Irreducible cosmological backgrounds of a real scalar with a broken symmetry

TL;DR

This paper investigates the cosmological effects of a real scalar field with a broken $ ext{Z}_2$ symmetry, highlighting its potential to produce gravitational waves, dark matter, and observable decay signatures through domain wall dynamics.

Contribution

It introduces three benchmark scenarios linking domain wall behavior with gravitational waves, dark matter production, and scalar decay signatures, offering new insights into scalar field cosmology.

Findings

Gravitational wave signals from domain walls could be detected by pulsar timing arrays for PeV-scale scalars.

Domain wall annihilation can produce dark matter with distinctive power spectrum features.

Unstable scalars from freeze-in decay can leave observable cosmological and terrestrial signatures.

Abstract

We explore the irreducible cosmological implications of a singlet real scalar field. Our focus is on theories with an approximate and spontaneously broken $\mathbb{Z}_2$ symmetry where quasi-stable domain walls can form at early times. This seemingly simple framework bears a wealth of phenomenological implications that can be tackled by means of different cosmological and astrophysical probes. We elucidate the connection between domain wall dynamics and the production of dark matter and gravitational waves. In particular, we identify three main benchmark scenarios. The gravitational wave signal observed by pulsar timing arrays can be generated by the domain walls if the mass of the singlet is $m_s \sim\,$PeV. For lower masses, but with $m_s \gtrsim 10\,$GeV, scalars produced in the annihilation of the domain walls can be dark matter with a distinctive feature in their power spectrum. Finally, the thermal bath provides an unavoidable source of unstable scalars via the freeze-in mechanism whose subsequent decays can be tested by their imprints on cosmological and terrestrial observables.