Dark Radiation and the Hagedorn Phase

TL;DR

This paper explores how an early universe Hagedorn phase for the Standard Model sector, separate from dark radiation, can result in low observable ΔN_eff, with implications for warped string compactifications and axion safety.

Contribution

It introduces the idea that a Hagedorn phase in the Standard Model sector can suppress dark radiation contributions in warped string scenarios.

Findings

Hagedorn phase can reduce dark radiation signals

Warped compactifications support the scenario

Bulk axions are less problematic during Hagedorn phase

Abstract

We point out that if the sector associated with the Standard Model degrees of freedom entered an open string Hagedorn phase in the early universe while the dark radiation sector was not part of this plasma, then this can lead to low values of the observable $\Delta N_{\text{eff}}$ (number of additional neutrinolike species) from the dark radiation. For explicit analysis, we focus on warped string compactifications with the Standard Model degrees of freedom at the bottom of a warped throat. If the Hubble scale during inflation is above the warped string scale associated with the throat, then the Standard Model sector will enter the Hagedorn phase. In this scenario, bulk axions are no longer dangerous from the point of view of dark radiation. While this article focuses on warped compactifications, the basic idea can be relevant to any scenario where the early universe entered a Hagedorn phase.