial black holes.

http://arxiv.org/abs/1407.0989
Black hole fireworks: quantum-gravity effects outside the horizon spark black to white hole tunneling
Hal M. Haggard, Carlo Rovelli
(Submitted on 3 Jul 2014 (v1), last revised 6 Jul 2014 (this version, v2))
We show that there is a classical metric satisfying the Einstein equations outside a finite spacetime region where matter collapses into a black hole and then emerges from a white hole. We compute this metric explicitly. We show how quantum theory determines the (long) time for the process to happen. A black hole can thus quantum-tunnel into a white hole. For this to happen, quantum gravity should affect the metric also in a small region outside the horizon: we show that contrary to what is commonly assumed, this is not forbidden by causality or by the semiclassical approximation, because quantum effects can pile up over a long time. This scenario alters radically the discussion on the black hole information puzzle.
10 pages, 5 figures http://inspirehep.net/record/1304671?ln=en

The title and abstract for Muxin Han's talk have not been posted. My guess is that his talk will be based on these two papers:

http://arxiv.org/abs/1506.03053
Encoding Curved Tetrahedra in Face Holonomies: a Phase Space of Shapes from Group-Valued Moment Maps
http://arxiv.org/abs/1412.7546
SL(2,C) Chern-Simons Theory, a non-Planar Graph Operator, and 4D Loop Quantum Gravity with a Cosmological Constant: Semiclassical Geometry

 

[Berlin]

I wonder what Wieland is doing! Mind you, he has not published in almost a full year! By the way, also very curious about the 'physics implications' paper of Connes et al ! Compare this to Smolin who is publishing all the time, not really sure about the physics progress though, and promising papers who often do not appear (cortes 'program', general dsr, ..). I think it would be good ethics if the physics community forbids referring to future papers. Just a Friday evening feeling.

Berlin

 

[marcus]

I wonder what Wieland is doing! Mind you, he has not published in almost a full year! By the way, also very curious about the 'physics implications' paper of Connes et al ! Compare this to Smolin who is publishing all the time, not really sure about the physics progress though, and promising papers who often do not appear (cortes 'program', general dsr, ..). I think it would be good ethics if the physics community forbids referring to future papers. Just a Friday evening feeling.

Berlin

How far is it from Berlin to Erlangen?

Any chance you could get down there during the conference? You'd ask the Loopsters some good questions!
Wieland is just a postdoc (and I understand a very nice person). Why wouldn't he take the time to explain to you what he is doing?

 

[Berlin]

Hi Marcus,

I am not from Berlin! Look at my picture! Don't you recognize the bold man with the moustache, from the first Solvay conference? No, not the other bold one from Berlin, who worked so hard in October 1900, chasing the quantum. On the buste the text says 'Door meten tot weten', and it stands in the city
where he found out that liquids are more fun,
where Einstein met his father in physics,
where the cosmos got its saddle point,
where the electron got its spin and
where we had the best physics teacher before Feynman.

berlin :)

 

[marcus]

"Through measurement to knowledge"
Heike Kamerlingh Onnes - Leyden

 

[marcus]

http://www.gravity.physik.fau.de/events/loops15/program.shtml
All the plenary session abstracts have now been posted, most recently at 22:35 hours central on 24 June.
For the latest, scroll down to the abstract of Muxin Han's Friday 10AM talk.

Plenary Session
Monday, 08:45 - 10:45
Opening
08:45 - 09:00, Joachim Hornegger (President FAU Erlangen, Germany)

Spacetime and matter in asymptotic safety
09:00 - 09:45, Astrid Eichhorn (Imperial College, United Kingdom)
I will explain the main conceptual ideas underlying the asymptotic safety scenario in quantum gravity. I will then summarize the status of this approach, and highlight some recent developments. In particular, I will discuss why matter matters in this model of quantum gravity and how this could potentially open a window into experimental tests of quantum gravity.

Aspects of Hamiltonian constraint in Loop Quantum Gravity
10:00 - 10:45, Alok Laddha (Chennai Mathematical Institute, India)
We review some of the recent progress that has been made in constructing the dynamical equations of canonical Loop Quantum Gravity in the Euclidean sector. We start be recalling some of the well known problems which plague the dynamical aspects of canonical theory and set out a strategy aimed at resolving them. After reviewing the lessons which have been obtained in various toy models, we define a continuum quantum Hamiltonian constraint of the theory and show some intriguing evidence that this constraint generates an anomaly free Quantum constraint algebra off-shell.

Plenary Session
Monday, 11:15 - 13:00
Covariant Loop Quantum Gravity: Models and Developments
11:15 - 12:00, Wolfgang Wieland (IGC Penn State, USA)
I start with a pedagogical introduction to covariant loop quantum gravity. I select a number of recent results and discuss the open problems in the field. I present, in particular, a program to derive the spinfoam amplitudes from a classical action for discretized gravity. This action, which is a one-dimensional integral over a system of auxiliary worldlines, has the same field content as loop quantum gravity, which explains its relevance for the theory.

Continuum limit and renormalization in spin foam models
12:15 - 13:00, Benjamin Bahr (University of Hamburg, Deutschland)
Spin foam models provide a tentative proposal for the path integral of quantum gravity. In recent years, there have been very successful candidates for such models, and while their properties on a microscopic level are very promising, little is known about the contuum limit of many building blocks.
In this talk I will give an overview of background-independent renormalization techniques, which can not only provide a framework for investigating the continuum limit, but also as a method to construct the analogue of low-energy effective actions for spin foam models. I will also discuss the connection with diffeomorphism-invariance.

Plenary Session
Tuesday, 09:00 - 10:45
Early-Universe Cosmology: Issues and Opportunities
09:00 - 09:45, Anna Ijjas (Princeton University, USA)
Inflationary cosmology is commonly considered as the “standard model" of the early-universe, though, it has several open issues. Some of the problems have been known since the introduction of inflationary theory in the early 80s but some of the problems have been realized first later as we learned to better understand the theory and gained more experimental data. I will present the main problems with inflation, discuss alternative approaches and point to the opportunities the current situation gives us.

Numerical loop quantum cosmology: overview and recent results
10:00 - 10:45, Parampreet Singh (Louisiana State Universitty, USA)
In this talk, we will discuss some of the recent developments in the investigations on singularity resolution using numerical simulations in loop quantum cosmology. Thanks to the introduction of new techniques, the bounce can now be established for a wide variety of states and the reliability of the effective dynamics can be checked rigorously. These results will be discussed in isotropic and anisotropic models.

Plenary Session
Tuesday, 11:15 - 13:00
Hybrid Loop Quantum Cosmology
11:15 - 12:00, Mercedes Martin-Benito (Radboud University Nijmegen, Netherlands)
I will present an overview of the hybrid approach to quantize inhomogeneous cosmologies in the framework of loop quantum cosmology. Though I will keep the discussion as general as possible, for definiteness I will focus on two particular examples largely studied so far: linearly polarized Gowdy cosmologies with spatial three-torus topology, which is the simplest inhomogeneous cosmological model, and the flat homogeneous and isotropic model minimally coupled to a scalar field and with cosmological perturbations. After explaining the hybrid quantization of these models, I will discuss some aspects of the resulting quantum dynamics, and also comment on approximations that one can introduce to extract physical results.

Loop quantum cosmology and alternatives to inflation
12:15 - 13:00, Edward Wilson-Ewing (Albert Einstein Institute, Germany)
High precision observations of the cosmic microwave background provide strong constraints on the dynamics of the early universe and raise the hope that it may be possible to detect quantum gravity effects. In this talk, I will focus on realizations of the ekpyrotic and matter bounce scenarios in loop quantum cosmology. These are alternatives to inflation where scale-invariant perturbations are generated in a contracting background Friedmann space-time which later bounces due to loop quantum cosmology effects. I will show how it is possible to explicitly calculate the evolution of the perturbations through the non-singular bounce and explain under what conditions scale-invariance is preserved. I will also discuss how loop quantum cosmology can affect observational quantities; one such effect is a damping of the amplitude of tensor modes.

Plenary Session
Wednesday, 09:00 - 10:45
Pathways in Quantum Gravity Phenomenology
09:00 - 09:45, Stefano Liberati (SISSA, Trieste, Italy)
In this talk I will offer a panoramic view on the lessons and the achievements gathered so far in the quest for probing the fabric of spacetime. In particular, I will discuss possible scenarios for the mesoscopic physics between our classical world and full quantum gravity regimes. In doing so I shall focus on the perspectives to test them so to gain some guidance in the development of quantum gravity.

Higher dimensional connection dynamics and applications
10:00 - 10:45, Norbert Bodendorfer (University of Warsaw, Poland)
A reformulation of general relativity in terms of connection variables constitutes the classical basis of loop quantum gravity. In this talk, we will review the construction of a set of connection variables which generalise those of Ashtekar and Barbero, and allow to apply the quantisation techniques of loop quantum gravity also to higher-dimensional gravitational theories. Recent developments such as the computation of black hole entropy in higher dimensions and the treatment of generalised gravity theories will be discussed. Future prospects and promising lines of research will be outlined.

Plenary Session
Wednesday, 11:15 - 13:00
Getting rid of the Barbero-Immirzi parameter?
12:15 - 13:00, Karim Noui (LMPT Tours, France)
The Barbero-Immirzi parameter plays a rather intriguing role in loop quantum gravity. At the classical level, it has no physical relevance whereas it plays a crucial role in the quantum theory (quantum geometry, black hole entropy, spin foams, etc). In a recent series of articles, we have shown that the right value of this parameter could be the complex number +/- i. We will review how this result appears in the contexts of black holes and three dimensional gravity. We will also discuss some consequences of this observation in the full theory.

Isolated and Dynamical Horizons: Overview and recent results
12:15 - 13:00, Jonathan Engle (Florida Atlantic University, USA)
The notion of isolated horizon encodes just that `minimum' of the properties of a stationary black hole necessary to ensure that the laws of black hole mechanics hold. It is quasi-local, and makes possible a well-defined canonical framework in which energy and angular momentum of a black hole can be defined. Dynamical horizons generalize this notion to the non-stationary case. I review the basic ideas of this framework, and discuss its applications, from the statistical mechanics of quantum black holes to numerical relativity. Both classic and more recent results and developments will be reviewed.

Plenary Session
Thursday, 09:00 - 10:45
A New Decay Mode for Black Holes
09:00 - 09:45, Hal Haggard (Bard College, USA)
The recent discovery of a metric satisfying the Einstein equations outside a finite spacetime region where matter collapses into a black hole and then emerges from a white hole has ignited interest in an alternative mode for black hole decay: through quantum tunneling of its geometry a black hole might turn into a white hole. I will discuss the current status, weaknesses and strengths, of this proposal and review potential phenomenological possibilities for the observation of decaying primordial black holes.

Group field theory for (loop) quantum gravity
10:00 - 10:45, Aristide Baratin (University of Waterloo, Canada)
Group field theory is a higher dimensional analogue of matrix models, which gives a prescription for summing over triangulations in spin foam models. I will give an overview of the framework and discuss some of the (numerous) recent developments in the field.

Plenary Session
Thursday, 11:15 - 13:00
On black hole design
11:15 - 12:00, Matteo Smerlak (Perimeter Institute, Canada)
Black hole design is the art of constructing black hole spacetimes from scratch, i.e. without solving the Einstein equations. This endeavor is motivated by Penrose's realization that classical black hole generically have singularities, and by the belief that these singularities must be resolved by quantum effects. I will share my perspective on the black hole design process, its upshots, and its pitfalls.

Loop quantization of vacuum spacetimes with spherical symmetry
12:15 - 13:00, Javier Olmedo (Louisiana State University, USA)
In this talk I will briefly review the quantization of spherically symmetric vacuum spacetimes. Concretely, I will discuss in more detail the quantization of this midisuperspace setting within loop quantum gravity. I will adopt a redefinition of the classical constraints characterized by an algebra free of structure functions. I will then adopt the Dirac quantization approach together with a representation mimicking the one of loop quantum gravity. I will provide the solutions to the constraints, a suitable inner product, and the quantum observables of the model (some of them without classical counterpart). I will eventually discuss about the applications and extensions to other midisuperspace models.

Plenary Session
Friday, 09:00 - 10:45
Gravity in the radial gauge
09:00 - 09:45, Wojciech Kaminski (University of Warsaw, Poland)
A prominent feature of General Relativity is the presence of diffeomorphism group as gauge transformations. As a result, evolution of the metric tensor in a given space point is not a well defined notion unless coordinate system is uniquely specified. The physically motivated choices are certain versions of normal or Fermi coordinates, which are determined by a single observer moving in space-time. The particularly simple form of the metric in these radial coordinate systems allows to eliminate spurious degrees of freedom. This geometrical meaning attracted a lot of attention. I will give a sketch of recent works (of N. Bodendorfer, P. Duch, J. Lewandowski, J. Swiezewski) on the hamiltonian formulation in terms of the radial gauge. The aim of the construction is to define relational observables.

4-dimensional Spinfoam Amplitude with Cosmological Constant, 3-Manifold, and Supersymmetric Gauge Theory
10:00 - 10:45, Muxin Han (FAU Erlangen, Germany)
In this talk, I give an overview of the recent progress of covariant LQG in 4-dimensions with cosmological constant, with emphasis on the interesting relations with other areas of physics and mathematics. The 4d spinfoam amplitude is written as a finite dimensional integral, which has nice relation with Chern-Simons theory on a (dual) 3-manifold. Moreover the 4d spinfoam amplitude can be formulated as the holomorphic block in 3d, which arises from the holomorphic factorization of a certain 3-dimensional N=2 supersymmetric gauge theory. This formulation relates covariant LQG to M5-brane dynamics and 6d (2,0) theory in String/M-theory.

Plenary Session
Friday, 11:15 - 13:00
Quantum Reduced Loop Gravity
11:15 - 12:00, Emanuele Alesci (Warsaw University, Poland)
We present Quantum Reduced Loop Gravity a gauge fixed version of LQG. We show how QRLG provides a promising framework for a consistent characterization of the early Universe.
The effective semiclassical dynamics, the differences with Loop Quantum Cosmology and the inclusion of a scalar field will be discussed.

Summary
12:00 - 13:00, Abhay Ashtekar, Carlo Rovelli, Jerzy Lewandowski

 

[marcus]

I mentioned several people earlier who have taken part in very interesting research recently but who were not on the list of plenary speakers. Now that the titles and abstracts for the PARALLEL SESSION talks are posted online we can see what those folks are going to be talking about too.

BTW I noticed that Suzanne Lanéry, who did that interesting series of papers with Thomas Thiemann, is one those who have already registered--so one can count on presentation of some of the recent Thiemann et al work in the parallel sessions.
... Gofreddo Chirco (of the CHRR) is another of those listed, who will most likely be presenting results in parallel session. Francesca Vidotto as well. I'm interested in that "compact phase space" result which comes out of incorporating the cosmological constant in simplicial QG.

I have time to mention only a few of the parallel session talks. I have highlighted ones by Haggard, Riello, Vidotto, and Geiller, about recent research discussed here at PF.
Parallel Session: Foundations of Canonical LQG
Monday, 16:30 - 18:00, Room: Lecture Hall
Chair: Alok Laddha
...
Fractal Coherent States
17:00 - 17:30, Suzanne Lanéry (FAU Erlangen, Germany)
Motivated by obstructions to the construction of semi-classical states on the holonomy-flux algebra, i will discuss how a discrete subalgebra can be extracted while preserving universality and diffeomorphism invariance. This paves the way for the construction of states whose semi-classicality is enforced step by step, starting from collective, macroscopic degrees of freedom and going down progressively toward smaller and smaller scales.
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Parallel Session: Foundations of Covariant LQG (Spin Foams)
Tuesday, 14:30 - 16:00, Room: Lecture Hall
Chair: Muxin Han
...
Encoding Curved Tetrahedra in Face Holonomies
15:30 - 16:00, Hal Haggard (Bard College, USA)
I will present a generalization of Minkowski’s classic theorem on the reconstruction of tetrahedra from algebraic data to homogeneously curved spaces. Euclidean notions such as the normal vector to a face are replaced by Levi-Civita holonomies around each of the tetrahedron’s faces. This new approach allows the reconstruction of both spherical and hyperbolic tetrahedra within a unified framework. Several interesting mathematical structures arise in setting up a phase space for these curved tetrahedra such as group-valued moment maps and quasi-Poisson spaces. Curved tetrahedra also provide a natural starting point for thinking about discrete and quantum gravity in spacetimes with a cosmological constant.
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Parallel Session: Foundations of Covariant LQG (Spin Foams)
Tuesday, 16:30 - 18:00, Room: Lecture Hall
Chair: Muxin Han
From a curved-space reconstruction theorem to a 4d Spinfoam model with a Cosmological Constant
16:30 - 17:00, Aldo Riello (Perimeter Institute, Canada)
I will discuss the first steps towards a definition of a spinfoam model for 4d gravity with a cosmological constant, via complex Chern-Simons theory with defects. The proposal hinges on a reconstruction theorem assessing the correspondence between a class of flat connections on a S3 graph complement (related to the 4-simplex 1-skeleton) and the geometries of a constant-curvature Lorentzian 4-simplex. The main result consists in showing that in the semiclassical approximation of the vertex amplitude the Regge action of simplicial general relativity correctly appears. This construction borrows ingredients from the EPRL/FK model and adapts them to the curved case. Time allowing I will also comment on the phase space structure of the boundary states of the model.
Compactification of LQG phase space
17:00 - 17:30, Francesca Vidotto (Radboud University Nijmegen, Netehrlands)
In order to introduce the cosmological constant in a simplicial geometry, simplex faces should be taken of constant curvature. This yields a compactification of the phase space and the finiteness of the Hilbert space for each link. Not only the intrinsic, but also the extrinsic geometry turns out to be discrete, pointing to discreetness of time, in addition to space.
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Parallel Session: Homogeneous and Hybrid Loop Quantum Cosmology (LQC)
Tuesday, 16:30 - 18:00, Room: Seminar Room 4
Chair: Edward Wilson-Ewing
LQC, Non-Gaussianity and CMB anomalies
16:30 - 17:00, Ivan Agullo (LSU, USA)
This talk will summarize the prediction of LQC for the spectrum of Non-Gaussianity and its role as a potential source for the power asymmetry observed at large angular scales in the CMB
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Parallel Session: Foundations of Covariant LQG (Spin Foams)
Thursday, 14:30 - 16:00, Room: Seminar Room 5
Chair: Benjamin Bahr
The area-law sector of loop quantum gravity
14:30 - 15:00, Eugenio Bianchi (Penn State, USA)
In this talk I present a concrete realization of the conjecture that semiclassical states in quantum gravity satisfy an area law. In loop quantum gravity the entanglement entropy of a random spin-network state scales linearly with the volume of a region of space. I describe a class of spin-network states that are fully characterized by the expectation value and the 2-point correlation function of geometric observables. Such states are semiclassical, have non-vanishing graviton-graviton correlations, and satisfy the area law. The framework extends previous results about the black hole horizon entropy to all regions of space.
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Parallel Session: Foundations of Canonical LQG
Thursday, 14:30 - 16:00, Room: Lecture Hall
Chair: Norbert Bodendorfer
...
Typicality and local thermalisation in spin networks
15:30 - 16:00, Goffredo Chirco (CPT - AMU, France)
We investigate the notion of quantum typicality in spin networks, by applying the general approach proposed by Popescu, Short and Winter in 2006, in the context of LQG. In particular, we focus on a basic spin network building block consisting in a N-valent SU(2) intertwiner with fixed total spin, the equivalent of a space of convex polyhedra with N face and fixed total boundary area at the classical level. On the fixed-area subspace of the intertwiner, we study the reduced state associated to a small region of the boundary surface.
By exploiting the "concentration of measure phenomenon", we show how the distribution for such a state is highly peaked around the "thermal state" for almost all pure states of the global intertwiner. We obtain a Gibbs state written in terms of the area preserving generator of the U(N) group, the area having the role played by the energy in the standard canonical picture. Local thermalisation arises as the result of the degree of correlations between local state and environment.
We study the temperature of the local surface patch state and we confront the specific structure of correlations of our result with the previous derivations of a single link thermal state present in the literature.
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Parallel Session: Foundations of Covariant LQG (Spin Foams)
Friday, 16:30 - 18:00, Room: Lecture Hall
Chair: Wolfgang Wieland
Graviton propagator of the "proper" vertex
16:30 - 17:00, Atousa Chaharsough Shirazi (Florida Atlantic University, USA)
The “proper” spin-foam vertex amplitude was obtained from the EPRL vertex by projecting out all but a single gravitational sector, in order to enable correct semi-classical behavior. We calculated the gravitational two-point function predicted by the proper spin-foam vertex to lowest order in the vertex expansion. We find the same answer as in the EPRL case, so that the theory is consistent with the predictions of linearized gravity in the regime of small curvature.
Proper Vertex asymptotics and Graviton Propagator
17:00 - 17:30, Ilya Vilensky (Florida Atlantic University, US)
The EPRL vertex amplitude provides a consistent formulation of dynamics of loop quantum gravity states. However, its semi-classical limit does not exactly match classical Regge calculus. We present a modification of the EPRL amplitude - the proper vertex amplitude - that has the correct semi-classical limit. We use the proper vertex amplitude to calculate graviton propagator and find that in semi-classical limit it agrees with the result from Lorentzian Regge calculus.
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Parallel Session: Group Field Theory and Tensor Models
Friday, 16:30 - 18:00, Room: Seminar Room 4
Chair: Aristide Baratin
A new representation for loop quantum gravity
16:30 - 17:00, Marc Geiller (ICG Penn State, USA)
One of the key results of loop quantum gravity is the existence of a diffeomorphism-invariant representation of the holonomy-flux algebra of observables, and the construction of a continuum inductive limit Hilbert space. After briefly recalling the properties and the role played by the so-called Ashtekar-Lewandowski vacuum state in this construction, I will describe how a dual formulation can be obtained by trading the roles of the holonomies and the fluxes. This dual representation is built upon a vacuum based on states of topological BF theory, and therefore cast canonical loop quantum gravity in a formulation closer to the spirit of spin foam models. Furthermore, this new vacuum allows for the construction of a continuum limit Hilbert space carrying a (unitarily inequivalent) representation of the holonomy-flux algebra, and gives a new perspective on the derivation of quantum geometry and on the extraction of physics from the theory.
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