29. Eye on ArXiv: 28 Apr 2008 - D3/D7 brane inflation

I would like to briefly discuss the paper

Michael Haack, Renata Kallosh, Axel Krause, Andrei Linde, Dieter Lust, Marco Zagermann, “Update of D3/D7-Brane Inflation on K3 x T^2/Z_2″

released last friday. As follows from the title, the paper is about D3/D7 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 T_2/Z_2 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 D3/D7 scenario

As we know, basically all stringly inspired D-brane inflationary modes (KKLMMT, for example) suffer from the \eta-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.

D3/D7 is instead a D-term inflationary model and does not suffer from the \eta-problem.

(Actually, in many respects D3/D7 compactified on K_3\times{}T^2/Z_2 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.)

D3/D7 has however another problem: cosmic string production in the end of inflation (when D-term flatness is restored and U(1) 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 U(1) coupling) or, if there is small amount of them, n_s turns out to be very close to 1.  One can still squeeze D3/D7 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 M_P). 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 D3/D7 inflationary model, due to the stringy corrections potential of the inflaton can acquire a maximum. Near the maximum potential has the form -m^2 \phi^2 and inflation is eternal.

(Also, as it turns out, the inflaton field range allowed in the model can be significantly larger than M_P if the torus T_2 is asymmetrical. )

Rate this:
2.5
These icons link to social bookmarking sites where readers can share and discover new web pages.
  • StumbleUpon
  • del.icio.us
  • bodytext
  • Reddit
  • Technorati
  • Facebook

Related posts:

18. Eye on ArXiv: 15 Apr 2008 - Transplanckian physics, brane cosmology and holographic mesons
20. More on KS throat cloaking…
12. Eye on ArXiv: 4 Apr 2008 - KS throat cloaking, non-gaussianity and Mathur’s fuzzballs
23. Eye on ArXiv: 21 Apr 2008 - SUSY/non-SUSY duality
46. Leaving for Planck 2008

28. Eye on ArXiv: 24 Apr 2008 - dS vacua in no-scale SUGRA; multifield DBI inflation

1. Laura Covi, Marta Gomez-Reino, Christian Gross, Jan Louis, Gonzalo A. Palma, Claudio A. Scrucca, “de Sitter vacua in no-scale supergravities and Calabi-Yau string models”

As 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 \alpha' and g_s 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 \alpha' and g_s 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 T and a no-scale Kahler potential

 K=-3\log (T+\bar{T})

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 W=W(T), 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.

2. David Langlois, Sebastien Renaux-Petel, Daniele A. Steer, Takahiro Tanaka, “Primordial fluctuations and non-Gaussianities in multi-field DBI inflation”

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
c_s, so that both get amplified at the sound horizon crossing. In the small c_s limit entropy modes are enhanced compared to the adiabatic ones, and they strongly affect both non-gaussianity level and final curvature perturbation.

Rate this:
2.5
These icons link to social bookmarking sites where readers can share and discover new web pages.
  • StumbleUpon
  • del.icio.us
  • bodytext
  • Reddit
  • Technorati
  • Facebook

Related posts:

57. Stability of de Sitter space: statement of the problem 1
19. Disorder on the landscape: non-technical intro
29. Eye on ArXiv: 28 Apr 2008 - D3/D7 brane inflation
15. Anderson localization. Just crossed my mind…
58. Stability of de Sitter space: dS as a perfect interferometer

Science Blogs - Blog Catalog Blog Directory