What is needed of a tachyon if it is to be the dark energy?

Abstract

We study a dark energy scenario in the presence of a tachyon field $ϕ$ with potential $V(ϕ)$ and a barotropic perfect fluid. The cosmological dynamics crucially depends on the asymptotic behavior of the quantity $λ=-M_pV_ϕ/V^{3/2}$. If $λ$ is a constant, which corresponds to an inverse square potential $V(ϕ) \propto ϕ^{-2}$, there exists one stable critical point that gives an acceleration of the universe at late times. When $λ\to 0$ asymptotically, we can have a viable dark energy scenario in which the system approaches an ``instantaneous'' critical point that dynamically changes with $λ$. If $|λ|$ approaches infinity asymptotically, the universe does not exhibit an acceleration at late times. In this case, however, we find an interesting possibility that a transient acceleration occurs in a regime where $|λ|$ is smaller than of order unity.