# Was the Universe Actually Radiation Dominated Prior to Nucleosynthesis?

## TL;DR

This paper investigates whether non-perturbative effects can prevent a prolonged matter-dominated epoch caused by light moduli in the early universe, concluding that such an epoch is likely unavoidable in string theory scenarios.

## Contribution

It analyzes the impact of non-perturbative effects on moduli dynamics, showing that a matter-dominated epoch prior to BBN is generally unavoidable unless moduli become strongly coupled.

## Key findings

- Non-perturbative effects are insufficient to eliminate the matter-dominated epoch in most scenarios.
- Light moduli near the TeV scale lead to a prolonged matter-dominated epoch until BBN.
- Strong coupling of moduli could potentially shorten or remove the matter-dominated phase.

## Abstract

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.