Loop Quantum Gravity: Zurück vor den Urknall" by Martin Bojowald

[atyy]

See the discussion beginning just before p13 of http://arxiv.org/abs/gr-qc/0702030 .

I guess the question should be is there time if there is no matter?

 

[marcus]

RUTA, The pointer that Atyy posted to page 13 of that Ashtekar article is apt to be just the right thing. I can't think of a better.

So the talks you attended were at the Institute for Gravitation and the Cosmos (IGC) Thanks for the link! I will add that to the list of links to be periodically checked for new source material, which I keep handy in the thread called "Introduction to LQG".
Here are past workshops and conferences held at IGC
http://www.gravity.psu.edu/events/workshops.shtml

I was glad to hear that they were recording the speakers for posting online. This has been the practice with other workshops on the above list.

For example if you click on "Abhayfest" you get the program for this summer's celebration of Ashtekar's 60th birthday, and there are links to slides and audio for the speakers. Hopefully the same will happen with the 2-day workshop you attended.

So I will check that list from time to time and see what talks they have online.

 

[marcus]

Here's a brief excerpt from the Introduction to Bojowald's book. It includes part of what I think is a key paragraph where he sets out what he will do in the rest of the book.

==quote from original German edition of Before the Big Bang==
...Was wir aber vor allem in den letzten Jahren gesehen haben, sind zahlreiche vielversprechende Indizien für ihre Eigenschaften, die bereits analysiert werden können. Die Situation, wie so oft in der Forschung, gleicht dem Anfangsstadium eines Puzzle-Spiels, in dem man das endgültige Bild vielleicht teilweise erahnen kann, dennoch aber auch auf einem Irrweg sein könnte. Unser derzeitiges Bild deutet an, was eine Vervollständigung der physikalischen Theorie bewerkstelligen kann: Sie erlaubt uns zu sehen, was während und sogar vor dem Urknall geschehen sein könnte. Wir erhalten Einblick in die früheste Urzeit unseres Universums und können erstmals analysieren, wie es wohl entstand.

In diesem Buch werden sowohl jüngste Resultate der Theorie als auch für die nähere Zukunft geplante Beobachtungen I am Weltraum erläutert, und es wird gezeigt, wie radikal sie unser Weltbild verändern können. Insbesondere mit der Schleifen-Quantengravitation, eine der Varianten, die derzeit für eine Kombination von Allgemeiner Relativitätstheorie und Quantentheorie gehandelt werden, sind Ansätze für eine nichtsinguläre Beschreibung des Urknalls erzielt worden. In diesem Rahmen existierte das Universum schon vor dem Urknall, und es lässt sich grob abschätzen, wie es sich damals in seinen Eigenschaften von den jetzigen unterschieden haben könnte. Durch den Einfluss auf spätere Phasen der kosmischen Expansion, die empfindlichen Beobachtungen offenstehen, kann man diese Urgeschichte des Universums untersuchen.
==endquote==

I'll try to translate parts of this, as a help in case any reader's knowledge is even more rudimentary than mine. Help would be welcome if anyone who knows the language wants to volunteer.

This book will describe not only of the most recent theoretical results but also the observations in space planned for the near future---and it will show how radically they could change our picture of the world. In the case of loop quantum gravity, one of the current approaches combining General Relativity with Quantum Theory, assumptions allow for a nonsingular description of the Big Bang. In this framework, the universe existed before the Big Bang, and one can roughly estimate how its characteristics then may have differed from those at present. Through its influence on subsequent phases of cosmic expansion, as detected by sensitive instruments, one can probe the prehistory of the universe.

 

[atyy]

He is going to describe how the effect of the existence of the universe before the big bang would have consequences that remain observable to this day - and which would distinguish such a theory from our current best.

Predictions! Really?

 

[marcus]

Atyy, I've been discussing this in the present thread, and others. Here's post #47 for example.
Any statement about the past has to be based on a model of time-evolutiuon that one can test (in some way) in the present.
The LQC model happens to go back before the start of expansion but the problem of validating it is like that with other models that evolve back into the past.

One has to derive predictions--e.g. of what future CMB space missions will find by observing the CMB at higher resolution (with polarization).

One has to say "if this reconstruction of the past, which goes back before expansion started, is correct, then Planck spacecraft will see this and this footprint". And then if the predicted features of the CMB are not found, the model is falsified.

This is the only way we have of seeing back into the past---construct a model and then test the model by observations made in the present.

So this is what people like Jack Mielczarek are doing for us---trying to derive predictions from the LQC model which will hopefully distinguish it and allow it to be falsified if it is wrong. And people like Aurelien Barrau and Julien Grain, whose papers I listed in the post here:

Earlier several questions were raised about the prospects for testable predictions from Loop Cosmology, which at least for now boils down to looking for "footprints in the CMB".

Several papers about this, from 2008 and 2009, were mentioned. The most recent ones were by Jack Mielczarek (I think the name is pronounced *myel-cha-rek*) and by two co-authors Aurelien Barrau and Julien Grain.

The Barrau-Grain "LQG footprint" paper gives an idea of the current status of efforts to work out tests for LQC. This week a follow-up appeared, a solo paper by Julien Grain. Here is a sample of Grain's recent work:

1. arXiv:0911.1625
Loop Quantum Cosmology corrections on gravity waves produced during primordial inflation
J. Grain
Comments: to be published in the AIP Proceedings of the 'Invisible Universe International Conference', UNESCO-Paris, June 29-July 3, 2009; 9 pp., 4 Figs

2. arXiv:0910.2892
Fully LQC-corrected propagation of gravitational waves during slow-roll inflation
J. Grain, T. Cailleteau, A. Barrau, A. Gorecki
Comments: 9 pages, submitted for publication to Phys. Rev. D

3. arXiv:0903.2350
Polarized CMB power spectrum estimation using the pure pseudo-cross-spectrum approach
J. Grain, M. Tristram, R. Stompor
Comments: 31 pages, 24 figures, typos corrected on Eq. 32, Appendix C clarified, published in Physical Review D
Journal-ref: Phys.Rev.D79:123515,2009

4. arXiv:0902.3605
Inverse volume corrections from loop quantum gravity and the primordial tensor power spectrum in slow-roll inflation
J. Grain, A. Barrau, A. Gorecki
Comments: 15 pages, 5 figures, published version with minor modifications, results unchanged
Journal-ref: Phys.Rev.D79:084015,2009

5. arXiv:0902.0145 [ps, pdf, other]
Cosmological footprints of loop quantum gravity
J. Grain, A. Barrau
Comments: Accepted by Phys. Rev. Lett., 7 pages, 2 figures
Journal-ref: Phys.Rev.Lett.102:081301,2009

I have highlighted the titles having to do with Loop Quantum Gravity, or with Loop Quantum Cosmology. Basically this has become an active area of CMB phenomenology. Researchers apparently have sensed a possibility of deriving predictions which can be tested by CMB observatories such as the ESO's Planck spacecraft that was launched this year and is recording data. So far I have only seen qualitative predictions, however, not hard numbers.

Bojowald has also been working on this problem---how to test LQC by observations of the microwave background---according to some quoted statements I've seen. He also has some papers attempting to put bounds on how much we can actually tell about pre-bang conditions---assuming the LQC model is right. It's not deterministic, after all, so there appear to be severe limits (but there has been some controversy about this---Bojowald has been on the pessimistic side.)

 

[atyy]

Atyy, I've been discussing this in the present thread, and others. Here's post #47 for example.

Yes, I'm aware of that. I was wondering if Bojowald's book itself said the same thing.

 

[marcus]

Yes, I'm aware of that. I was wondering if Bojowald's book itself said the same thing.

Well it's "through a glass, darkly" because German. I think the untranslated passage I just quoted gives some idea of the tone of his statements---a certain degree of caution and qualification. So I'll attempt another snippet of translation:

"Above all, what we've seen in the last few years has been numerous significant indications of its characteristics which can actually be analyzed. The situation, as so often happens in research, is like the beginning stages of solving a jig-saw puzzle---in which one might perhaps guess the final picture, but nevertheless could still be on the wrong track.

Our current picture indicates what a completion of the physical theory can accomplish: It allows us to see what could have happened during and even before the Big Bang. We obtain insight into our universe's earliest prehistory and for the first time are able to analyze how it actually arose."

====That was just a rough translation so I'll repeat the original for comparison====
...Was wir aber vor allem in den letzten Jahren gesehen haben, sind zahlreiche vielversprechende Indizien für ihre Eigenschaften, die bereits analysiert werden können. Die Situation, wie so oft in der Forschung, gleicht dem Anfangsstadium eines Puzzle-Spiels, in dem man das endgültige Bild vielleicht teilweise erahnen kann, dennoch aber auch auf einem Irrweg sein könnte.

Unser derzeitiges Bild deutet an, was eine Vervollständigung der physikalischen Theorie bewerkstelligen kann: Sie erlaubt uns zu sehen, was während und sogar vor dem Urknall geschehen sein könnte. Wir erhalten Einblick in die früheste Urzeit unseres Universums und können erstmals analysieren, wie es wohl entstand.
==endquote==

 

[marcus]

He is going to describe how the effect of the existence of the universe before the big bang would have consequences that remain observable to this day - and which would distinguish such a theory from our current best.

Predictions! Really?

Well in terms of the amount of research being done and the citations to it, objective criteria in other words, I guess you could say that the LQC that Bojowald is talking about IS our current best theory of how our expanding universe got started.

At least if by "current" you mean "since 2005" or so.

Folks have been working on the LQC bounce picture since 2001. It does appear able to make predictions about what we can observe in the present (e.g. with the Planck spacecraft ) which would distinguish it from our current NEXT best. Or next next best, or in any case from various alternatives, if the alternatives made any predictions.

The alternatives are things like Linde's eternal inflation, Steinhardt's brane clash, Hawking's ideas of the 1980s, Veneziano stringy "pre-big-bang" scenario of the 1990s. The alternatives to LQC either are not currently being much worked on, or are rather nebulous and don't make much in the way of prediction.

To recall what Bojowald says about it, I'll put the two snippets of translation together as they appear in the introduction.

==quote introduction==
...Above all, what we've seen in the last few years has been numerous significant indications of its characteristics which can actually be analyzed. The situation, as so often happens in research, is like the beginning stages of solving a jig-saw puzzle---in which one might perhaps guess the final picture, but nevertheless could still be on the wrong track.

Our current picture indicates what a completion of the physical theory can accomplish: It allows us to see what could have happened during and even before the Big Bang. We obtain insight into our universe's earliest prehistory and for the first time are able to analyze how it actually arose.

This book will describe not only of the most recent theoretical results but also the observations in space planned for the near future---and it will show how radically they could change our picture of the world. In the case of loop quantum gravity, one of the current approaches combining General Relativity with Quantum Theory, assumptions allow for a nonsingular description of the Big Bang. In this framework, the universe existed before the Big Bang, and one can roughly estimate how its characteristics then may have differed from those at present. Through its influence on subsequent phases of cosmic expansion, as detected by sensitive instruments, one can probe the prehistory of the universe.
==endquote==
Here's the spires search for "quantum cosmology" papers since 2005:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=dk+quantum+cosmology+and+date%3E2005&FORMAT=WWW&SEQUENCE=citecount%28d%29

If you look at the top 50 or 75 papers they are almost all LQC. There are a couple by Hawking and Hartle that made it into the top 75 (they are numbers 57 and 60). Very few Steinhardt ("ekpyrotic" or "cyclic" brane clash) papers.
You may want to expand the search and see what is going on in more detail.

I think objectively what we have here is arguably "current best". I'll post the German original in case anyone wants to check the translation against it.

==quote from original German edition of Before the Big Bang==
...Was wir aber vor allem in den letzten Jahren gesehen haben, sind zahlreiche vielversprechende Indizien für ihre Eigenschaften, die bereits analysiert werden können. Die Situation, wie so oft in der Forschung, gleicht dem Anfangsstadium eines Puzzle-Spiels, in dem man das endgültige Bild vielleicht teilweise erahnen kann, dennoch aber auch auf einem Irrweg sein könnte. Unser derzeitiges Bild deutet an, was eine Vervollständigung der physikalischen Theorie bewerkstelligen kann: Sie erlaubt uns zu sehen, was während und sogar vor dem Urknall geschehen sein könnte. Wir erhalten Einblick in die früheste Urzeit unseres Universums und können erstmals analysieren, wie es wohl entstand.

In diesem Buch werden sowohl jüngste Resultate der Theorie als auch für die nähere Zukunft geplante Beobachtungen I am Weltraum erläutert, und es wird gezeigt, wie radikal sie unser Weltbild verändern können. Insbesondere mit der Schleifen-Quantengravitation, eine der Varianten, die derzeit für eine Kombination von Allgemeiner Relativitätstheorie und Quantentheorie gehandelt werden, sind Ansätze für eine nichtsinguläre Beschreibung des Urknalls erzielt worden. In diesem Rahmen existierte das Universum schon vor dem Urknall, und es lässt sich grob abschätzen, wie es sich damals in seinen Eigenschaften von den jetzigen unterschieden haben könnte. Durch den Einfluss auf spätere Phasen der kosmischen Expansion, die empfindlichen Beobachtungen offenstehen, kann man diese Urgeschichte des Universums untersuchen.
==endquote==