  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.
