Is the Cyclic Model of the Universe a Viable Alternative to the Big Bang Theory?

[Naty1]

Paul Steinhardt and Neil Turok, Princeton and Cambridge, respectively, explain their new cyclic model of the universe in THE ENDLESS UNIVERSE, 2007. Like most on this forum, I took the big bang and subsequent inflation as the best explanation for how this universe got started. I now see these guys have a great alternative THEORY; Here's a quick overview.

Perhaps the biggest surprise is that by combining Heterotic M theory (with it's two parallel branes as envisioned by Witten and Horava), a cyclic universe, endlessly repeating, can be theorized in ten dimensional space between the branes. Dark energy plays three fascinating roles at different cycle stages: it speeds up the expansion rate, as currently observed, it acts as a stabilizer by acting as a shock absorber (analogous to an automatic pneumatic door close cylinder), and it decays over long periods, shutting itself off. Dark energy is dynamic! It means the universe does NOT have to expand forever.

One of the things I clearly don't understand is how the second law of thermodynamics (entropy must increase) is not violated. But they say:

It has been shown that a combination of branes and an extra dimension, with regular assists from gravity and dark energy can cause the universe to repeatedly replenish itself ...at regular intervals,,,without violating the second law of thermodynamics.

"..(Brane) collisions are unavoidable and fill the branes with hot but finite and nearly uniform radiation. ..Brane kinetic energy is converted to matter and radiation. Dark energy dominates before the bang, stretching the branes making them flat,smooth and parallel…

...A key assumption of the cyclic model is that dark energy can decay..smoothly and slowly. Dark energy is in mostly a “potential energy” form at the beginning of an expansion cycle and over time turns to a mix of potential and kinetic energy while its gravitational effect on space reverses…it acquires the properties of a high pressure gas that dominates the contracting phase of the cycle model and plays the same role of solving the homogeneity and flatness problems that inflationary energy does in an expanding universe….and a surprise is that the high pressure form of dark energy can acquire the scale invariant density variations before the final contraction which are of just the right scale invariant form…but almost opposite that of the inflationary model: slow contraction with high pressure dark energy versus rapid expansion with inflationary energy..."

So they envision a varying, evolving dark energy rather than patching together "inflation" at one stage with "dark energy" from another stage; and unlike inflationary theory, no fine tuning is required in their model, a big plus. So far, no experimental observations can detect any differences between their cyclic and the alternative inflationary model. I did not see any specific proposed experimental differences which could prove one model over the other.

 

[LURCH]

That does sound fascinating. I've got some time off coming, maybe I'll try and read that.

I don't know as I'll understand all of it, but I can suggest this; Entropy is not violated by any oscilating universe model. Entropy is the tendency of energy to spread itself out more and more evenly within the universe (and by "universe" I am referring to the current spacetime continuum). What happens after this universe ends and another begins is not within the scope of that law.

 

[Naty1]

Entropy is not violated by any oscilating universe model.

Acording to the authors, all prior cyclic models DID violate entropy...

 

[Naty1]

Opps, I missed a few pages at the back of the book: the key experimentally verifable difference between this cyclic model and inflationary models are gravitational waves. The spectrum of gravitational waves between the two models is very different. Long wavelength gravitational waves are minsicule compared with the inflationary model and are not scale invariant as are the inflationary model predictions.

Five other key tests are virtually identical between the two models...and experimental results so far cannot distinguish between the models.

 

[cristo]

the key experimentally verifable difference between this cyclic model and inflationary models are gravitational waves.

I was about to mention this, when I read above that you didn't find any prediction different between standard inflationary cosmology and the ekpyrotic model. Namely, the amplitude of the spectrum of primordial tensor perturbations in the latter decreases as the wavelength increases. So, there are ways to tell the two models apart.

 

[gabrielh]

I'll definitely be adding this to my reading list. I do have a question though. Does the book address the issue of when this type of Cyclic Universe began, as in, when was the first universe made? Or has this been an infinite cycle?

 

[marcus]

Just for a reality check, almost no research on cyclic/ekpyrotic besides stuff by Steinhardt/Turok and their co-authors, so we can get a pretty good listing of the well-cited papers, at least, just by searching for Steinhardt, and ordering the list by cites.
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+STEINHARDT+AND+DATE+%3E+2001&FORMAT=www&SEQUENCE=citecount%28d%29

I have selected out Steinhardt's cyclic/ekpyrotic papers as best I could in a quick pass. I will highlight the publication year to give a rough notion of the timeframe. Maybe I could do this smarter and cleaner but anyway here goes:

2) A cyclic model of the universe.
P.J. Steinhardt (Princeton U., Plasma Physics Lab.) , N. Turok (Newton Inst. Math. Sci., Cambridge) . 2002.
Published in Science 296:1436-1439,2002.
Cited 148 times

4) Cosmological perturbations in a big crunch / big bang space-time.
Andrew J. Tolley, Neil Turok (Cambridge U., DAMTP) , Paul J. Steinhardt (Princeton U.) . Jun 2003. 47pp.
Published in Phys.Rev.D69:106005,2004.
e-Print: hep-th/0306109
Cited 88 times

6) Designing cyclic universe models.
Justin Khoury (ISCAP, New York) , Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP) . Jul 2003. 4pp.
Published in Phys.Rev.Lett.92:031302,2004.
e-Print: hep-th/0307132
Cited 69 times

9) Conditions for generating scale-invariant density perturbations.
Steven Gratton (Princeton U.) , Justin Khoury (Princeton U. & Columbia U., Astron. Astrophys.) , Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP) . Jan 2003. 6pp.
Published in Phys.Rev.D69:103505,2004.
e-Print: astro-ph/0301395
Cited 64 times

11) Kasner and mixmaster behavior in universes with equation of state w >= 1.
Joel K. Erickson, Daniel H. Wesley (Princeton U.) , Paul J. Steinhardt (Princeton U. & Princeton, Inst. Advanced Study) , Neil Turok (Cambridge U., DAMTP) . Dec 2003. 25pp.
Published in Phys.Rev.D69:063514,2004.
e-Print: hep-th/0312009
Cited 58 times

14) Inflation versus cyclic predictions for spectral tilt.
Justin Khoury (Columbia U.) , Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP) . Feb 2003. 4pp.
Published in Phys.Rev.Lett.91:161301,2003.
e-Print: astro-ph/0302012
Cited 48 times

15) M theory model of a big crunch / big bang transition.
Neil Turok, Malcolm Perry (Cambridge U., DAMTP) , Paul J. Steinhardt (Princeton U. & Princeton, Inst. Advanced Study) . Aug 2004. 39pp.
Published in Phys.Rev.D70:106004,2004, Erratum-ibid.D71:029901,2005.
e-Print: hep-th/0408083
Cited 48 times

16) The Cosmic gravitational wave background in a cyclic universe.
Latham A. Boyle, Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP.) . Jul 2003. 4pp.
Published in Phys.Rev.D69:127302,2004.
e-Print: hep-th/0307170
Cited 44 times

17) Generating ekpyrotic curvature perturbations before the big bang.
Jean-Luc Lehners (Cambridge U., DAMTP) , Paul McFadden (Amsterdam U.) , Neil Turok (Cambridge U., DAMTP) , Paul J. Steinhardt (Princeton U.) . DAMTP-2007-14, ITFA-2007-12, Feb 2007. 21pp.
Published in Phys.Rev.D76:103501,2007.
e-Print: hep-th/0702153
Cited 43 times

20)The Cyclic model simplified.
Paul J. Steinhardt (Princeton U. & Princeton, Inst. Advanced Study) , Neil Turok (Cambridge U.) . Apr 2004. 12pp.
Published in New Astron.Rev.49:43-57,2005.
e-Print: astro-ph/0404480
Cited 40 times

 

[Naty1]

Marcus,
thanks for the references...this current incarnation of a cyclic model via an ekpyrotic model is rather new but feedback at major conferences is beginning to address it:

In the WMAP feedback conference in his section of reporting experimental results Lyman Page (I think one of the WMAP project leaders) announced " The pattern we have detected is pure E-mode, consistent with both the inflationary and ekpyrotic predictions." Other results ruled out a number of promising inflationary models, but many possibilities remain. The tests apparently showed that further refinements via new technology COULD distinguish between the inflationary and ekpyrotic models.

The authors descriptions reflect that a number of components of their ekpyrotic model have resulted from mathematical formulations developed as long as 25 years ago which failed to fit inflationary models. These authors have previously published papers on inflationary models, models which they assert have ultimately now led to a very unsatisfactory cosmological projections, and have caused them to seek better cosmological models.

 

[marcus]

...this current incarnation of a cyclic model via an ekpyrotic model is rather new but feedback at major conferences is beginning to address it:
...

I'm confused by what you say.
http://en.wikipedia.org/wiki/Ekpyrotic_universe

My impression is that the ekpyrotic universe is OLDER not newer. My impression is that Steinhardt initially christened his "brane clash" scenario Ekpyrotic.

Then either he just renamed it Cyclic, or there was some minor change in the model, perhaps a generalization, that necessitated the new term.

For what it's worth, Wikipedia says:
"The ekpyrotic model is a precursor to, and part of the cyclic model."

Please let me know if Wiki and I are wrong.

=====================

I'd be curious to see the discussion you referred to where Lyman Page said something.
Do you have a link?

======================

Marcus,
thanks for the references...

You are welcome! I hope you realize that I wasn't suggesting you read that stuff, or even check out the one paragraph abstracts! The point I was trying to make is that at this point Ekpyrotic/Cyclic has been around for quite a few years and is getting a bit stale. Look at the dates of the papers. I highlighted the years on them.

The heyday, the sort of peak interest, was, I think, around 2002-2005.

I think (I haven't checked) that 2002-2005 is the time where you would find Ekpyotic/Cyclic being addressed in major conferences. And I mean major, international conferences with several hundred participants stretching over a work-week, like GR16 (2004), and Strings 2003, Strings 2004, and like the Marcel Grossmann meetings of 2003 and 2006. If not these, then some equally major, in the same 2002-2005 frame.

Of course interest could pick up again! But I don't think Lyman Page mentioning Ekpyrotic counts for much. I'd have to see a new batch of highly cited papers. Which of course could happen!

For now, when I look at Steinhardt's output, I see his interest shifting into other channels and in other directions. One thing is is clear about though, he doesn't like rolled up extra dimensions! He doesn't like the anthropic principle and the string landscape. He's definitely not inclined to accept the Multiverse.
The question is how do you combat those things. It looked for a while as if promoting the Ekpyrotic/Cyclic model was an effective way to combat Inflation (and its attendent Bubble-Multiverse disease.) But maybe that has become less urgent, or he has found another research focus.

 

[marcus]

The earliest version seems to have been called Ekpyrotic, then apparently they made some improvement and for some reason renamed it Cylic. I missed the big 2001 Ekpyrotic papers in my previous listing. So here is a more complete list, you get by searching for Steinhardt, ordering the list by cites, and selecting out only the brane-clash cosmo papers.
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+STEINHARDT+AND+DATE+%3E+2000&FORMAT=www&SEQUENCE=citecount%28d%29


1) The Ekpyrotic universe: Colliding branes and the origin of the hot big bang.
Justin Khoury (Princeton U.) , Burt A. Ovrut (Pennsylvania U.) , Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP) . Mar 2001. 63pp.
Published in Phys.Rev.D64:123522,2001.
e-Print: hep-th/0103239
Cited 562 times

3) From big crunch to big bang.
Justin Khoury (Princeton U.) , Burt A. Ovrut (Pennsylvania U.) , Nathan Seiberg (Princeton, Inst. Advanced Study) , Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP) . Aug 2001. 16pp.
Published in Phys.Rev.D65:086007,2002.
e-Print: hep-th/0108187
Cited 325 times

4) Cosmic evolution in a cyclic universe.
Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP) . Nov 2001. 49pp.
Published in Phys.Rev.D65:126003,2002.
e-Print: hep-th/0111098
Cited 309 times

5) Density perturbations in the ekpyrotic scenario.
Justin Khoury (Princeton U.) , Burt A. Ovrut (Pennsylvania U.) , Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP) . Sep 2001. 36pp.
Published in Phys.Rev.D66:046005,2002.
e-Print: hep-th/0109050
Cited 166 times

6) A cyclic model of the universe.
P.J. Steinhardt (Princeton U., Plasma Physics Lab.) , N. Turok (Newton Inst. Math. Sci., Cambridge) . 2002.
Published in Science 296:1436-1439,2002.
Cited 148 times

9) A Cyclic model of the universe.
Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP) . Nov 2001. 19pp.
e-Print: hep-th/0111030
Cited 89 times

10) Cosmological perturbations in a big crunch / big bang space-time.
Andrew J. Tolley, Neil Turok (Cambridge U., DAMTP) , Paul J. Steinhardt (Princeton U.) . Jun 2003. 47pp.
Published in Phys.Rev.D69:106005,2004.
e-Print: hep-th/0306109
Cited 88 times

12) Designing cyclic universe models.
Justin Khoury (ISCAP, New York) , Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP) . Jul 2003. 4pp.
Published in Phys.Rev.Lett.92:031302,2004.
e-Print: hep-th/0307132
Cited 69 times

15) Conditions for generating scale-invariant density perturbations.
Steven Gratton (Princeton U.) , Justin Khoury (Princeton U. & Columbia U., Astron. Astrophys.) , Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP) . Jan 2003. 6pp.
Published in Phys.Rev.D69:103505,2004.
e-Print: astro-ph/0301395
Cited 64 times

18) Kasner and mixmaster behavior in universes with equation of state w >= 1.
Joel K. Erickson, Daniel H. Wesley (Princeton U.) , Paul J. Steinhardt (Princeton U. & Princeton, Inst. Advanced Study) , Neil Turok (Cambridge U., DAMTP) . Dec 2003. 25pp.
Published in Phys.Rev.D69:063514,2004.
e-Print: hep-th/0312009
Cited 58 times

23) Inflation versus cyclic predictions for spectral tilt.
Justin Khoury (Columbia U.) , Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP) . Feb 2003. 4pp.
Published in Phys.Rev.Lett.91:161301,2003.
e-Print: astro-ph/0302012
Cited 48 times

24) M theory model of a big crunch / big bang transition.
Neil Turok, Malcolm Perry (Cambridge U., DAMTP) , Paul J. Steinhardt (Princeton U. & Princeton, Inst. Advanced Study) . Aug 2004. 39pp.
Published in Phys.Rev.D70:106004,2004, Erratum-ibid.D71:029901,2005.
e-Print: hep-th/0408083
Cited 48 times

25) The Cosmic gravitational wave background in a cyclic universe.
Latham A. Boyle, Paul J. Steinhardt (Princeton U.) , Neil Turok (Cambridge U., DAMTP.) . Jul 2003. 4pp.
Published in Phys.Rev.D69:127302,2004.
e-Print: hep-th/0307170
Cited 44 times

 

[Naty1]

Does the book address the issue of when this type of Cyclic Universe began, as in, when was the first universe made? Or has this been an infinite cycle?

Their model is based on unending, infinite, cycles...but the authors say nothing is inconsistent with it having a beginning...Like the inflationary model, I suspect a realistic starting mechanism has not been developed...

 

[Chronos]

A universe from 'nothing' [Sean Carroll] remains plausible, depending upon how you define 'nothing'.

 

[Naty1]

Posted by Marcus:

I'm confused by what you say.
http://en.wikipedia.org/wiki/Ekpyrotic_universe

My impression is that the ekpyrotic universe is OLDER not newer. My impression is that Steinhardt initially christened his "brane clash" scenario Ekpyrotic...

For what it's worth, Wikipedia says:
"The ekpyrotic model is a precursor to, and part of the cyclic model."

Please let me know if Wiki and I are wrong.

I am only reflecting my interpretation of the T&S book: for example in the preface of the 2007 edition they say " This new theory of a cyclic universe"...now what that REALLY means, if anything, I have no idea; perhaps they overcame a major prior objection. Or maybe it was poor editing. In various parts of the book they make reference to how detailed studies by the authors led to answers within the model that both met available observational evidence on one hand and NO STUDY REQUIRED SPECIAL FINE TUNING of the model on the other.

Beginning on page 130, he describes thermodynamic objections to "oscillatory" models and refers to papers exchanged between Richard Tolman and Albert Einstein. Tolman showed that higher temperatures after each successive bang results in a greater expansion rate than the prior cycle...and explains how oscillatory models were "killed again" in the 1990's when astronomical evidence showed matter density is not enough to cause cosmological collapse and the universe was found flat and presumably open...then new ideas emerged around 1995 that caused "a new kind of cyclic model" to be proposed. ..in the "new theory" only the extra dimension contracts...an indication I think he likes small dimensions for the writing of this book...
=====================

I'd be curious to see the discussion you referred to where Lyman Page said something.
Do you have a link?

======================


I missed this historical comment earlier: the authors a cite a March 16, 2006 WMAP conference in Europe as a "live or die moment for the cyclic model"...that's the one where Lyman Page made reference to both inflationary and ekpyrotic models...

The point I was trying to make is that at this point Ekpyrotic/Cyclic has been around for quite a few years and is getting a bit stale.

could be, but the authors appear VERY interested as of 2006...

The heyday, the sort of peak interest, was, I think, around 2002-2005...Of course interest could pick up again! But I don't think Lyman Page mentioning Ekpyrotic counts for much. I'd have to see a new batch of highly cited papers. Which of course could happen!

Could be, I have no perspective on this history of this thing. Because i have read a half dozen or so books which reflect cosmological models maybe I was misled into thinking its newer than it really is...but I think what may be going on here is not only oscillatory, cyclic, ekpyrotic models, and perhaps each with different incarnations within.

One thing is is clear about though, he doesn't like rolled up extra dimensions! He doesn't like the anthropic principle and the string landscape. He's definitely not inclined to accept the Multiverse.

Well in this writing he LIKES branes and extra dimensions; but did NOT appear to like the multiverse... because he/they think it would make scientific explanation impossible...if ALL possibilities exist, there would be no logical way to explain our own universe.

It looked for a while as if promoting the Ekpyrotic/Cyclic model was an effective way to combat Inflation (and its attendent Bubble-Multiverse disease.) But maybe that has become less urgent, or he has found another research focus.

I do not know...the single concept I found most interesting was variable dark energy which changes from potential to kinetic forms and also slowly decays...powered by energy exchanges with gravity...I had not seen that idea before and it's a cornerstone of their ekpyrotic model. He clearly positions the idea as an assumption, but for me it seemed worthwhile because it leads to finite "bangs" which naturally evolve into a universe with characteristics similar to inflation and our observable universe.

I have a question posted under Classical Physics asking how such energy exchanges might take place...but no responses yet...Cyclic Model of the Universe: Thermodynamics,
https://www.physicsforums.com/showthread.php?t=318009

 

[marcus]

Naty, thanks for the additional comment and detail! It helps considerably to fill out the picture and understand better where you are coming from on this.

In particular I was glad to get some context on the Lyman Page thing:

I missed this historical comment earlier: the authors a cite a March 16, 2006 WMAP conference in Europe as a "live or die moment for the cyclic model"...that's the one where Lyman Page made reference to both inflationary and ekpyrotic models...

They would have been discussing WMAP3 data (the three-year WMAP results).

The next cycle came in March 2008 when the WMAP5 data and preprints of a half-dozen or so reports were presented. The reports were commented and corrected and eventually published in early 2009. We could look at the relevant report and see what they say about cyclic this time around, if you are interested.

Each time around, each data cycle, they have had one particular report which addresses cosmology. In the WMAP3 case it was subtitled "Implications for Cosmology" and in the more recent case "Cosmological Interpretations".

For future reference, to get it you can google "komatsu WMAP cosmology".

Yeah notice Lyman Page is one of the co-authors, as likewise Ned Wright and Princeton's David Spergel. The preprint is here:
http://arXiv.org/abs/0803.0547
The co-authors contain a number of bigtime cosmo leaders (should also mention Charles Bennett, Joanna Dunkley).

So it would be good to look thru and see if they mention Steinhardt&Turok brane-clash model this time around.
======================

I note that in the previous cycle the main report DID mention ekpyrotic. That would be Spergel et al 2006
http://arxiv.org/abs/astro-ph/0603449
On page 45 they say:
"... to explore the implications of the data for inflationary models. The results of the third year analysis are consistent with the conclusions from the first year data: while the data rule out large regions of parameter space, there are also wide range of possible inflationary models consistent with our current data. One of the most intriguing features of Figure 14 is that the data now disfavors the exact Harrison-Zel’dovich-Peebles spectrum (n_s = 1, r = 0). For power law inflationary models, this suggests a detectable level of gravity waves. There are, however, many inflationary models that predict a much smaller gravity wave amplitude. Alternative models, such as the ekpyrotic scenario (Khoury et al. 2001, 2002) also predict an undetectable level of gravity waves."

The ekpyrotic/cyclic papers referenced were:
Khoury, J., Ovrut, B. A., Seiberg, N., Steinhardt, P. J., & Turok, N. 2002, Phys. Rev., D65, 086007, e-Print: hep-th/0108187
Khoury, J., Ovrut, B. A., Steinhardt, P. J., & Turok, N. 2001, Phys. Rev., D64, 123522, e-Print: hep-th/0103239

What I'm wondering is whether they did in the corresponding report--the Komatsu et al 2009.

YES! I found TWO mentions of Steinhardt's ekpyrotic/cyclic work one on page 7:

"...On the other hand, an inflationary expansion may not be the only way to solve cosmological puzzles and create primordial fluctuations. Contraction of the primordial universe followed by a bounce to expansion can, in principle, make a set of predictions that are qualitatively similar to those of inflation models (Khoury et al. 2001, 2002a,b; Khoury et al. 2003; Buchbinder et al. 2007, 2008; Koyama & Wands 2007; Koyama et al. 2007; Creminelli & Senatore 2007), although building concrete models and making robust predictions have been challenging (Kallosh et al. 2001,?; Linde 2002; Kallosh et al. 2008)..."

and another on page 17:

"...There is also a possibility that non-Gaussianity can be used to test alternatives to inflation. In a collapsing universe followed by a bounce (e.g., New Ekpyrotic scenario), f ^local_{N L} is given by the inverse (as well as inverse-squared) of slow-roll parameters; thus, f^local_{N L} as large as of order 10 to 10^2 is a fairly generic prediction of this class of models (Koyama et al. 2007; Buchbinder et al. 2008; Lehners & Steinhardt 2008; Lehners & Steinhardt 2008)..."

 

[marcus]

OK I've been looking closer at this stuff and I find that I have to yield some and reconsider--and maybe adopt a different position. Because by my own measures I see that Steinhardt's brane-clash cosmology is doing better than I thought it was.

I still have no intuitive hunch or suspicion that it might describe nature, but I always try to balance my subjective with some kind of objective measure of what the professional researchers are doing and I'm impressed with what I see.

I will get arxiv links to the papers by Buchbinder and Koyama and so on. That way we can see what impact they are having. This is not my cup of tea but I want to give it a fair assessment.

Komatsu et al refer to these brane-clash papers:

Buchbinder et al. 2007, 2008;
Koyama & Wands 2007;
Koyama et al. 2007;
Creminelli & Senatore 2007
Lehners & Steinhardt 2008
Lehners & Steinhardt 2008

We have to look up which papers they are actually talking about by consulting their bibliography.
This is not meant as reading for anyone! I just want to have a handle on the ekpyrotic papers that the Komatsu WMAP5 report referenced so that anyone who wants to can check them out, see which ones are getting cited and so forth. It can help eliminate some guesswork about what is actually going on with ekpyrotic.

To me it seems important that somebody besides Steinhardt and Turok is working on this. For example the next paper is by Buchbinder, Khoury, Ovrut.
That may not seem important from someone else's perspective. But it impresses me that there are papers by totally other people, with neither Steinhardt or Turok as a co-author. Also that the main WMAP5 report on cosmology referred to this stuff. That is, it is visible to some of the mainstream core cosmologists. I already gave some quotes from the WMAP5 report that were about this.

http://arxiv.org/abs/hep-th/0702154 [ps, pdf, other]
http://arxiv.org/cits/hep-th/0702154
New Ekpyrotic Cosmology
Evgeny I. Buchbinder, Justin Khoury, Burt A. Ovrut
Comments: 41 pages, 4 figures. v2: minor corrections, references added. v3: small modifications in bounce section, references added. v4: version published in PRD
Journal-ref: Phys.Rev.D76:123503,2007

http://arxiv.org/abs/0710.5172 [ps, pdf, other]
http://arxiv.org/cits/0710.5172
Non-Gaussianities in New Ekpyrotic Cosmology
Evgeny I. Buchbinder, Justin Khoury, Burt A. Ovrut
Comments: 4 pages.
Journal-ref: Phys.Rev.Lett.100:171302,2008

http://arxiv.org/abs/hep-th/0702165 [ps, pdf, other]
http://arxiv.org/cits/hep-th/0702165
A smooth bouncing cosmology with scale invariant spectrum
Paolo Creminelli (ICTP, Trieste), Leonardo Senatore (Harvard U.)
Comments: 20 pages, 1 fig.
Journal-ref: JCAP0711:010,2007

http://arxiv.org/abs/hep-th/0108187 [ps, pdf, other]
http://arxiv.org/cits/hep-th/0108187
From Big Crunch to Big Bang
Justin Khoury, Burt A. Ovrut, Nathan Seiberg, Paul J. Steinhardt, Neil Turok
Comments: 16 pages
Journal-ref: Phys.Rev. D65 (2002) 086007

http://arxiv.org/abs/hep-th/0103239 [ps, pdf, other]
http://arxiv.org/cits/hep-th/0103239
The Ekpyrotic Universe: Colliding Branes and the Origin of the Hot Big Bang
Justin Khoury, Burt A. Ovrut, Paul J. Steinhardt, Neil Turok
Comments: 67 pages, 4 figures.
Journal-ref: Phys.Rev.D64:123522,2001

http://arxiv.org/abs/hep-th/0109050 [ps, pdf, other]
http://arxiv.org/cits/hep-th/0109050
Density Perturbations in the Ekpyrotic Scenario
Justin Khoury, Burt A. Ovrut, Paul J. Steinhardt, Neil Turok
Comments: 36 pages
Journal-ref: Phys.Rev. D66 (2002) 046005

http://arxiv.org/abs/astro-ph/0302012 [ps, pdf, other]
http://arxiv.org/cits/astro-ph/0302012
Inflation versus Cyclic Predictions for Spectral Tilt
Justin Khoury, Paul J. Steinhardt, Neil Turok
Comments: 4 pages
Journal-ref: Phys.Rev.Lett. 91 (2003) 161301
Subjects: Astrophysics (astro-ph); High

http://arxiv.org/abs/hep-th/0703040 [ps, pdf, other]
http://arxiv.org/cits/hep-th/0703040
Ekpyrotic collapse with multiple fields
Kazuya Koyama, David Wands
Comments: 13 pages, 1 figure
Journal-ref: JCAP0704:008,2007

http://arxiv.org/abs/0804.1293 [ps, pdf, other]
http://arxiv.org/cits/0804.1293
Intuitive understanding of non-gaussianity in ekpyrotic and cyclic models
Jean-Luc Lehners, Paul J. Steinhardt
Comments: 10 pages, 4 figures.
Journal-ref: Phys.Rev.D78:023506,2008

http://arxiv.org/abs/0712.3779 [ps, pdf, other]
http://arxiv.org/cits/0712.3779
Non-Gaussian Density Fluctuations from Entropically Generated Curvature Perturbations in Ekpyrotic Models
Jean-Luc Lehners, Paul J. Steinhardt
Comments: 5 pages, 2 figures.
Journal-ref: Phys.Rev.D77:063533,2008

I'd welcome it if someone else wants to comment or interpret. Or if someone who knows about these models can explain anything. One thing that puzzles me is what Buchbinder et al call "New Ekpyrotic". Is this still a clash of branes? Several of the papers talk about "bounce" cosmology. I don't see a mathematical representation of the branes.

 

[cristo]

To me it seems important that somebody besides Steinhardt and Turok is working on this. For example the next paper is by Buchbinder, Khoury, Ovrut.
That may not seem important from someone else's perspective. But it impresses me that there are papers by totally other people, with neither Steinhardt or Turok as a co-author.

I think such an observation should be taken with a pinch of salt since I think it's a good guess that Khoury was Steinhardt's student.

One thing that puzzles me is what Buchbinder et al call "New Ekpyrotic". Is this still a clash of branes? Several of the papers talk about "bounce" cosmology. I don't see a mathematical representation of the branes.

I think the new ekpyrotic model has the same mechanism as the original one, but with a few additions to account for the problems of the first model (namely, a singularity after the original contraction, and a non scale invariant spectrum of the curvature perturbation). From what I gather, they introduce some fields to solve these problems. I guess the details are in 0702154.

 

[Chronos]

It's all 'turtles' to me. I prefer to take the universe at face value. We see all that is possible to see and deduce from there. Observational evidence favoring the existence of 'fairies' is required before they need be included in the model. I admit I might be a little hard core, and I've never been accused of being a string apologist. Anyone wandering down this path should change their major to philosophy, IMO.

 

[Ithryndil]

I am currently reading their book Endless Universe...and I think I either missed something or it's just not in there. I remember reading some of Michio Kaku's books and I believe in one of them he mentioned that there might be an infinite number of branes floating in a higher dimension. Either way, I am wondering why in the Endless Universe only two branes are mentioned. I suppose that's how they are designing their theory, but it still raises the question: "Why only two branes?" I suppose I should skim back through the book, but I have been reading it and looking for an explanation for that so far, and haven't found one.

 

[Ithryndil]

I am wondering if anyone knows the answer to my above question. Basically I am wondering why Neil and Paul seem to only consider two branes in their model. I remember reading something that allows for multiple branes in higher dimensional space. Seems to me that there could be a large to infinite amount of these branes floating in a higher dimension. So why did they pick two? Thanks.