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

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 cosmic string power spectrum and correct previous CMB analyses with domain walls that overestimated the constraints. 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 five. Finally, we assess the impact of Nambu-Goto string loops on CMB anisotropies in light of both current and future observations.

Comments: 42 pages, 14 figures. v3: corrected a typo inherited from previous literature, which slightly updates the limit on the domain wall tension. Matches the version published in JCAP