Correlations between cosmic strings and extra relativistic species

TL;DR

This study investigates the relationship between cosmic strings and extra relativistic species, finding that cosmic strings can account for additional radiation in the early universe and are mildly favored by CMB data.

Contribution

It introduces a joint analysis of cosmic strings and neutrino-like radiation, revealing their correlation and the potential of strings to explain extra relativistic species.

Findings

CMB data favor cosmic strings at 2sigma level

Strings can account for all extra radiation needed

Inclusion of non-CMB data reduces preferences for strings and extra species

Abstract

The recent observation that the Cosmic Microwave Background (CMB) may prefer a neutrino excess has triggered a number of works studying this possibility. The effect obtained by the non-interacting massless neutrino excess could be mimicked by some extra radiation component in the early universe, such as a cosmological gravitational wave background. Prompted by the fact that a possible candidate to source those gravitational waves would be cosmic strings, we perform a parameter fitting study with models which considers both cosmic strings and the effective number of neutrinos as free parameters, using CMB and non-CMB data. We find that there is a correlation between cosmic strings and the number of extra relativistic species, and that strings can account for all the extra radiation necessary. In fact, CMB data prefer strings at a 2sigma level, paying the price of a higher extra radiation component. CMB data also give a moderate preference for a model with ns=1. The inclusion of non-CMB data lowers both the preference for strings and for the extra relativistic species.