The moduli problem at the perturbative level

TL;DR

This paper investigates how moduli fields during inflation can generate isocurvature perturbations, leading to new constraints on their properties that worsen the longstanding moduli problem, especially at high inflationary scales.

Contribution

It derives new bounds on moduli parameters from isocurvature constraints, highlighting the severity of the perturbative moduli problem at high inflation scales.

Findings

Constraints on moduli mass and vacuum expectation value are tightened.

High inflationary scales exacerbate the moduli problem.

Certain particle physics models are ruled out by these bounds.

Abstract

Moduli fields generically produce strong dark matter -- radiation and baryon -- radiation isocurvature perturbations through their decay if they remain light during inflation. We show that existing upper bounds on the magnitude of such fluctuations can thus be translated into stringent constraints on the moduli parameter space m_\sigma (modulus mass) -- \sigma_{inf} (modulus vacuum expectation value at the end of inflation). These constraints are complementary to previously existing bounds so that the moduli problem becomes worse at the perturbative level. In particular, if the inflationary scale H_{inf}~10^{13} GeV, particle physics scenarios which predict high moduli masses m_\sigma > 10-100 TeV are plagued by the perturbative moduli problem, even though they evade big-bang nucleosynthesis constraints.