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| Paper IPM / P / 8035 |
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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/V3/2. 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.
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