Accelerated expansion of the universe driven by tachyonic matter

Abstract

It is an accepted practice in cosmology to invoke a scalar field with potential V (φ) when 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 (φ), it is always possible to find a scalar field potential which will reproduce a given evolution. I provide a recipe for determining V (φ) from a(t) in two cases: (i) Normal scalar field with Lagrangian L = (1/2)∂aφ∂aφ−V (φ) used in quintessence/dark energy models; (ii) A tachyonic field with Lagrangian L = −V (φ)[1−∂aφ∂aφ] 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. This suggests a string theory based nterpretation of the current phase of the universe with tachyonic condensate acting as effective cosmological constant.