Dark radiation as a probe for phase transition in the early universe

TL;DR

This paper proposes that dark radiation can serve as a probe for early universe phase transitions by analyzing its density and spectrum, revealing details about the early cosmological constant and the phase transition characteristics.

Contribution

It introduces a novel method to infer the temperature and strength of early universe phase transitions through dark radiation observations, linking cosmological constant dynamics to observable signals.

Findings

Dark radiation density and spectrum are significantly affected by early cosmological constant.

Observations of dark radiation can determine the temperature of the phase transition.

Dark radiation spectrum provides insights into the strength of the phase transition.

Abstract

The cosmological constant is not necessarily small in the early universe. If a scalar field obtains a vacuum expectation value after a phase transition (PT), a possibly large cosmological constant could present before PT. The early cosmological constant (ECC) and the PT process may be detectable from dark radiation (DR) today, such as in the cosmic axion background. We show that for a broad class of DR models, the DR density and spectrum are significantly modified by the presence of an ECC. From the density and the spectrum of the DR today, we can deduce the temperature and the strength of the PT.