Was the Universe Actually Radiation Dominated Prior to Nucleosynthesis?
  Maybe not. String theory approaches to both beyond the Standard Model and
Inflationary model building generically predict the existence of scalars
(moduli) that are light compared to the scale of quantum gravity. These moduli
become displaced from their low energy minima in the early universe and lead to
a prolonged matter-dominated epoch prior to BBN. In this paper, we examine
whether non-perturbative effects such as parametric resonance or tachyonic
instabilities can shorten, or even eliminate, the moduli condensate and
matter-dominated epoch. Such effects depend crucially on the strength of the
couplings, and we find that unless the moduli become strongly coupled the
matter-dominated epoch is unavoidable. In particular, we find that in string
and M-theory compactifications where the lightest moduli are near the TeV-scale
that a matter-dominated epoch will persist until the time of Big Bang
Nucleosynthesis.
