Accelerated expansion of the universe driven by tachyonic matter

It is an accepted practice in cosmology to invoke a scalar field with a potential V () when the observed evolution of the universe cannot be reconciled with theoretical prejudices. Since one function degree of freedom in the expansion factor a (t) can be traded off for the function V (), it is always possible to find a scalar field potential which will reproduce a given evolution. I provide a recipe for determining V () from a (t) in two cases: (i) a normal scalar field with the Lagrangian L= (1/2) ₀^a-V () used in quintessence or dark energy models; (ii) a tachyonic field with the Lagrangian L=-V () 1-₀^a^1/2, motivated by recent string theoretic results. In the latter case, it is possible to have accelerated expansion of the universe during the late phase in certain cases.