The Axiverse induced Dark Radiation Problem

TL;DR

The paper discusses the dark radiation problem arising from the Axiverse in string/M theory, where decay of moduli fields produces too many relativistic axions, conflicting with observational limits on effective neutrino species.

Contribution

It identifies the dark radiation problem in the Axiverse scenario and proposes a solution involving specific decay channels of the lightest modulus, constraining the moduli Kahler potential.

Findings

The decay of heavy moduli produces excessive dark radiation.

The problem conflicts with observational bounds on N_eff.

A potential solution involves the lightest modulus decaying only into its axion superpartner and Standard Model particles.

Abstract

The string/$M$ theory Axiverse -- a plethora of very light Axion Like Particles (ALPs) with a vast range of masses -- is arguably a generic prediction of string/$M$ theory. String/$M$ theory also tends to predict that the early Universe is dominated by moduli fields. When the heavy moduli decay, before nucleosynthesis, they produce dark radiation in the form of relativistic ALPs. Generically one estimates that the number of relativistic species grows with the number of axions in the Axiverse, in contradiction to the observations that $N_{eff} \leq 4$. We explain this problem in detail and suggest some possible solutions to it. The simplest solution requires that the lightest modulus decays only into its own axion superpartner plus Standard Model particles and this severely constrains the moduli Kahler potential and mass matrix.