Null orbifolds in AdS, Time Dependence and Holography

We study M/D-branes in a null-brane background. By taking a near horizonlimit, one is left with cosmological models in the corresponding Poincar\'epatches. To deal with their usual horizons, we either extend these models toglobal AdS or remain in the Poincar\'e patch and apply a T-dualitytransformation whenever the effective radius of the compact dimensionassociated with the null-brane probes distances smaller than the string scale.The first scenario gives rise to null orbifolds in AdS spaces, which aredescribed in detail. Their conformal boundaries are singular. The second has adual gauge theory description in terms of Super Yang-Mills in the null-branebackground. The latter is a good candidate for a non-perturbative definition ofstring theory in a time-dependent background.Comment: 1+25 pages, no figure