Gauge theory duals of cosmological backgrounds and their energy momentum tensors

We revisit type IIB supergravity backgrounds with null and spacelike singularities with natural gauge theory duals proposed in and. We show that for these backgrounds there are always choices of the boundaries of these deformed AdS5×S5 space-times, such that the dual gauge theories live on flat metrics and have space-time dependent couplings. We present a new time dependent solution of this kind where the effective string coupling is always bounded and vanishes at a spacelike singularity in the bulk, and the space-time becomes AdS5×S5 at early and late times. The holographic energy momentum tensor calculated with a choice of flat boundary is shown to vanish for null backgrounds and to be generically nonzero for time dependent backgrounds.