LQG and the Big Bounce: Does Accelerating Expansion Matter?

[TheSwager]

This may have been posted somewhere else but, I would like to know.

How do LQG theorists reconcile the fact that the universe is expanding at an accelerating rate,
and the LQG prediction of the big bounce. Also I have heard that they say the universe is still expanding because of inertia from the last "bounce", how do they explain the acceleration or do they ignore it.

thanks

 

[marcus]

in a typical Loop quantum cosmology (LQC) model there is one bounce
and soon after the bounce the quantum model converges to the standard cosmology, so it fits the observational data.it is a common misconception that a bounce cosmology needs to have several bounces if it has one, but it doesn't

you can play with the parameters so that you can get repeated bounces, but then it will not fit our universe---the astronomical observations---so well.
In the research literature they study various cases including ones that don't fit our universe, but you can learn from them too.

the best LQC match to reality is basically the case where you have standard classical Lambda CDM (LCDM) cosmology, which has a cosmological constant Lambda so you get accelerating expansion at the present time, but the LCDM model is put on a quantum basis so there is no singularity.
In that case, as I suggested earlier, what you get is a classical spacetime contracting, leading to a bounce, and then expanding (and matching our observed reality).

If you want to delve into the professional research literature, here is a search engine link that will get the most cited quantum cosmology papers published since 2005.
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=find+dk+quantum+cosmology+and+date+%3E+2004&FORMAT=WWW&SEQUENCE=citecount%28d%29
there you will find both LQC and stringy papers. And among the LQC you will find some that study the "k=1" repeated bounce case, and others that study the one-bounce case. Often they run computer simulations of the quantum universe doing a bounce. Let me know if you want specific papers recommended. The PDFs are all free online so it is easy to explore and check out the current research.

 

[TheSwager]

thanks

 

[marcus]

absolutely! it's an exciting field. lots going on right now.
keep going back to the Spires search engine every few months
you can set the order preferences so it puts most recent papers at the top, instead of the most highly cited, so it can help you keep up with what's going on

there are also video lectures in Loop quantum gravity, if you want.
one that comes to mind was the LQG basics for string theorists talk that Carlo Rovelli gave at this year's String-2008 conference. We could probably come up with several others, maybe more focused on cosmology and the bounce---but I can't think of anything introductory right at the moment.

 

[TheSwager]

LQG basics for string theorists

where is that video i could not find it?

 

[marcus]

where is that video i could not find it?

I put the link in this post
https://www.physicsforums.com/showthread.php?p=1851114#post1851114
(it is post #62 in a general QG news thread)

that was back in August, right after the annual Strings conference. I don't know if the video link still works, so i will try it.

Yes! the video still works. It is a good idea to download the slides first and have them ready to scroll through or page through as you watch the video, because in the video the slides projected in the background are not so legible.
Video:
http://cdsweb.cern.ch/record/1121957?ln=en
Slides:
http://indico.cern.ch/getFile.py/access?contribId=30&resId=0&materialId=slides&confId=21917

 

[Max™]

There is a way to reconcile an expanding universe with a bounce model.Think about it for a second and consider: what within an expanding universe would be contracting to an extreme state?
A black hole!

"Um, Max, there are billions of black holes inside of this Universe, probably. Are you saying they are all new Universes?"

Not yet, wait for this Universe to experience a heat death. Then I suspect all of the black holes (now without event horizons, WOO! TOPLESS SPACETIME DEFECTS BARELY LEGAL) will do what any region of extremely compressed and stressed spacetime would do.

Unfurl dramatically.

 

[Coin]

in a typical Loop quantum cosmology (LQC) model there is one bounce
and soon after the bounce the quantum model converges to the standard cosmology, so it fits the observational data.

The big bounce is required by LQC?

Is LQC required to be true if LQG is true or are they different in some way?

 

[francesca]

Is LQC required to be true if LQG is true or are they different in some way?

The answer is no, in the following sense. The relationship between LQG and LQC is still under discussion. There have been some deep mathematical work to clarify how LQC is embedded in the full theory (I refer in particular to Fleishhack and Brunnemann works: On the Configuration Spaces of Homogeneous Loop Quantum Cosmology and Loop Quantum Gravity, ArXiv 0709.1621). Consider that there are two ways to end up to a quantum cosmological theory: first symmetry reduction and then quantization of the survived degrees of freedom, otherwise take a full quantum theory of gravity and select a cosmological sector (defining what is the symmetric sector of the theory, see Engle works). In every case, following both these paths, one would like to end up with the same theory (technically this is possible only having almost periodic functions).
Usual LQC take the first point of view (reduction-->quantization) as it has been usual in quantum cosmology starting from Wheeler-DeWitt-Misner works. One should note that doesn't exist just one theory derived from Loop quantization, but a forest of Loop-inspired cosmologies: metric variables quantization, polymer quantization, lattice refinement... Corichi and Singh have recently shown that the only quantization that gives viable physical results is the so-called improved $\bar\mu$-quantization.
Ok, let me cite a tentative starting from the full theory to recover quantum cosmology by Rovelli and Vidotto .

The big bounce is required by LQC?

It's not required but it's a general feature appearing when a fundamental discreteness is introduce. Actually what is quite general is just the remotion of the singularity, while one should define exactly what it's meant by bounce. With bounce we mean also some conditions on the dynamics, encoded in the scale factor and in its derivative. So the answer is again no.

 

[francesca]

how do they explain the acceleration or do they ignore it.

LQG and inflation are currently studied by the Quantum Cosmology Group in Notthingam:

http://arxiv.org/abs/0708.1261"
*Super-inflation in Loop Quantum Cosmology*
E. J. Copeland, D. J. Mulryne, N. J. Nunes, M. Shaeri
(Submitted on 9 Aug 2007) 11 pages, 2 figures
Abstract:
We investigate the dynamics of super-inflation in two versions of Loop Quantum Cosmology, one in which the Friedmann equation is modified by the presence of inverse volume corrections, and one in which quadratic corrections are important. Computing the tilt of the power spectrum of the perturbed scalar field in terms of fast-roll parameters, we conclude that the first case leads to a power spectrum that is scale invariant for steep power law negative potentials and for the second case, scale invariance is obtained for positive potentials that asymptote to a constant value for large values of the scalar field. It is found that in both cases, the horizon problem is solved with only a few e-folds of super-inflationary evolution.


http://arxiv.org/abs/0810.0104
*The gravitational wave background from super-inflation in Loop Quantum Cosmology*
E. J. Copeland, D. J. Mulryne, N. J. Nunes, M. Shaeri
(Submitted on 1 Oct 2008)8 pages, 3 figures
Abstract:
We investigate the behaviour of tensor fluctuations in Loop Quantum Cosmology, focusing on a class of scaling solutions which admit a near scale-invariant scalar field power spectrum. We obtain the spectral index of the gravitational field perturbations, and find a strong blue tilt in the power spectrum with <a href="javascript:;" onClick="latexPopup('tex', 'n_t \\approx 2')"><img src="latex_images/preview0-2.png" border=0 align=top alt='LaTeX Code: n_t \\approx 2 '></a>. The amplitude of tensor modes are, therefore, suppressed by many orders of magnitude on large scales compared to those predicted by the standard inflationary scenario where <a href="javascript:;" onClick="latexPopup('tex', 'n_t \\approx 0')"><img src="latex_images/preview0-3.png" border=0 align=top alt='LaTeX Code: n_t \\approx 0 '></a>.