Domain walls in light of Cosmic Microwave Background and Pulsar Timing Array data

TL;DR

This paper examines how current gravitational wave observations constrain models of domain walls, revealing tight bounds on their energy scales and conditions needed for them to explain recent pulsar timing array signals.

Contribution

It provides a detailed analysis of the constraints on biased domain wall scenarios from CMB and PTA data, highlighting the fine-tuning required for domain walls to account for observed gravitational waves.

Findings

CMB bounds slightly improve constraints on domain wall density.

Domain wall energy scales are tightly constrained even with early decay.

Explaining PTA signals with domain walls requires near dominance of their energy density.

Abstract

In this paper, we study the compatibility of biased domain wall scenarios with current gravitational wave data. We show that the Cosmic Microwave Background bounds on the fractional density of gravitational waves at the time of decoupling may only slightly improve on the constraints that result from requiring that domain walls never dominate the cosmic energy budget. We show that, despite this, the range of energy scales of the domain-wall forming phase transitions are already quite constricted, even if the networks decay early in cosmological history. We also show that, if domain walls are to provide an explanation to the stochastic gravitational wave background that was recently detected by pulsar timing arrays, they not only have to decay early in the radiation dominated era but also their energy density would have to be close to dominating the energy density of the universe, which would require some fine tuning of the parameters of the models.