Six themes for QG in 2015 (developments to watch for)

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marcus
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Not in any particular order (all are active important research initiatives).

1. Algebraic geomatter (google "geometry and the quantum: basics")
2. LQG with constant curvature simplices (embodying the cosmological curvature constant Λ)
3. Flux formulation LQG
4. LambdaCDM bounce
5. Projective LQG
6. Starbounce (google "planck star")

1. refers to Sep and Dec 2014 papers by Chamseddine Connes Mukhanov where quanta of both GR geometry and StdMdl matter seem to grow from the same algebraic root
2. refers to December 2014 paper by Haggard Han Kaminsky Riello (google "4D loop quantum gravity with a cosmological constant", part of the title of their paper) and February 2015 paper by Rovelli Vidotto ("compact phase space, cosmological constant, discrete time")
3. google "flux formulation LQG" for Dittrich&Geiller new formulation of LQG, likely opening to the GR limit.
4. google "LambdaCDM bounce" for standard cosmology combined with matter bounce/Loop bounce.
5. google "projective LQG" for massive work by Lanery&Thiemann reformulating LQG.
6. google "planck star" for work by Barrau, Rovelli, Vidotto on prospects of seeing collapse rebound explosions

I will get some links. These are research topics that I think may see some development in 2015. One wants to have looked at some of the relevant 2014 papers, or at least know of their existence, and be on the look-out for further progress this year. BTW nobody says "geomatter" or "starbounce". Those are just concise descriptive verbal handles I use for convenience and concreteness sake.

The wording of theme #2 was edited to make it less technical and more directly understandable.
EDIT: in the case of theme#2 a 2009 paper by Bahr&Dittrich proposing simplices with constant curvature should be mentioned (was this a "sleeper" in the sense of not getting sufficiently noticed and followed-up in the intervening 5 years?)
http://arxiv.org/abs/0907.4325
Regge calculus from a new angle
 
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  • #2
marcus
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atyy
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I didn't know whether to laugh or cry when I read what you had named the approach of Chamseddine, Connes and colleagues. :):L
 
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marcus
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heh heh, I guess it is a bit crude. We could try for a more polite name: "Spectral geometry and the Standard Model"
I'm personally not partial to something with a lot of syllables like "Noncommutative geometry and the standard model" (15 syllables).

If you just laugh I'm happy but I don't want anyone to be aggrieved or grossed out.
 
  • #5
atyy
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Oh I mean cry :L in a good way :). In my country we like corny jokes.
 
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marcus
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As in this country one might groan and look to the Heavens in response to a really awful pun. It is a form of ironical appreciation or at least acknowledgment.
Algebraic geomatter sounds a bit like algebraic geometry. But let's pretend we don't notice that. The main thing is they have somehow merged dynamical geometry and matter
so that quanta of geometry and quanta of matter blend into a common quantum geomatter
and they accomplished the merger in a highly algebraic way. So it is algebraic geomatter :w
Please clue me in anytime you see better terminology or decide some of these terms sound too dumb. We can go with them on a provisional basis.

Atyy I am kind of interested in which of these 6 themes will be in evidence at the February EFI Winter Conference at the ski resort of Tux in Austrian Alps.
http://www.gravity.physik.fau.de/events/tux3/tux3.shtml
For some reason I do not see Lanery and Thiemann on the list of participants.
But I see Ed Wilson-Ewing (so maybe LambdaCDM bounce will be discussed)
and I see Wolfgang Wieland (his family is Austrian so how could he miss this one) so maybe Causal Spinfoams might be discussed
and Starbounce for sure.
So that is 3 out of 6.
I'll check to see if Dittrich or Geiller will be there, if so they might touch on the "flux formulation" theme as well.

Yes Marc Geiller will be there, so there is a chance. Dittrich is not listed though.
So that means possibly four out of the six.
The program for the EFI Winter Conference should appear on line sometime in the next two weeks or so, I imagine.
 
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  • #7
atyy
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An older theme, but one which I'm hoping to see more of is the Chirco-Haggard-Riello-Rovelli (CHRR) approach to entanglement, gravity and thermodynamics. The Bianchi-Myers paper indicated this was a common interest for loops and strings. The stringy side is represented by the ongoing efforts to understand AdS/CFT, while CHRR seems to be the main loopy development of Bianchi-Myers. I think you pointed this out in the other thread, and I owe you a reply, but it needed an intelligence far beyond mine, so I'm just hoping some of those guys will add connections.

Also, what is the status of EPRL?
 
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An older theme, but one which I'm hoping to see more of is the Chirco-Haggard-Riello-Rovelli (CHRR) approach to entanglement, gravity and thermodynamics. The Bianchi-Myers paper indicated this was a common interest for loops and strings. The stringy side is represented by the ongoing efforts to understand AdS/CFT, while CHRR seems to be the main loopy development of Bianchi-Myers. I think you pointed this out in the other thread, and I owe you a reply, but it needed an intelligence far beyond mine, so I'm just hoping some of those guys will add connections.

Also, what is the status of EPRL?

Let me paste in a couple of abstracts as reminders of the CHRR paper and the more recent collaboration of Haggard and Riello (with two others). I need a little time to think about where CHRR might lead. As far as what's happening with EPRL some things that occur to me (as a not too well-informed onlooker) are:
1. Wieland's new action is a spin foam action and he argues it is an improvement
2. Dittrich's flux formulation is argued to be an improvement. I'm not sure why these researchers think that EPRL should evolve.
3. Haggard and Riello (with Han and Kaminski) have a new improved EPRL with Lambda (constant curvature in the bulk interior of the simplexes, not flat ones) and they think that is better. They call it "LambdaEPRL" sometimes, I think. I need to refresh my memory. Let me put up those abstracts

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
Hal M. Haggard, Muxin Han, Wojciech Kamiński, Aldo Riello
(Submitted on 23 Dec 2014)
We study the expectation value of a nonplanar Wilson graph operator in SL(2,C) Chern-Simons theory on S3. In particular we analyze its asymptotic behaviour in the double-scaling limit in which both the representation labels and the Chern-Simons coupling are taken to be large, but with fixed ratio. When the Wilson graph operator has a specific form, motivated by loop quantum gravity, the critical point equations obtained in this double-scaling limit describe a very specific class of flat connection on the graph complement manifold. We find that flat connections in this class are in correspondence with the geometries of constant curvature 4-simplices. The result is fully non-perturbative from the perspective of the reconstructed geometry. We also show that the asymptotic behavior of the amplitude contains at the leading order an oscillatory part proportional to the Regge action for the single 4-simplex in the presence of a cosmological constant. In particular, the cosmological term contains the full-fledged curved volume of the 4-simplex. Interestingly, the volume term stems from the asymptotics of the Chern-Simons action. This can be understood as arising from the relation between Chern-Simons theory on the boundary of a region, and a theory defined by an F2 action in the bulk. Another peculiarity of our approach is that the sign of the curvature of the reconstructed geometry, and hence of the cosmological constant in the Regge action, is not fixed a priori, but rather emerges semiclassically and dynamically from the solution of the equations of motion. In other words, this work suggests a relation between 4-dimensional loop quantum gravity with a cosmological constant and SL(2,C) Chern-Simons theory in 3-dimensions with knotted graph defects.
54+11 pages, 9 figures

The earlier Haggard Riello was the CHRR one that you mentioned, Atyy. Let me get that. Sometimes following people could help follow ideas, I think. As I recall that was not SPECIFICALLY about EPRL, it was more general. Any QG degrees of freedom that satisfied certain conditions could be the basis of a Jacobson-like analysis. So you could be starting either from EPRL or an improved version of EPRL or some other simplicial QG? I'm not sure but I think it was not tied to a particular spinfoam model. Let's see.
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
 
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  • #9
atyy
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Yes, I dind't mean to associate EPRL with CHRR. That was a separate question. Is there a problem with EPRL that is motivating flux LQG or projective LQG?
 
  • #10
marcus
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Atyy you jogged my memory when you mentioned the CHRR paper, and also current developments with EPRL, even if not directly associated.
This made me realize that we couldn't leave out the OTHER Haggard&Riello paper: the one that appeared in December 2014 after their ILQGS online seminar talk in November. I copied the abstract a couple of posts back.

It's an important development and actually chimes conceptually with what you were talking about because it involves a relation between one theory on a boundary and another in bulk. E.g. notice the last sentence of the HHKR abstract I just posted:
"In other words, this work suggests a relation between 4-dimensional loop quantum gravity with a cosmological constant and SL(2,C) Chern-Simons theory in 3-dimensions with knotted graph defects."
 
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marcus
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I was wondering earlier which of these 6 themes (algebraic geomatter, chern-simons spinfoam, flux formulation, lambdaCDM bounce, projectiveLQG, and "starbounce") are likely to be represented at the 16-20 February 2015 EFI Winter Conference.
Han will be there so they will hear about Chern-Simons spinfoam with cosmological constant. Putting positive Lambda into EPRL was one of the main motivations for hooking up with Chern-Simons. Han has been working on that several years and gave a talk at Perimeter in 2011.

Marc Geiller will be there and he has been working with Dittrich on flux formulation of LQG (the authors say it is closer to EPRL than past formulations of LQG)

Edward Wilson-Ewing will be at the conference---so LambdaCDM bounce cosmology may be discussed.

Rovelli and Vidotto will be there so one way or another the Planck star idea will come up. So that's four out of the 6 topics. Planck star theme is important because related to possible observations (gamma ray bursts, unexplained brief radio-frequency bursts). But it is also possible that one or the other could surprise by giving a talk on something unexpected, say related to the Chamseddine, Connes, Mukhanov work. Rovelli co-authored a paper with Connes some years back about thermal time, he could have gotten interested in the CCM work---which appears to be unifying quantum GR with StdMdl matter--- and have something to say about it. Then 5 out of the 6 themes would be covered.

It will be interesting to see the EFI Winter conference program when it is posted. Also there's Loops 2015 coming up, at Erlangen this year.
http://www.gravity.physik.fau.de/events/tux3/tux3.shtml
http://www.gravity.physik.fau.de/events/loops15/loops15.shtml
 
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  • #13
atyy
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What is your country? :)

I'm a Singaporean who is a postdoc in the US. My country only refers to the former, I don't intend to impugn my hosts! :)
 
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So, are we already getting TOE from "eigen deformations" of the geometry of space time, or like coupled fields to the geometry of it?
 
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a nice extra resource, we have the Cai&Wilson-Ewing paper ( http://arxiv.org/abs/1412.2914 ) and now there is a video lecture at Perimeter, by Wilson-Ewing, to go with it:
http://pirsa.org/15010074/
A ΛCDM Bounce Scenario
Speaker(s): Edward Wilson-Ewing
Abstract:
We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. Assuming that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, we calculate the spectrum of scalar and tensor perturbations after the bounce, assuming initial quantum vacuum fluctuations. We find that the modes that exit the (sound) Hubble radius during matter-domination when the effective equation of state is slightly negative due to the cosmological constant will be nearly scale-invariant with a slight red tilt, in agreement with observations. The tensor perturbations are also nearly scale-invariant, and the predicted tensor-to-scalar ratio is small. Finally, as this scenario predicts a positive running of the scalar index, it can be differentiated from inflationary models.
21/01/2015 - 4:00 pm

Also possibly of interest, a pirsa video talk by Karim Noui titled "Getting rid of the Barbero-Immirzi parameter in LQG"
http://pirsa.org/15010130/
http://pirsa.org/displayFlash.php?id=15010130
 
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The program for the Feb 2015 EFI winter school in Tirol ski country has been posted. I want to see if it reflects some of the themes mentioned, and if others stand out as well.
Here is a list of talks to be given in the afternoons. Mornings are left free for ski. The time slots seem to be mostly 40 minutes.

Mehdi Assanioussi: Construction of a hamiltonian operator in LQG

Tomasz Pawlowski: Interfacing loop quantum gravity with cosmology

Beatriz Elizaga: Effective homogeneous and isotropic scenarios emerging from states of the hybrid Gowdy model

Maciej Dunajski: Non-relativistic twistor theory and Newton-Cartan geometry

Giuseppe Sellaroli: Spinor operators in 3D Lorentzian gravity

Muxin Han: Chern-Simons Theory, Flat Connections and 4d Quantum Geometry

Marcin Kisielowski: First-order Dipole Cosmology

Guillermo Mena Marugan: Mukhanov-Sasaki equations in Loop Quantum Cosmology

John Schliemann: Coherent Quantum Dynamics: What Fluctuations Can Tell

Ilkka Mäkinen: Coherent state operators in loop quantum gravity

Ivan Agullo: Unitarity and ultraviolet-regularity in QFT in curved spaces

Maximillian Hanusch: Symmetry Actions and Invariance Conditions in LQG

Edward Wilson-Ewing: A Lambda-CDM Bounce Scenario

Andrzej Dragan: Ideal clocks - convenient fiction

Goffredo Chirco: Thermally correlated states in LQG

Martin Ammon: Recent developments in AdS/CFT and higher spin gravity

Jorge Pullin: Recent results in spherically symmetric LQG

Mercedes Martin-Benito: More information about the early Universe than meets the eye

Carlo Rovelli: Can we test quantum gravity with black hole explosions?

Benjamin Bahr: Background-independent renormalization in Spin Foam models

Maite Dupuis: Towards the Turaev-Viro amplitudes from a Hamiltonian constraint

Jedrzej Swiezewski: Radial gauge - reduced phase space of General Relativity

Xiangdong Zhang: Loop quantum cosmology in 2+1 dimensions

Simone Speziale: First order gravity on the light front

Wolfgang Wieland: Covariant Loop Quantum Gravity as the Scattering of Defects

Florian Girelli: Turaev Viro GFT and its Fourier transform

Norbert Bodendorfer: A quantum reduction to Bianchi I models in LQG

Lacina Kamil: The problem of time in background independence

Andrea Dapor: Rainbows from Quantum Gravity

Francesca Vidotto: The compact phase space of Loop Quantum Gravity

Marc Geiller: Flux formulation of loop quantum gravity

http://www.gravity.physik.fau.de/events/tux3/program.pdf
http://www.gravity.physik.fau.de/events/tux3/tux3.shtml


So, four out of our 6 themes and theme #2 is treated in two papers :w
I have five of the scheduled talks highlighted in blue. Another one that interests me is Wieland's. I do not know what it entails. His most recent paper is the July 2014 one with arXiv number
14070025 in which an action for spinfoamQG is constructed as a kind of "scattering of tetrahedra", where a conserved fluid of tetrahedra flows timewards through a network. I have not seen a "scattering of defects" paper by him so I cannot see how to connect the Tux talk with the previous paper. I have highlighted the talk in green as exemplifying an unknown theme---maybe one to add to our list.
 
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marcus
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For me, the Wilson-Ewing recorded talk was actually an improvement over his paper with Cai ( http://arxiv.org/abs/1412.2914 ) that appeared last month.
Since I did not get the video, I downloaded the slides PDF and then turned on the audio MP3. Highly satisfactory. Clear rapid-fire delivery. Emphasis on TESTABILITY.
W-E mentioned some ways the scenario could definitely be FALSIFIED. It replaces inflation with the effects of the prior contraction in the presence of dark matter, radiation, and the Lambda constant. This leads to slightly different predictions, so the scenario is DISTINGUISHABLE by observations which are practical to make and some of which are already being made.

The inflation paradigm, as the BICEP events showed, is overly flexible and seems to adapt to whatever new observations, so it was refreshing to hear clear prospects of testability emphasized, in a model which is alternative to inflation---achieves the general results which originally motivated cosmologists to dream up inflation and invent the never-observed exotic "inflaton" field, but does not need to invoke the fabulous. I liked the talk a lot---here's the link and abstract again:

http://pirsa.org/15010074/
A ΛCDM Bounce Scenario
Speaker(s): Edward Wilson-Ewing
Abstract:
We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. Assuming that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, we calculate the spectrum of scalar and tensor perturbations after the bounce, assuming initial quantum vacuum fluctuations. We find that the modes that exit the (sound) Hubble radius during matter-domination when the effective equation of state is slightly negative due to the cosmological constant will be nearly scale-invariant with a slight red tilt, in agreement with observations. The tensor perturbations are also nearly scale-invariant, and the predicted tensor-to-scalar ratio is small. Finally, as this scenario predicts a positive running of the scalar index, it can be differentiated from inflationary models.
21/01/2015 - 4:00 pm
 
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  • #18
julian
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Thiemann has always been a constructionist of new mathematical machinery. He is indeed making progress in the construction of more satisfactory coherent states that allows for a more balanced treatment of the holonomy and flux variables. As I understand there are remain obstructions to the completion of this project.

I wonder to what extent this construction will eventually address old outstanding problems with his previous attempts of the construction of coherent states which suffered the following issues:

(i) The semi-classical machinery developed was only appropriate to non-graph-changing operators, however, Thiemann's Hamiltonian constraint is a graph-changing operator - the new graph it generates has degrees of freedom upon which the coherent state does not depend and so their quantum fluctuations are not suppressed.

(i) There is also the restriction, as far as I know, that these coherent states are only defined at the Kimematic level, and now one has to lift them to the level of the Diff and Phys Hilbert space

which prevented a proper evaluation of the semi-classical limit of canonical LQG. Will his/their new construction address or circumvention these issues in some sense? What then would constitute a successful establishment of LQG having the correct semi-classical limit in the context of their new construction? What physical applications could this new construction have? Have other researchers started looking into this question? Perhaps these reasons and the obstructions and because it is so recent is why it is not prominent there in the talks? Even though as we know Thiemann et al are always people to watch...
 
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  • #19
marcus
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Thiemann has always been a constructionist of new mathematical machinery. He is indeed making progress in the construction of more satisfactory coherent states that allows for a more balanced treatment of the holonomy and flux variables...
Perhaps... why it is not prominent there in the talks? Even though as we know Thiemann et al are always people to watch...
It's a good question to be asking! Thiemann will deliver his message the week before:
http://www.gravity.physik.fau.de/events/cosmo2015/cosmo2015.shtml
there will be some partial overlap of participants and topics between this workshop at Erlangen 9-13 Feb and the conference at Tux 16-20 Feb
http://www.gravity.physik.fau.de/events/tux3/tux3.shtml
Invited speakers at the Cosmology and Quantum Gravity workshop include:
Ivan Agullo (Louisiana State University)
Abhay Ashtekar (Pennsylvania State University)
Latham Boyle (Perimeter Institute)
Edmund Copeland (University of Nottingham)
Raphael Flauger (Princeton University)
Steffen Gielen (Imperial College London)
Jean-Luc Lehners (Albert Einstein Institute Golm)
Thomas Thiemann (University of Erlangen-Nürnberg)
Edward Wilson-Ewing (Albert Einstein Institute Golm)

It seems like a good idea to put energy into a "Cosmology and Quantum Gravity" meeting because there is a strong trend (I sense) away from speculative untestable paradigms involving stuff we can't see and that may be made up. Cosmology is richly observational and it will be good for QG as a field to join in with Cosmology. the two lines of research have implications for each other. It's time for QG to be seen as empirical, as observation-based.

There was that strongly worded article in Nature recently, by George Ellis and Joe Silk.
Scientific method: Defend the integrity of physics. You probably read it and may have already commented, Julian. If anyone wants it
google: "Ellis Silk integrity physics Nature". That should get it.

Basically IMHO for Lanery and Thiemann's reformulation of LQG to succeed in a real sense, it should go all the way and get rid of the cosmological singularity---which most likely means predicting a bounce, as you go back in time. And when you put the cosmological constant and dark matter and radiation in, then it should make some predictions we can check about early universe things we can see.
 
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  • #20
marcus
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Julian, I'm glad you referred to the Lanery&Thiemann Projective LQG work which is one of the six research directions I especially interested in watching.
The program for the Cosmology&QG Workshop 9-13 February has not been posted yet. But to me it seems likely that TWO of our themes will surface there.
Thiemann is giving a talk (which could very well be about Projective LQG) and also there is to be one by Latham Boyle.
Boyle has had dealings with the Connes Chamseddine Spectral Geometry and Standard Model stuff
http://arxiv.org/abs/1408.5367
Rethinking Connes' approach to the standard model of particle physics via non-commutative geometry
http://arxiv.org/abs/1401.5083
Non-Commutative Geometry, Non-Associative Geometry and the Standard Model of Particle Physics
http://arxiv.org/abs/1303.1782
Non-Associative Geometry and the Spectral Action Principle
So it's hardly a sure thing but he might touch on what I called "algebraic geomatter".

Having seen the EFI Winter School program we know that 4 of the six are on the agenda at Tux 16-20 February
and it's not impossible that the other 2 will be come up at the CosmologyQG Workshop at Erlangen 9-13 February.
I'm optimistic :w
 
  • #21
After reading about the idea of Planck Stars, I can't help but wonder how that proposition is being received by the scientific community. It seems to be a very daring concept, but creative and possible nonetheless. Thoughts?
 
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marcus
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After reading about the idea of Planck Stars, I can't help but wonder how that proposition is being received by the scientific community. It seems to be a very daring concept, but creative and possible nonetheless. Thoughts?
I agree! Daring, creative, and possible.
It is still early (the idea first appeared in january 2014) but several authors besides the original ones have already contributed to developing the idea. However, physical confirmation depends on being able to witness the delayed explosions of primordial black holes (PBH) formed in the very early universe, the existence of which is only a conjecture.

Actually the possibility that we might see evidence (including end-of-life fireworks) of primordial BH has been discussed by many in the physics community over the past 30 years. Even conventional Hawking evaporation produces a radiation spike at the very end, in theory bright enough to be detected perhaps a few lightdays away. In that sense the possibility of PBH explosions has had fairly wide acceptance. In this version, the final burst, though impressive by our standards, occurs only after evaporation has exhausted most of the BH mass.

According to the model proposed by Rovelli and friends, the explosion of a black hole occurs earlier, while it still has a large fraction (a third to a half) of its original mass. Conventional Hawking evaporation is not allowed to finish, because a bounce occurs. So the predicted explosion is much larger---detectable hundreds of light years away.
But still the lifespan (before the explosion occurs) is proportional to the cube of the mass, and for a stellar mass BH would exceed the present expansion age by orders of magnitude. Our only chance to witness BH explosions according to the proposed model depends on ample numbers of miniature BH having arisen in the dense conditions around the start of expansion.

At this point AFAICS the ball is in the Observation court. Past data on Gamma Ray Bursts (GRB) needs to be analyzed to see if any of the recorded events could be from comparatively small explosions with the right wavelength signature, coming from comparatively nearby (distances of a few hundred LY). Even better would be launching spacecraft specifically adapted to look for the indicated near-small GRB.

Since the idea is recent there has not been enough time to see much of a response from observational astrophysicists.

However there was a curious doppelgänger appearance in June-September 2014. Researchers in the US and Canada came up with a proposal that looks remarkably similar:
http://arxiv.org/abs/1406.1525
Backreaction of Hawking Radiation on a Gravitationally Collapsing Star I: Black Holes?
Laura Mersini-Houghton
8 pgs
Physics Letters B, 16 September 2014
http://inspirehep.net/record/1299557?ln=en notes 8 citations, 7 by other authors.

http://arxiv.org/abs/1409.1837
Back-reaction of the Hawking radiation flux on a gravitationally collapsing star II: Fireworks instead of firewalls
Laura Mersini-Houghton, Harald P. Pfeiffer
(Submitted on 5 Sep 2014)
A star collapsing gravitationally into a black hole emits a flux of radiation, knowns as Hawking radiation. When the initial state of a quantum field on the background of the star, is placed in the Unruh vacuum in the far past, then Hawking radiation corresponds to a flux of positive energy radiation travelling outwards to future infinity. The evaporation of the collapsing star can be equivalently described as a negative energy flux of radiation travelling radially inwards towards the center of the star. Here, we are interested in the evolution of the star during its collapse. Thus we include the backreaction of the negative energy Hawking flux in the interior geometry of the collapsing star and solve the full 4-dimensional Einstein and hydrodynamical equations numerically. We find that Hawking radiation emitted just before the star passes through its Schwarzschild radius slows down the collapse of the star and substantially reduces its mass thus the star bounces before reaching the horizon. The area radius starts increasing after the bounce. Beyond this point our program breaks down due to shell crossing. We find that the star stops collapsing at a finite radius larger than its horizon, turns around and its core explodes. This study provides a more realistic investigation of the backreaction of Hawking radiation on the collapsing star, that was first presented in [1].
9 pages, 6 figures
http://inspirehep.net/record/1315316?ln=en notes 5 citations by other authors.
 
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  • #23
marcus
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As a check I'll list last year's Planck Star papers.
http://arxiv.org/abs/1412.6015 http://inspirehep.net/record/1334933
http://arxiv.org/abs/1409.4031 http://inspirehep.net/record/1316456 (3) Physical Review D
http://arxiv.org/abs/1407.0989 http://inspirehep.net/record/1304671?ln=en (8)
http://arxiv.org/abs/1404.5821 http://inspirehep.net/record/1291921?ln=en (7) Physics Letters B
http://arxiv.org/abs/1401.6562 http://inspirehep.net/record/1278812?ln=en (19) Int.J.Mod.Phys.
Aurelien Barrau, Tommaso De Lorenzo, Hal Haggard, Costantino Pacilio , Carlo Rovelli, Simone Speziale, Francesca Vidotto

 
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  • #24
marcus
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Since we just turned a page, I will bring forward the start of the thread, to make clear what we're discussing:
==quote==
Not in any particular order (all are active important research initiatives).
1. Algebraic geomatter (google "geometry and the quantum: basics")
2. LQG with constant curvature simplices (embodying the cosmological curvature constant Λ)
3. Flux formulation LQG
4. LambdaCDM bounce
5. Projective LQG
6. Starbounce (google "planck star")

1. refers to Sep and Dec 2014 papers by Chamseddine Connes Mukhanov where quanta of both GR geometry and StdMdl matter seem to grow from the same algebraic root
2. refers to December 2014 paper by Haggard Han Kaminsky Riello (google "4D loop quantum gravity with a cosmological constant", part of the title of their paper) and February 2015 paper by Rovelli Vidotto ("compact phase space, cosmological constant, discrete time")
3. google "flux formulation LQG" for Dittrich&Geiller new formulation of LQG, likely opening to the GR limit.
4. google "LambdaCDM bounce" for standard cosmology combined with matter bounce/Loop bounce.
5. google "projective LQG" for massive work by Lanery&Thiemann reformulating LQG.
6. google "planck star" for work by Barrau, Rovelli, Vidotto on prospects of seeing collapse rebound explosions

I will get some links. These are research topics that I think may see some development in 2015. One wants to have looked at some of the relevant 2014 papers, or at least know of their existence, and be on the look-out for further progress this year. BTW nobody says "geomatter" or "starbounce". Those are just concise descriptive verbal handles I use for convenience and concreteness sake.

Links to sample papers, illustrating these 6 lines of research:
1. http://arxiv.org/abs/1411.0977
2. http://arxiv.org/abs/1412.7546 http://arxiv.org/abs/1502.00278
3. http://arxiv.org/abs/1412.3752
4. http://arxiv.org/abs/1412.2914
5. http://arxiv.org/abs/1411.3592
6. http://arxiv.org/abs/1401.6562 http://arxiv.org/abs/1404.5821 http://arxiv.org/abs/1409.4031
==endquote==
EDIT: in connection with theme#2 LQG with constant curvature simplices, embodying the cosmological curvature constant Lambda, I should include a link to the earlier paper by Bahr and Dittrich http://arxiv.org/abs/0907.4325
 
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  • #25
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I mentioned earlier that there are two relevant meetings coming up in February
Cosmology and Quantum Gravity Workshop (Erlangen, 9-13 February)
EPI Winter School in Quantum Gravity (Tux, 16-20 February)
http://www.gravity.physik.fau.de/events/cosmo2015/cosmo2015.shtml
http://www.gravity.physik.fau.de/events/tux3/tux3.shtml
http://www.gravity.physik.fau.de/events/tux3/program.pdf

Last week when the program for Tux was posted I noticed that FOUR of the six themes will serve as topics of invited talks (2,3,4, and 6)

Talk titles for the Cosmology&QG workshop have not been posted, but Edward Wilson-Ewing is one of the invited speakers listed, so there will likely be a talk on LambdaCDM bounce cosmology.
Another of the speakers at the workshop is Latham Boyle who has recently co-authored papers on topic 1 of our list (spectral geometry and the standard model: "algebraic geomatter").
There is at least a chance he will discuss that during his participation in the workshop.
 
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  • #26
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Two more papers on the "bounce cosmology" theme (#4 on our list of six) appeared yesterday.
http://arxiv.org/abs/1501.06591
Superbounce and Loop Quantum Ekpyrotic Cosmologies from Modified Gravity: F(R), F(G) and F(T) Theories
S.D. Odintsov, V.K. Oikonomou, Emmanuel N. Saridakis
(Submitted on 26 Jan 2015)
We investigate the realization of two bouncing paradigms, namely of the superbounce and the loop quantum cosmological ekpyrosis, in the framework of various modified gravities. In particular, we focus on the F(R), F(G) and F(T) gravities, and we reconstruct their specific subclasses which lead to such universe evolutions. These subclasses constitute from power laws, polynomials, or hypergeometric ansatzes, which can be approximated by power laws. The qualitative similarity of different effective gravities which realize the above two bouncing cosmologies, indicates to some universality lying behind such a bounce. Finally, performing a linear perturbation analysis, we show that the obtained solutions are conditionally or fully stable.
31 pages.

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

Also it may be recalled that the example given for this theme was the "LambdaCDM Bounce" paper by Cai and Wilson-Ewing, and the latter author will be presenting this work at two meetings in February:
On Thursday, 12 February at the Erlangen Cosmology and Quantum Gravity Workshop.
And then the next week on Wednesday 18 February at the Tux EPI Winter School.

Bounce cosmology often dispenses with inflation and it may be a more natural alternative to inflation. The singularity has to be replaced by something, and quantum GR-based models often seem predisposed to replace it with a bounce. If one accepts what quantizing GR offers, then, one does not have to invent mythical "inflaton" fields in order to get the observed cosmic features. So this theme increasingly comes up in the research literature.
 
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  • #27
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The appearance of those two "bounce cosmology with no inflationary need" papers yesterday reminded me to list some others that appeared recently (October November December 2014). There has been a flurry of research into that branch of cosmology. Some versions don't need an inflation episode at all, others involve inflation, but get it without needing a (so far unobserved) "inflaton field".

http://arxiv.org/abs/1410.8183
Matter Bounce Loop Quantum Cosmology from F(R) Gravity
S.D. Odintsov, V.K. Oikonomou
(Submitted on 29 Oct 2014)
Using the reconstruction method, we investigate which F(R) theories, with or without the presence of matter fluids, can produce the matter bounce scenario of holonomy corrected Loop Quantum Cosmology. We focus our study in two limits of the cosmic time, the large cosmic time limit and the small cosmic time limit. For the former, we found that, in the presence of non-interacting and non-relativistic matter, the F(R) gravity that reproduces the late time limit of the matter bounce solution is actually the Einstein-Hilbert gravity plus a power law term. In the early time limit, since it corresponds to large spacetime curvatures, assuming that the Jordan frame is described by a general metric that when it is conformally transformed to the Einstein frame, produces an accelerating Friedmann-Robertson-Walker metric, we found explicitly the scalar field dependence on time. After demonstrating that the solution in the Einstein frame is indeed accelerating, we calculate the spectral index derived from the Einstein frame scalar-tensor counterpart theory of the F(R) theory and compare it with the Planck experiment data. In order to implement the resulting picture, we embed the F(R) gravity explicitly in a Loop Quantum Cosmology framework by introducing holonomy corrections to the F(R) gravity. In this way, the resulting inflation picture corresponding to the F(R) gravity can be corrected in order it coincides to some extent with the current experimental data.
28 pages.

http://arxiv.org/abs/1411.3475
Quasi-matter domination parameters in bouncing cosmologies
Emili Elizalde, Jaume Haro, Sergei D. Odintsov
(Submitted on 13 Nov 2014)
For bouncing cosmologies, a fine set of parameters is introduced in order to describe the nearly matter dominated phase, and which play the same role that the usual slow-roll parameters play in inflationary cosmology. It is shown that, as in the inflation case, the spectral index and the running parameter for scalar perturbations in bouncing cosmologies can be best expressed in terms of these small parameters. Further, they explicitly exhibit the duality which exists between a nearly matter dominated Universe in its contracting phase and the quasi de Sitter regime in the expanding one. The results obtained also confirm and extend the known evidence that the spectral index for a matter dominated Universe in the contracting phase is, in fact, the same as the spectral index for an exact Sitter regime in the expanding phase. Finally, in both the inflationary and the matter bounce scenarios, the theoretical values of the spectral index and of the running parameter are compared with their experimental counterparts, obtained from the most recent PLANCK data, with the result that the bouncing models here discussed do fit well accurate astronomical observations.
14 pages.

http://arxiv.org/abs/1412.8195
Loop Quantum Cosmology Matter Bounce Reconstruction from F(R) Gravity Using an Auxiliary Field
V.K. Oikonomou
(Submitted on 28 Dec 2014)
Using the reconstruction technique with an auxiliary field, we investigate which F(R) gravities can produce the matter bounce cosmological solutions. Owing to the specific functional form of the matter bounce Hubble parameter, the reconstruction technique leads, after some simplifications, to the same Hubble parameter as in the matter bounce scenario. Focusing the study to the large and small cosmic time t limits, we were able to find which F(R) gravities can generate the matter bounce Hubble parameter. In the case of small cosmic time limit, which corresponds to large curvature values, the F(R) gravity is F(R)∼R+αR2, which is an inflation generating gravity, and at small curvature, or equivalently, large cosmic time, the F(R) gravity generating the corresponding limit of the matter bounce Hubble parameter, is F(R)∼1/R, a gravity known to produce late-time acceleration. Thus we have the physically appealing picture in which a Jordan frame F(R) gravity that imitates the matter bounce solution at large and small curvatures, can generate Starobinsky inflation and late-time acceleration. Moreover, the scale factor corresponding to the reconstruction technique coincides almost completely to the matter bounce scenario scale factor, when considered in the aforementioned limiting curvature cases. This is scrutinized in detail, in order to examine the validity of the reconstruction method in these limiting cases, and according to our analysis, exact agreement is achieved.

Also as regards theme #1 ("algebraic geomatter") Alain Connes gave a 2 hour presentation of the Connes Chamseddine Mukhanov synthesis of gravity and the Standard Model at the Hausdorff Institute of Mathematics, Nicoo shared the YouTube link with us, for which thanks!
 
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  • #28
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Theme #2 (recall the HHKR paper, LQG with constant curvature simplexes to embody Λ)!
The authors here, in their acknowledgement, thank Haggard and Han, and say the present work was directly inspired by the intrinsic curved simplices approach developed by HHKR to embody the cosmological [curvature] constant.
http://arxiv.org/abs/1502.00278
Compact phase space, cosmological constant, discrete time
Carlo Rovelli, Francesca Vidotto
(Submitted on 1 Feb 2015)
We study the quantization of geometry in the presence of a cosmological constant, using a discretization with constant-curvature simplices. Phase space turns out to be compact and the Hilbert space finite dimensional for each link. Not only the intrinsic, but also the extrinsic geometry turns out to be discrete, pointing to discreetness of time, in addition to space. We work in 2+1 dimensions, but these results may be relevant also for the physical 3+1 case.
6 pages

This will definitely be presented and discussed at the EFI Winter School meeting in the mountains at Tux Austria this month (13-20 Feb).

Fun paper. A lot going on here. Could turn out to be one of 2015's most important papers. Since the cosmological constant is a small CURVATURE, a kind of inherent "vacuum curvature"which geometry has, it is not proper to represent it in a theory with FLAT simplexes, the authors say. So they employ Haggard Han Kaminski Riello formulation (http://arxiv.org/abs/1412.7546) that uses simplices with a slight uniform built-in curvature. The cosmological constant becomes inherent in the tools. That is what theme #2 was about. It is nice to see a paper following this lead so soon!
 
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  • #29
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This is an amazing paper. It is conceptually not too complicated but it pulls together a number of ideas: a minimal nonzero measurable angle, a minimal measurable time interval, a maximal acceleration. Also a kind of conjugate momentum of the geometry so that one can describe a phase space namely the state of the geometry together with its conjugate momentum, whereupon this phase space turns out to be compact. The authors say to expect a related paper by Haggard Han Kaminski Riello to appear soon, dealing with the dimension 3+1 case. Vidotto will be giving a talk based on this "compact phase space" paper this month at the EFI Winter School
http://www.gravity.physik.fau.de/events/tux3/tux3.shtml
http://www.gravity.physik.fau.de/events/tux3/program.pdf
 
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A key thing about the "Compact Phase Space, Cosmological Constant" paper is this reference to a May 2014 paper by Nozari et al. The authors initials are NGHV. It could be important, because introducing Λ into quantum spacetime geometry leads to a compact phase space:
http://arxiv.org/abs/1405.4083
Natural Cutoffs via Compact Symplectic Manifolds
Kourosh Nozari, M. A. Gorji, V. Hosseinzadeh, B. Vakili
(Submitted on 16 May 2014)
It is well-known that the ultraviolet and infrared natural cutoffs can be realized from a noncommutative algebra in phase space. In this paper we propose a noncanonical structure on a symplectic manifold that generates a noncommutative algebra. According to the Darboux theorem, one can always find a local chart in which any structure takes the canonical form as the corresponding algebra becomes commutative. Therefore, the local noncommutativity cannot be treated as the origin of the natural cutoffs. We find that symplectic manifolds with compact topology are naturally cutoff-regularized, independent of the chart in which the physical system is considered. By taking this fact and universality of quantum gravity effects into account, we conclude that the cutoffs should be global (topological) properties of the symplectic manifold. Finally, we justify our claim by considering three well-known examples of the deformed phase space: the Moyal, Snyder and polymerized phase spaces.
9 pages,
 

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