Cosmic strings and domain walls: the impact of CMB $B$-mode data

TL;DR

This paper uses the latest CMB data to constrain cosmic strings and domain walls, finding no significant evidence but setting tighter limits, and forecasts future improvements with upcoming observatories.

Contribution

First comprehensive analysis combining Planck 2018 and BICEP/Keck 2018 data to constrain cosmic defects, including detailed modeling and future forecast assessments.

Findings

No significant defect detection, mild preference for non-zero string tension.

Constraints improved up to a factor of two over previous limits.

Future observatories will significantly tighten constraints, especially for domain walls.

Abstract

We analyse CMB constraints on stable networks of cosmic strings and domain walls using for the first time full Planck 2018 data together with BICEP/Keck 2018 $B$-mode measurements. The defect-induced anisotropies are computed using the Unconnected Segment Model for Nambu-Goto and Abelian-Higgs strings, as well as for stable domain walls, and included in a full Markov Chain Monte Carlo analysis jointly varying all $\Lambda$CDM parameters, the tensor-to-scalar ratio, and the string/domain wall tension. No statistically significant evidence for defects is found, although we observe a mild preference for non-zero cosmic string tension. Our results improve previous constraints on the defect power spectrum by up to a factor of two. In the particular case of strings, the improvement is driven by the $B$-mode data, and is especially pronounced for Abelian-Higgs strings. We also present forecasts for the Simons Observatory and the LiteBIRD satellite: the former will improve string tension constraints by about a factor of three, while the latter those on the domain wall tension by about a factor of ten. Finally, we assess the impact of Nambu-Goto string loops on CMB anisotropies in light of both current and future observations.