2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/179400It 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 {\it 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 ${\cal L} = (1/2)\partial_aϕ\partial^aϕ- V(ϕ)$ used in quintessence/dark energy models. (ii) A tachyonic field with Lagrangian ${\cal L} = -V(ϕ) [ 1- \partial_aϕ\partial^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 interpretation of the current phase of the universe with tachyonic condensate acting as effective cosmological constant.4 pages; uses revtex4High Energy Physics - TheoryAstrophysicsGeneral Relativity and Quantum Cosmologytext