Tachyon field in cosmology

We study cosmological effects of homogeneous tachyon field as dark energy. We concentrate on two different scalar field potentials, the inverse square potential and the exponential potential. These models have a unique feature that the matter density parameter and the density parameter for tachyons remain comparable for a large range in redshift. It is shown that there exists a range of parameters for which the universe undergoes an accelerated expansion and the evolution is consistent with structure formation requirements. For a viable model we require fine tuning of parameters comparable to that in ΛCDM or in quintessence models. For the exponential potential, the accelerated phase is followed by a phase with a(t) ∝ t thus eliminating a future horizon. This report is based on recent work in collaboration with J. S. Bagla and T. Padmanabhan [1]. In this paper, we construct cosmological models with homogeneous tachyon matter [2] to provide the dark energy component which drives acceleration of the universe (for a recent review of dark energy models see [3]). We assume that the tachyon matter coexists with normal nonrelativistic matter and radiation (for other work on cosmological aspects of tachyon field, see[4]). For a spatially flat universe, the Friedman equations are