Null orbifolds in AdS, Time Dependence and Holography

TL;DR

This work analyzes time-dependent cosmologies arising from null-brane backgrounds through the lens of AdS/CFT. It develops two complementary holographic pictures: (i) null orbifolds in AdS produced by null rotations and their singular conformal boundaries, and (ii) a gauge theory description via 4D Super Yang-Mills on the null-brane, offering a potential non-perturbative definition of string cosmology. The study extends to string/M-theory embeddings, including AdS_{p+2}×S^{D-p-2} vacua with fluxes, and examines null-brane quotients on branes, revealing reduced supersymmetry (typically 1/2 or 1/4) and rich geometric structures. The paper also discusses the prospects and challenges of a non-perturbative formulation of string cosmology using T-duality, M-theory lifts, and gauge/gravity duality, aiming to characterize dynamics across cosmological necks and singular boundaries.

Abstract

We study M/D-branes in a null-brane background. By taking a near horizon limit, one is left with cosmological models in the corresponding Poincaré patches. To deal with their usual horizons, we either extend these models to global AdS or remain in the Poincaré patch and apply a T-duality transformation whenever the effective radius of the compact dimension associated with the null-brane probes distances smaller than the string scale. The first scenario gives rise to null orbifolds in AdS spaces, which are described in detail. Their conformal boundaries are singular. The second has a dual gauge theory description in terms of Super Yang-Mills in the null-brane background. The latter is a good candidate for a non-perturbative definition of string theory in a time-dependent background.