Opinions about April SciAm article about inflation inflation by Steinhardt?

[Oldfart]

Well, I was impressed, anyway...

Anybody?

 

[mathman]

Since Steinhardt is a major contributor to inflation theory, his ideas on the subject have to be respected.

 

[marcus]

Please give a link to the article if there is one open for non-subscribers.

I just was able to find two paragraphs that didn't say much. Was there a link to more there that I didn't see?

Could either of you summarize what the main point is that he is making?

Is he now skeptical about inflation, or does he argue that it is inconsistent with some other popular ideas (so that one or the other would have to go, either inflation or some other popuar idea)?

I'm curious, would like a link. I respect Steinhardt a lot. But I'm not a subscriber to SciAm and I'm not near a bookstore or public library for the time being.

 

[bcrowell]

The article consists of two main sections: the case for inflation and the case against.

The case for:

There are lots of fields believed to exist (but not necessarily the ones in the standard model?) that should have acted as inflaton fields. Solves the fine-tuning problems related to observed flatness and uniformity. Makes successful predictions, e.g., scale-invariance of structure.

The case against:

Although it gets rid of some fine-tuning problems, it creates others. E.g., fine-tuning is required to keep temperature variations from being too big. The anthropic principle doesn't help, because larger temperature variations might have made the universe *more* hospitable to life. Penrose argues that obtaining a flat universe with no inflation is higher in probability than obtaining one with inflation.

Eternal inflation robs the theory of its predictive value. Attempts to fix this by applying a measure don't work, because the choice of measure is ad hoc, has no explanatory value, is itself in need of explanation.

He has a quantum-mechanical model which he thinks may work better than inflation.

He refers to some popular-level books plus this paper: http://arxiv.org/abs/hep-th/0609095

 

[marcus]

The case for:
... observed flatness and uniformity. Makes successful predictions, e.g., scale-invariance of structure.

The case against:
Although it gets rid of some fine-tuning problems, it creates others. E.g., fine-tuning is required to keep temperature variations from being too big. The anthropic principle doesn't help, because larger temperature variations might have made the universe *more* hospitable to life. Penrose argues that obtaining a flat universe with no inflation is higher in probability than obtaining one with inflation.
...
He refers to some popular-level books plus this paper: http://arxiv.org/abs/hep-th/0609095

Thanks for the summary of Steinhardt's article! By coincidence, the technical paper you mention he refers to, namely Gibbons and Turok http://arxiv.org/abs/hep-th/0609095, is one that has been answered by Ashtekar and Sloan from a Loop Cosmology perspective.

The Measure Problem in Cosmology
G.W. Gibbons, Neil Turok
22 pages, 6 figures.
(Submitted on 13 Sep 2006)
"... We further show that, for some quantities of interest, the measure is very insensitive to the details of how the identification is made. One such quantity is the probability of inflation in simple scalar field models.
We find that, according to our implementation of the canonical measure, the probability for N e-folds of inflation in single-field, slow-roll models is suppressed by of order exp(-3N) and we discuss the implications of this result."

Probability of Inflation in Loop Quantum Cosmology
Abhay Ashtekar, David Sloan
34 pages, 3 figures
(Submitted on 12 Mar 2011)
Inflationary models of the early universe provide a natural mechanism for the formation of large scale structure. This success brings to forefront the question of naturalness: Does a sufficiently long slow roll inflation [e.g. 65 e-foldings] occur generically or does it require a careful fine tuning of initial parameters? In recent years there has been considerable controversy on this issue.

In particular, for a quadratic potential, ... Gibbons and Turok have argued that this probability is suppressed by a factor of ~ 10^-85. We first clarify that such dramatically different predictions can arise because the required measure on the space of solutions is intrinsically ambiguous in general relativity. We then show that this ambiguity can be naturally resolved in loop quantum cosmology (LQC) because the big bang is replaced by a big bounce and the bounce surface can be used to introduce the structure necessary to specify a satisfactory measure.

The second goal of the paper is to present a detailed analysis of the inflationary dynamics of LQC using analytical and numerical methods. By combining this information with the measure on the space of solutions, we address a sharper question than those investigated in the literature: What is the probability of a sufficiently long slow roll inflation WHICH IS COMPATIBLE WITH THE SEVEN YEAR WMAP DATA? We show that the probability is very close to 1..."

 

[marcus]

To clarify, in case anyone is is new to this, and curious about the numbers. For inflation to provide a satisfactory explanation for observed features of the universe, the scale factor of the universe has to increase by something like a factor of e⁶⁵----by "65 e-folds" as they say.

With a kind of generic inflaton potential of a sort commonly assumed, G and T found that the probability of such a satisfactory inflation is quite small, on the order of one over
e^3N = e¹⁹⁵ = 10⁸⁵.

So without some fine tuning or fancy extra assumption, they were suggesting, a satisfactory inflation is improbable like one over "1 followed by 85 zeros".

The intuitive reason is that in order for inflation to ever stop (so we can have a regular noninflating universe to live in) the inflaton field has to gradually decay.
But then, if it is going to decay, it has to start in a very "pumped up" condition in order for it to provide the full 65 e-folds of expansion!

In ordinary inflation scenarios, this "pumping up" happens by chance. G&T calculated that an inadequate pumping up is much more likely. When some random fluctuation causes inflation, it is likely to be unsatisfactory and not give the required expansion. Adequate inflation is very UNlikely.

Ashtekar and Sloan recently responded to this by arguing that:

1. Classic GR does not provide a good framework for stating probabilities about initial conditions because it has a breakdown ("singularity") at the start of expansion. So the framework is no good and you will never settle anything that way.

2. Loop cosmology improves the situation in three ways:
No breakdown, a bounce replaces the former singularity. So you can meaningfully describe initial conditions at the time of bounce. Probabilities can be defined. So at least there is something to discuss.

Also in the Loop model the bounce causes a natural inflation episode that does not require any inflaton or any further assumptions. Quantum corrections to the standard equations of gravity make gravity briefly repulsive instead of attractive. The result is called "superinflation".

Finally, if a generic inflaton of the usual sort is added to the picture, the bounce, with its "superinflation" episode would "pump up" that inflaton so adequate ordinary inflation becomes much more likely. G&T estimate its probability at near one, using a measure on conditions at the bounce which they argue is reasonable to adopt as prior.

 

[Oldfart]

Marcus, thanks for your replies!

Two quick questions: (1) Are Ashtekar/Sloan (and you) politely saying that Stienhardt's position on inflation is highly questionable, and (2) Is the introduction of bounce in the loop model a respectable thing to do now-a-days, or more of an unsubstantiated way of ducking the singularity issues?

 

[marcus]

Marcus, thanks for your replies!

Two quick questions: (1) Are Ashtekar/Sloan (and you) politely saying that Steinhardt's position on inflation is highly questionable, and (2) Is the introduction of bounce in the loop model a respectable thing to do now-a-days, or more of an unsubstantiated way of ducking the singularity issues?

I'm something of a Steinhardt fan, partly because of his hardheaded opposition to Multiverse fantasies. But mainly just out of respect for his rigorous intelligence in general. (I don't like his "clashing branes" cosmology, but that doesn't prevent me from valuing the man.)

The game in cosmology (and maybe science in general) is not to get people to believe stuff, but to produce testable theories and test them.

It may be that Loop Cosmology is right and there was a bounce, or it could be wrong and there wasn't. Loop might make inflation more viable and both could be wrong. Or both could be right.

There are a number of phenomenologists who have written papers recently discussing how to test for a bounce by close examination of the CMB. The Planck spacecraft will help but another similar mission to study polarization of the CMB would be needed. The bounce, and LQC, might be falsified, or might pass the test.

If LQC passes some observational tests, that would make inflation more tenable than it is now.
(Essentially because the bounce has a natural brief "superinflation" phase that can provide a proper start for inflation.)

I haven't read the SciAm article. I don't know all the arguments Steinhardt raises. I know the G&T paper and I know some papers of Steinhardt, but can't speak about the SciAm article.
=========================

About respectability, people who don't know what's going on habitually heap scorn on LQC, just because string theorists made it fashionable to scorn anything Loop.
But Loop is getting the attention of phenomenologists (professional theory-testers) and is experiencing a period of rapid growth in papers, citations, number of researchers, jobs, and faculty positions.

It's a small field with only 200 or so active researchers world-wide, so very few people encounter evidence of this growth in their own departments. An increasing number of universities are taking on Loop PhDs, but the number is still small, so a typical guy in a physics department will not be aware of this.

The key reason for growth (I think) is that in Loop there is roughly speaking one main theory, and it is testable.
(There are variants as always but for the Loop Cosmology application it's basically whatever Ashtekar says, for the full Loop Gravity theory it is basically what Rovelli says, and they exchange postdocs and collaborators.)

I can give you links to 14 recent papers about testing Loop bounce by CMB observations, if you want. A simple "Spires" search for stuff after 2008.

Another way to see the field is just to do a Spires search for "quantum cosmology" ranked by citation count. In quantum cosmology it is the Loop papers that are getting cited, so they predominate in the first 50 or so on the list.

So about respectability, it depends on how aware you are of the real world. I'd say very respectable if you know what's going on. Not respectable if you're the kind of person who just repeats what they hear in the average physics department coffeeroom.

I live in Berkeley. This year UC Berkeley will produce its first Loop PhD. One of the most promising PhD students in the physics department just happened somehow to get the notion, and find an advisor who just happened to be willing. That kind of thing doesn't happen all that much! They both went to Marseille for several weeks and spent time with Rovelli's group--in 2008 and 2009---and it worked out.

You still have to be fairly original and have some gumption to do that, unless you happen to be in one of the handful of universities that already have programs. Or perhaps worldwide it is a dozen. Have to go.

 

[marcus]

I remember a phenomenologist named Aurelien Barrau gave a paper last year on the connection between Inflation and Loop Cosmology.
This was at the 35th annual International Conference on High Energy Physics (Paris 2010) a respectable gathering
http://arxiv.org/abs/1011.5516
Inflation and Loop Quantum Cosmology
Aurelien Barrau
5 pages
(Submitted on 24 Nov 2010)
"On the one hand, inflation is an extremely convincing scenario: it solves most cosmological paradoxes and generates fluctuations that became the seeds for the growth of structures. It, however, suffers from a 'naturalness' problem: generating initial conditions for inflation is far from easy. On the other hand, loop quantum cosmology is very successful: it solves the Big Bang singularity through a non-perturbative and background-independent quantization of general relativity. It, however, suffers from a key drawback: it is extremely difficult to test. Recent results can let us hope that inflation and LQC could mutually cure those pathologies: LQC seems to naturally generate inflation and inflation could allow us to test LQC."

I'm beginnig to appreciate the mentality of phenomenologists. They don't believe or disbelieve, they don't commit to this or that. They just want to test, however it comes out.

Here are some 29 recent articles mostly by phenomenologists, they appeared 2009 or later. It's a kick to read some of these guys' abstracts (the summary at the beginning of the article) because the focus is definitely on testing.

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+POWER+SPECTRUM+or+dk+cosmic+background+radiation%29+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

 

[skydivephil]

I just read the whole article here:
http://www.nature.com/scientificamerican/journal/v304/n4/full/scientificamerican0411-36.html
What confused me was he was very critical of inflation leading to a multiverse but it appears to me his own alternative of a cyclic model does the same. Its just the multiple universes exist one after another rather than at the same time. But I don't see why this makes a qualitve difference. Same with Penrose CCC.
Anyone think this does make a qualitive difference? if so why?
He also didnt mention attempts to test eternal inflation via bubble collisions.
http://arxiv.org/abs/1012.1995
If he thinks this line of reasearch is not legit, that's fine, but making his cirticisms known would add to our understanding, ignoring it does not.
Same with LGC as Marcus said.
So I feel he over inflated the case against inflation. In his book "Endlesss Universe" he essentially said inflation has passed 5 out of 6 tests and one could determine which theory (inflation or his cyclic model) is true by observation of primodial gravitational waves, the 6th test. The tone in this piece seemed to be less sympathetic. I would have preferred it if an article about "the inflation debate" would have actually been one, maybe Steinhardt versus Guth.

 

[dougal217]

There's been a growing restlessness with Inflation as originally developed. I'm surprised Steinhardt didn't mention the Weeks article/paper/study at all, although that DID appear in a competitor's magazine.:) Even Guth admits that if Weeks is right, Inflation is dead. I come to not bury Guth, but to praise him... his work represents an important milestone from which we all can travel further