What is the Rationale Behind Eternal Inflation?

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Discussion Overview

The discussion revolves around the concept of "eternal inflation" and its implications for the existence of multiverses, as well as critiques of standard inflationary models. Participants explore theoretical perspectives, critiques, and alternative models such as ekpyrosis and cyclic theories, examining the rationale behind eternal inflation and its acceptance in the cosmological community.

Discussion Character

  • Debate/contested
  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant references Max Tegmark's argument for infinite multiverses based on the concept of eternal inflation, expressing a desire to understand its rationale.
  • Another participant critiques standard inflationary models, suggesting that the "Steinhardt schism" provides a valid critique of the multiverse predictions associated with inflation.
  • Some participants note that Steinhardt has long advocated for alternatives like ekpyrosis and cyclic models, questioning the consensus around inflationary models.
  • Concerns are raised about the validity of eternal inflation, with one participant expressing relief that their objections are recognized by leading cosmologists.
  • Discussion includes clarification on the classification of inflation models based on potential types and the distinction between ordinary cosmic expansion and the concept of eternal inflation.
  • One participant acknowledges confusion regarding terminology, specifically the use of "superluminal" in relation to inflation, while others clarify the nature of exponential expansion in inflationary models.
  • References are made to past discussions on cyclic models and their predictions regarding dark energy and gravitational waves, with some noting that current data from Planck are consistent with these models.

Areas of Agreement / Disagreement

Participants express a range of views on the validity of eternal inflation and its implications, with some supporting it and others advocating for alternative models. The discussion remains unresolved, with multiple competing perspectives present.

Contextual Notes

Participants highlight limitations in the current understanding of inflation models, including unresolved mathematical steps and the dependence on specific definitions of inflation and expansion.

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I am currently reading Max Tegmark's book "Our Mathematical Universe" in which he argues for an infinite number of parallel multiverses. It seems to me that the basis for his postulating the multiverses is that "eternal inflation" occurs and continues to occur in space regions even after a particular space region (like ours) undergoes a big bang. It also seems to me that the infinity of multiverses and all its dramatic consequences such as "everything that can occur will occur" and do so an infinite number of times is already inherent and simply a consequence of the concept of "eternal" expansion. Therefore before I suspend my belief in the finite and accept the seeming bizarre claims of the infinite (e'.g. that there are multiple copies of nearly me in the spectrum of multiverses) I'd like some help in understanding the rationale of "eternal" inflation. Thanks kindly.
 
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I don't see the point.
google "Steinhardt schism" for a good critique of standard textbook inflation and the recent modification (in response to recognized problems like multiverse-prediction problem) that the authors call "post-modern" inflation.

Steinhardt and Loeb are among the top cosmology people at their respective institutions (princeton and harvard)

Actually "Loeb schism" is easier to type and it will also get you their recent article
http://arxiv.org/abs/1402.6980
click on PDF to get the full text, it's free.

Nice article, written to be clearly understood by wide audience, not just by specialists.
 
Steinhardt's been banging the anti-inflationary drum for years in favor of ekpyrosis and cyclic models. I would not consider his view to be consensus despite his illustrious creds.
 
bapowell said:
Steinhardt's been banging the anti-inflationary drum for years in favor of ekpyrosis and cyclic models...

I haven't heard very much about those (ekpyr. and cyclic) ideas lately. As I recall those proposals go back to around 2001, maybe earlier. Steinhardt has not been pushing them much of late. Loeb, of course, has no vested interest in those alternatives at all. Both of them seem seriously concerned to point out flaws and dangers of inflation models. I don't detect any ulterior motive or "hidden agenda". I think in this case they are criticizing inflation ideologies because they see serious problems with them and science requires this kind of critical reflection. I can't imagine why Avi Loeb would be involved otherwise.

The logic of their critique is pretty solid and stands on its own. BTW this paper is the SECOND with Loeb, that is purely a critique of various inflation models--particularly in light of Planck2013 findings, which they argue make standard textbook inflation quite unlikely.
 
Thanks Marcus for referencing the article, which I found very helpful. I feel relieved that the objections I felt on eternal inflation (objections which were not rigorous but mostly based on my reluctance to accept the science of measurement and probability predictability in an infinite multiverse) are indeed objections that leading cosmologists recognize as valid.
 
I thought the ΛCDM cosmological model forecast 'eternal expansion' using 'classic' inflation...that is, superluminal expansion during a brief period of ['slow roll'] false vacuum.

Is this how Steinhardt and Loeb use the term 'classic' inflation?
 
You'll have to read the paper. You'll see they classify inflation models by the type of potential. E.g. a simple quadratic potential (like a parabola) versus more complicated ones with plateaux or terraces . each kind has its own unlikelinesses and or troubles.

I don't know what you have in mind by "eternal expansion". In the ordinary cosmic model of what we are in, expansion doesn't stop. this is not the same as what inflation ideologues call "eternal inflation".

The later is for instance where you postulate that inflation starts by a quantum accident kicking the inflation field up its potential which it gradually "rolls" down and during the time is energized there is exponential expansion. But then other random fluctuations can occur in other regions and the process can keep happening producing a multiplicity of universes. In infinite patchwork of inflations in various stages going on all over the place. That's not the exact idea but it suggests it. There is also the business of stopping. Maybe it stops in different regions at different stages, so there is a lot of variety. How do you arrange for it to stop after just the right amount. Excuse the informality, got to run an errand….
Inflation is not defined as "superluminal". We have superluminal expansion going on right now. The key thing is high rate of nearly EXPONENTIAL expansion.
 
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Inflation is not defined as "superluminal". We have superluminal expansion going on right now. The key thing is high rate of nearly EXPONENTIAL expansion.

DUH! I confused myself...I actually knew that and used the wrong term...
thanks...

bapowell:
Steinhardt's been banging the anti-inflationary drum for years in favor of ekpyrosis and cyclic models...

I posted about Steinhardt and Turok's 2007 book on the subject here...it is a very interesting read:

Cyclic Model of the Universe

https://www.physicsforums.com/showthread.php?t=317772&highlight=turok

Wow, that post was almost five years ago...Among other things, their cyclic theory hinges on the smooth decay of dark energy and according to them would be confirmed experimentally by the detection of "B mode" in the spectrum of early era gravitational waves...
Had either of those actually been detected, I'm guessing Steinhardt and Turok would be out in front of an entire band.
 
Naty1 said:
Wow, that post was almost five years ago...Among other things, their cyclic theory hinges on the smooth decay of dark energy and according to them would be confirmed experimentally by the detection of "B mode" in the spectrum of early era gravitational waves...
Had either of those actually been detected, I'm guessing Steinhardt and Turok would be out in front of an entire band.
Cyclic models in the ekpyrotic setup predict undetectable B modes. So currently, Planck data are consistent with these models. But we'll see.
 

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