29. Eye on ArXiv: 28 Apr 2008 - D3/D7 brane inflation
DmitryI would like to briefly discuss the paper
released last friday. As follows from the title, the paper is about 
inflationary scenario ( and it is huge).
The scenario works as follows: D3 and D7 branes get attracted to each other due to the presence of a non-self-dual world volume flux on a D7-brane. If both branes are spacetime-filling, and the D7-brane wraps K3, the transverse interbrane distance on 
plays the role of the inflaton. Compactification geometry is not warped.
There are two issues in this paper being of particular interest to me.
a) Cosmic strings in scenario
As we know, basically all stringly inspired D-brane inflationary modes (KKLMMT, for example) suffer from the 
-problem (the second slow roll parameter turns out generally of the order of 1 in this models). This feature is generic for all SUGRA F-term inflationary models.
is instead a D-term inflationary model and does not suffer from the -problem.
(Actually, in many respects compactified on 
can be considered a sane model compared to KKLMMT and other warped brane inflationary models - computations of moduli stabilization are so much more under control, SUSY breaking is spontaneous etc. As I understand, this was the main reason for Keshav Dasgupta and Renata Kallosh with collaborators to introduce this model back in 2002.)
has however another problem: cosmic string production in the end of inflation (when D-term flatness is restored and 
symmetry gets spontaneously broken). Based on results of WMAP5 observations, we do not want too many cosmic strings (just basically one per Hubble volume). The authors spend a lot of time discussing this issue.
The bottom line of their study is that there are either too many cosmic strings (the case of large coupling) or, if there is small amount of them, 
turns out to be very close to 1. One can still squeeze model into WMAP5 rigid bounds, but one needs to push really hard for that.
b) Eternal inflation is possible
As you remember, the inflaton field range in D-brane inflationary models is typically not very wide, since it is related to the overall value of CY (and the latter is in turn related to the value of the 4-dimensional Planckian mass 
). As a result, it is really hard for D-brane inflation to become eternal (in KKLMMT there is very small interval of field values corresponding to eternal inflation, please see our paper on disorder on the landscape).
As authors show, in inflationary model, due to the stringy corrections potential of the inflaton can acquire a maximum. Near the maximum potential has the form 
and inflation is eternal.
(Also, as it turns out, the inflaton field range allowed in the model can be significantly larger than if the torus 
is asymmetrical. )
| 2.5 |
Related posts:
28. Eye on ArXiv: 24 Apr 2008 - dS vacua in no-scale SUGRA; multifield DBI inflation
DmitryAs we know today thanks to Maldacena and Nunez, it is impossible to find smooth compactifications of 10- and 11-dimensional SUGRA admitting stable solutions of Einstein equations corresponding to positive cosmological constant. One way to bypass arguments by Maldacena and Nunez is to consider full string theory taking into account 
and 
corrections to SUGRA effective acion. Moduli can be stabilized by taking non-pert. corrections to effective action into account (see KKLT paper), but the SUSY ground state is either AdS or Minkowski. Again, by playing with model, we can uplift the vacuum energy, but this will make the ground state metastable.
Naively, one can get metastable vacua with broken SUSY and positive cosmological constant without considering and corrections to SUGRA - just by studying behavior of moduli (volume or Kahler); dynamics of the latter should be rich enough to admit existence of such vacua. However, it was shown that that for N = 1 SUGRA corresponding to string compactifications with a single volume modulus 
and a no-scale Kahler potential
stationary points of a positive scalar potential V generated only by F-terms are always characterised by the existence of a tachyonic direction, for any superpotential 
, so that the corresponding vacua are not long-living.
Authors study the issue of tachyonic instability in details and find that the vacuum metastability depends on the mass of the scalar superpartners of the Goldstino (other scalar fields can be made arbitrarily massive by appropriately choosing the superpotential). Syperpotential does not define the masses of sGoldstinos since in the global SUSY limit Goldstino is massless, and sGoldstino masses are generated by the SUSY breaking mechanism. As a result, they do not have to be positive, and only if they are, the given dS vacuum is long-living.
Using this observation, authors show that many classes of N=1 SUGRA do not admit metastable dS vacua irrespectively of superpotential of moduli VEVs.
Multiple field models automatically imply existence of entropy perturbations. As authors show, in multiple field DBI models adiabatic and entropy modes propagate with the same speed of sound
, so that both get amplified at the sound horizon crossing. In the small limit entropy modes are enhanced compared to the adiabatic ones, and they strongly affect both non-gaussianity level and final curvature perturbation.
| 2.5 |
Related posts:
