Trying to relate string and inflation

[marcus]

http://arxiv.org/abs/0709.0002
Searching for Inflation in Simple String Theory Models: An Astrophysical Perspective
Mark P. Hertzberg (MIT), Max Tegmark (MIT), Shamit Kachru (Stanford), Jessie Shelton (Rutgers), Onur Ozcan (MIT)
24 pages, 5 figs
(Submitted on 3 Sep 2007 (v1), last revised 3 Sep 2007 (this version, v2))

"...We analyze three explicit string models from the recent literature, each containing an infinite number of "vacuum" solutions. Our numerical investigation of some natural candidate inflatons, the so-called "moduli fields", fails to find inflation. We also find in the simplest models that, after suitable field redefinitions, vast numbers of these vacua differ only in an overall constant multiplying the effective inflaton potential, a difference which affects neither the potential's shape nor its ability to support slow-roll inflation. This illustrates that even having an infinite number of vacua does not guarantee having inflating ones. This may be an artifact of the simplicity of the models that we study. Instead, more complicated string theory models appear to be required, suggesting that explicitly identifying the inflating subset of the string landscape will be challenging." http://space.newscientist.com/article/dn12628-can-string-theory-accommodate-inflation.html
Can string theory accommodate inflation?

 

[marcus]

Hello marcus,
I see what you are getting at...

Hi Kurt, actually I am not sure how to interpret this paper and I'm hoping to learn by seeing how other people respond. Your response already helps clarify it for me, so thanks.

Something I noticed is that Shamit Kachru is one of the most highly cited string theorists less than, say 40 years old. If you look at papers since 2000, say, he is one of the stars.

Another thing I noticed is the quotes in the New Scientist article. I normally don't read NewSci because of its careless over-sensational style, but this article seemed better journalism than usual for them and the quotes were quite interesting----for instance from Paul Steinhardt.

And from the lead author Mark Hertzberg.

Another thing is the article SAYS up front that it has a definite political/pedagogical agenda of inducing more real astrophysicists, more mainstream working cosmologists, to look at stringy stuff. It says that it is a LANGUAGE barrier that has been keeping conventional astronomy people from taking string seriously and it declares that its purpose is to show by example how to get over that barrier.

But actually the example they present would seem to have something of the opposite effect. And their "dictionary" is almost funny. So here we have a paper by among other prominent people a young star of string, Kachru, that SAYS up front that it has a definite agenda, but that doesn't seem to go according. It even seems to put string public relations (with other fields like astro) at risk. So at first look, I am baffled.

 

[marcus]

The first paper was accepted for publication in Physical Review D.
A second Hertzberg et al paper came out today about the same stuff, the difficulty of having an inflationary stage in cosmology in the context of certain models

http://arxiv.org/abs/0711.2512
Inflationary Constraints on Type IIA String Theory
Mark P. Hertzberg (MIT), Shamit Kachru (Stanford), Washington Taylor (MIT), Max Tegmark (MIT)
(Submitted on 16 Nov 2007)

"We prove that inflation is forbidden in the most well understood class of semi-realistic type IIA string compactifications: Calabi-Yau compactifications with only standard NS-NS 3-form flux, R-R fluxes, D6-branes and O6-planes at large volume and small string coupling. With these ingredients, the first slow-roll parameter satisfies epsilon >= 27/13 whenever V > 0, ruling out both inflation (including brane/anti-brane inflation) and de Sitter vacua in this limit. Our proof is based on the dependence of the 4-dimensional potential on the volume and dilaton moduli in the presence of fluxes and branes. We also describe broader classes of IIA models which may include cosmologies with inflation and de Sitter vacua. The inclusion of extra ingredients, such as NS 5-branes and geometric or non-geometric NS-NS fluxes, evades the assumptions used in deriving the no-go theorem. We focus on NS 5-branes and outline how such ingredients may prove fruitful for cosmology, but we do not provide an explicit model. We contrast the results of our IIA analysis with the rather different situation in IIB."

There was another related thread, not specifically about inflation but about rolled-up dimensions being unstable.
https://www.physicsforums.com/showthread.php?t=197056