  It is an accepted practice in cosmology to invoke a scalar field with
potential $V(\phi)$ 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 $V(\phi)$, it is
{\it always} possible to find a scalar field potential which will reproduce a
given evolution. I provide a recipe for determining $V(\phi)$ from $a(t)$ in
two cases:(i) Normal scalar field with Lagrangian ${\cal L} =
(1/2)\partial_a\phi \partial^a\phi - V(\phi)$ used in quintessence/dark energy
models. (ii) A tachyonic field with Lagrangian ${\cal L} = -V(\phi) [ 1-
\partial_a\phi \partial^a\phi]^{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 interpretation of the current phase of the universe with tachyonic
condensate acting as effective cosmological constant.
