Inflationary Models: A Dead End?

[Naty1]

Following are some descriptions of Inflationary Model(s) of the Universe which are quite different than I have read elsewhere. My own view, I now believe, was likely a bit myopic. These comments reflect Paul Steinhardt and Neil Turoks' descriptions in their book ENDLESS UNIVERSE where they propose a cyclic model they call the ekpyrotic model. This is a cyclic, repeating, model that competes with inflationary, one time bang, models.

I'd be interested in comments from others regarding the general validity of these views of inflationary models.

(These are all from Chapter 10 of their book.)

To a large degree the (wide) acceptance of the inflationary model is based on the simpler picture of inflation that emerged 25 years ago.

and they imply the inflationary model has not addressed the cosmological constant
problem:

Having failed to find a mechanism for making the cosmological constant small, they concluded that it must be large in most places, and that the observed universe must be an anomaly.

..the inflationary model is incomplete because there is no compelling reason for the universe to emerge in an inflationary state...Guth had not realized the eternal nature of inflation when he initially proposed the idea...(but) he has become a leading advocate of its importance..."

Something strange has happened in the inflationary picture. ..What began as an efficient mechanism for apparently explaining the simplicity of the universe-that is the smoothness and uniformity of the observable universe- has been turned into a run away process that seems extraordinarily wasteful and unpredictable in its use of space and time.

In the inflatonary model, most of cosmic history is spent creating more and more vacuum... regions of space with galaxies and stars occur only in rare pockets that are separated...by unimaginably vast expanses of empty space.

So far, eternal inflation has been described as if every pocket universe it creates is exactly like ours...but in recent years...theorists have realized that an inflationary multiverse(results) in which physical conditions vary greatly from pocket to pocket...this multiverse (beyond our own universe) is neither predictive nor verifiable...inflation populates all possibilities to an infinite degree, making it impossible to explain why the observable universe has the particular physical properties it has.

Please note I'm not advocating one model over another, only sharing ideas and trying to gain a better perspective.

 

[marcus]

Following are some descriptions of Inflationary Model(s) of the Universe which are quite different than I have read elsewhere...

Thanks for assembling this collection of quotes. Here we see Steinhardt and Turok (S&T) arguing against socalled Eternal Inflation a fantasy process that dominates when you include an hypothetical "inflaton" field in the picture which can be turned on by a random fluctuation.

The lean-and-mean technical meaning of inflation is simply a brief period of exponential expansion, which may happen for other reasons besides an "inflaton", and may occur outside the context of Eternal Inflation. More generally, a brief period of rapidly accelerating expansion, imagined occurring at the start of expansion, sufficient to explain the degree of smoothness, flatness, uniformity that is observed.

My subjective leaning is I tend to have a lot of respect for Paul Steinhardt (even though I don't buy his cyclic "brane clash" cosmos) and not such high regard for Andrei Linde, the main promoter of the Baroque multiverse Eternal Inflation picture. I think it is a pity that Linde's EI vision has come to dominate what people think of in connection with inflation and has pushed out the simple lean-and-mean concept. So my personal attitude makes me appreciate these quotes you collected.

For that reason I will recopy these excerpts using the indent rather than the quote box.
The indent button is at the top of the "reply to thread" panel, in the line below the row of buttons that let you choose fonts, sizes, color and smilies. It is right below "sizes". Indented material survives when someone chooses to reply to your post. So it is more readily usable by others.

Excerpts from Steinhardt and Turok's book Endless Universe (courtesy Naty)
(These are all from Chapter 10 of their book.)

"To a large degree the (wide) acceptance of the inflationary model is based on the simpler picture of inflation that emerged 25 years ago."​

[My comment: Steinhardt was himself instrumental in gaining acceptance for the earlier simple inflation picture, on which he collaborated. Later he found that inflation has a deep incompatibility with the compactified dimensions of superstring/M, which in turn motivated S&T's invention of the brane clash model as a way of saving stringery. His position on both string and inflation is complex and nuanced.]

"Having failed to find a mechanism for making the cosmological constant small, they concluded that it must be large in most places, and that the observed universe must be an anomaly."​

"...the inflationary model is incomplete because there is no compelling reason for the universe to emerge in an inflationary state...Guth had not realized the eternal nature of inflation when he initially proposed the idea...(but) he has become a leading advocate of its importance..."​

"Something strange has happened in the inflationary picture. ..What began as an efficient mechanism for apparently explaining the simplicity of the universe--that is the smoothness and uniformity of the observable universe--has been turned into a run away process that seems extraordinarily wasteful and unpredictable in its use of space and time."​

"In the inflatonary model, most of cosmic history is spent creating more and more vacuum... regions of space with galaxies and stars occur only in rare pockets that are separated...by unimaginably vast expanses of empty space."​

"So far, eternal inflation has been described as if every pocket universe it creates is exactly like ours...but in recent years...theorists have realized that an inflationary multiverse(results) in which physical conditions vary greatly from pocket to pocket...this multiverse (beyond our own universe) is neither predictive nor verifiable...inflation populates all possibilities to an infinite degree, making it impossible to explain why the observable universe has the particular physical properties it has."​

 

[Naty1]

Marcus:
Although I did not refer to it here, the "lean and mean" brief inflationary period you refer to, and the one I have been exposed to in various popular cosmology books, originally from Alan Guth, has led to the broader eternal inflation view...one T&S write Guth now subscribes to! and even quote him in such support...

 

[marcus]

Marcus:
...the "lean and mean" brief inflationary period ... has led to the broader eternal inflation view...

Yes, "has led to" but not necessarily by a logical process. Those for whom it led to that may simply have been unable at the moment to think of an alternative way that early accelerated expansion might have occurred. For instance a brief acceleration occurring around the time of a bounce, under those bounce conditions, does not imply that multiple inflations must occur all over the place in a vast bubble multiverse Other ways have occurred to people since the time back when Andrei Linde sold people on the eternal inflation idea. Linde and Guth are old now, and from an earlier cosmology generation. They don't have current highly-cited papers. I would not automatically follow their leads.

Naty, there is a really important Steinhardt paper you should know about.
http://arxiv.org/abs/0811.1614

It was published just recently (May 2009, I think) in the American Physical Society's Physical Review D
http://link.aps.org/doi/10.1103/PhysRevD.79.104026

He and a guy at Cambridge, Dan Wesley, show that under broad assumptions the compactified extra dimensions of string theories are incompatible with inflation, and not only with inflation but with the slowly accelerating expansion we observe today.

It seems to me that Steinhardt has swung back towards an acceptance of (at least a simple barebones version of) inflation---but to the detriment of string/M.

So if we follow Steinhardt, it may not be Inflationary Models per se that are to be viewed as a dead end (as mentioned in your thread title.)

There appear to be other ways besides early universe accelerated expansion to explain the near flatness and uniformity.
(Certain types of bounce cosmology are being studied in that connection, for instance.)

There are models where early accelerated expansion shows up naturally, but which don't invoke the kind of "inflaton" explanation that leads to a multiverse of multi-inflations.
(A certain amount of early accelerated expansion occurs naturally in some bounce models, without an inflaton field being needed.)

So it's complicated. To see where Steinhardt is heading, I am waiting for his next paper about the incompatibility of string extra dimensions with inflation (and today's dark energy)
In the recently published paper in Physical Review D, he and his co-author Dan Wesley say they have a follow-up paper in preparation. I want to see what that says.

 

[Naty1]

Marcus,
I'll read that paper thanks.

In the Endless Universe of 2007 Steinhardt and Turok explain their Ekpyrotic (cyclic) model does NOT rely on inflation... the Heterotic M theory they began their model from makes inflation unnecessary...in other words they can explain homogeneity, inhomogenaity, and flatness via string/brane and extra dimension mechanisms...all finite transitions...which result in the different type of gravitational waves experimentalists might be able to detect in the next generation of WMAP type studies.

"It seems to me that Steinhardt has swung back towards an acceptance of (at least a simple barebones version of) inflation---but to the detriment of string/M."

His 2007 book DOES attempt to explain today's "slowly accelerating expansion"...via astronomically slow decay of dark energy which eventually reverses cosmological expansion...so it will be interesting to see how his newer work you reference compares!

It IS complicated, as you say, and I post brief excerpts here not attempt to explain it all, which I could not do anyway, but to provide a flavor so anyone who wants more can read the details and draw their own conclusions.

 

[Naty1]

First, let me say I'm not sure I'm qualified to interpret what Steinhardt means but
here are a few quotes from his new paper:

From the CONCLUSIONS section of
http://arxiv.org/abs/0811.1614

The added complexity is disappointing. Inflation and dark energy in 4d have always had the problem that they require special degrees of freedom and fine-tuning. One would have hoped that extra dimensions, which are introduced to simplify the unification of fundamental forces, would also alleviate the conditions needed for inflation. The no-go theorems say the opposite...

The fact that NEC violation is required to have inflation in theories with extra dimensions is unexpected since this was not a requirement in the original inflationary models based on four dimensions only. Curiously, a criticism raised at times about models with bounces from a contracting phase to an expanding phase, such as the ekpyrotic and cyclic alternatives to inflationary cosmology, is that the bounce requires a violation of the NEC (or quantum gravity corrections to GR as the FRW scale factor a(t) → 0 that serve the same function). Now we see that, although the details are different, all of these cosmologies require NEC violation when incorporated into theories with extra dimensions.

(my boldface)

To me the above implies (a) Steinhardt recognizes "violation" criticisms about his ekpyrotic models and (b) he says such violations unexpectedly apply to inflation models with similar extra dimensions.

I wonder if he recognized ekypyrotic "violations" back when his book was published? And if not, would he still promulgate such a model?? He did not shove his ekpyrotic model under the bus in the above quote so maybe he did forsee that.

My own limited understanding is that compactified extra dimensions have always had to be held constant in order for theoretical characteristics of extra dimensional models to match observations...otherwise basic physical constants would vary for example...and that such constraint has always appeared to violate Einsteins view of malleable space...and has never been a desireable aspect of extra dimensional modles.

 

[Naty1]

And here is another CONCLUSION which is rather cryptic:

We note that, thus far, we have restricted the analysis to no-go theorems that are simple to express and simple to prove. There are numerous other relations that must be satisfied to have cosmic acceleration that will be considered in future work.

Stay tuned!

 

[marcus]

...
Stay tuned!

I think you are getting a lot out of Steinhardt's no-go extra dimensions paper and also you have selected a bunch of very interesting Steinhardt quotes. I urge you to use INDENT when you quote these really pithy thought-provoking things. When you use the ordinary quote button it goes away when I reply to your post, so I don't have anything to comment on!

So I copy the quotes by hand:

The added complexity is disappointing. Inflation and dark energy in 4d have always had the problem that they require special degrees of freedom and fine-tuning. One would have hoped that extra dimensions, which are introduced to simplify the unification of fundamental forces, would also alleviate the conditions needed for inflation. The no-go theorems say the opposite...​

In fact as he shows, extra dimensions make the problems considerably more severe! Even with fine-tuning and even with relaxing the NEC it seems that compactified extra dimensions don't get along with inflation---or the currently observed accelerated expansion over the long haul.

The fact that NEC violation is required to have inflation in theories with extra dimensions is unexpected since this was not a requirement in the original inflationary models based on four dimensions only. Curiously, a criticism raised at times about models with bounces from a contracting phase to an expanding phase, such as the ekpyrotic and cyclic alternatives to inflationary cosmology, is that the bounce requires a violation of the NEC (or quantum gravity corrections to GR as the FRW scale factor a(t) → 0 that serve the same function). Now we see that, although the details are different, all of these cosmologies require NEC violation when incorporated into theories with extra dimensions.
​

What I recall from the paper is that even with NEC violation, compactified extra dimensions are no-go. Steinhardt and Wesley consider both cases--with NEC and with NEC violated---and in both cases they found compactified extra dimensions incompatible with inflation and dark energy.

Maybe I should go back and hunt down some more quotes, the paper has several sections.
They actually prove a range of no-go theorems based on a range of different assumptions.

We note that, thus far, we have restricted the analysis to no-go theorems that are simple to express and simple to prove. There are numerous other relations that must be satisfied to have cosmic acceleration that will be considered in future work.​

Yes! Definitely "stay tuned" as you say. The paper they have in preparation is reference [25]. I am looking forward to seeing it.