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 big bang nucleosynthesis (BBN). In this paper, we examine whether nonperturbative 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, a matter-dominated epoch will persist until the time of big bang nucleosynthesis.
Giblin, John T. and al., et, "Was the Universe Actually Radiation Dominated Prior to Nucleosynthesis?" (2017). Physical Review D: Covering Particles, Fields, Gravitation, and Cosmology 96(4): 043525-1-043525-6. Faculty Publications. Paper 256.
Physical Review D: Covering Particles, Fields, Gravitation, and Cosmology