Cosmology

We study generic single-field dark energy models, by a parametrization of the most general theory of their perturbations around a given background, including higher derivative terms. In appropriate limits this approach reproduces standard quintessence, k-essence and ghost condensation. We find no general pathology associated to an equation of state w_Q < -1 or in crossing the phantom divide w_Q = -1. Stability requires that the w_Q < -1 side of dark energy behaves, on cosmological scales, as a k-essence fluid with a virtually zero speed of sound. This implies that one should set the speed of sound to zero when comparing with data models with w_Q < -1 or crossing the phantom divide. We summarize the theoretical and stability constraints on the quintessential plane (1+w_Q) vs. speed of sound squared

I will review the string gas scenario of structure formation, stressing the role which holography plays. I will also discuss another way of obtaining a scale-invariant spectrum of cosmological perturbations (with specific signatures in the bispectrum) which may be realizable in scenarios based on the AdS/CFT correspondence which can resolve the cosmological singularity.

An update is given on AdS/CFT models of cosmological singularities, in particular on models in which a big crunch instability of the bulk theory is induced by an unstable multi-trace deformation of the dual field theory.

Certain classes of cosmological backgrounds in asymptotically anti-de-Sitter space-times have hologropically dual descriptions in terms of gauge theories. We analyse such backgrounds with smooth initial conditions, where curvatures become strong in a finite time and the gravity equations break down. We will show how the dual gauge theory can be used to continue time evolution in this singular region and speculate on the nature of space-time at late times.