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What is needed of a tachyon if it is to be the dark energy?
(2011-07-06) Copeland, E. J.; Garousi, M. R.; Sami, M.; et al.
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 = −MpVφ/V 3/2 . If λ is a constant, which corresponds to an inverse square potential V (φ) ∝ φ−2 , there exists one stable critical point that gives an acceleration of the universe at late times. When λ → 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.