Dark Energy as an off-shell Tachyon Background in four-dimensional Strings

TL;DR

This paper demonstrates that an off-shell tachyon background in a time-dependent bosonic string can produce a power-law expanding universe, offering a string-theoretic explanation for dark energy without extra dimensions.

Contribution

It shows that off-shell tachyon backgrounds in four-dimensional strings naturally lead to accelerating or decelerating universes, without requiring detailed knowledge of the full string effective action.

Findings

Power-law expansion can be accelerating or decelerating.

Dark energy can be modeled via off-shell tachyon backgrounds.

No extra dimensions needed for consistent four-dimensional backgrounds.

Abstract

We consider a time-dependent bosonic string in graviton, dilaton and tachyon backgrounds, for which it was already shown that conformal invariance is respected to all orders in alpha' and in any space-time dimension. Assuming that the tachyon is off-shell, we show in this note that the specific time-homogeneity of the corresponding space time effective action leads to a power-law expanding Universe, that can be accelerating or decelerating, depending on the range of parameter space considered. We interpret this result as dark energy. We arrive at this result without requiring the knowledge of the structure of the string effective action, which is not known to all orders in alpha'. Moreover, in our approach, the background configurations are consistent in a four-dimensional space time, without any need for extra dimensions.