Nearly seventy per cent of the energy density in the universe is unclustered and exerts negative pressure. This conclusion — now supported by numerous observations — poses the greatest challenge for theoretical physics today. I discuss this issue with special emphasis on the cosmological constant as the possible choice for the dark energy. Several curious features of a universe with a cosmologi...

We investigate cosmological consequences of nonlinear sigma model coupled with a cosmological fluid which satisfies the continuity equation. The target space action is of the de Sitter type and is composed of four scalar fields. The potential which is a function of only one of the scalar fields is also introduced. We perform a general analysis of the ensuing cosmological equations and give vari...

We investigate the evolution of a universe with a decaying cosmological term (vacuum energy) that is assumed to be a function of the scale factor. In this model, while the cosmological term increases to the early universe, the radiation energy density is lower than the model with the cosmological "constant". We find that the effects of the decaying cosmological term on the expansion rate at the...

This article briefly summarizes the increasingly precise observational estimates of the cosmological parameters. After three years on the stump, the ΛCDM model is still the leading candidate. Although the Universe is expanding , our picture of it is coming together.

For vacua in type IIB string moduli space parameterized by the coupling constant, we propose a scheme for interpretation of the cosmological term in type IIB superstrings as a Goldstone mode from the spontaneous symmetry breaking of SL(2, Z) to a Fuchsian subgroup of degree one, Γh. The moduli space Σh = Γh\H has a radial direction with a massive Goldstone mode, and the contribution to the posi...