Correlations between cosmic strings and extra relativistic species

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 noninteracting 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 consider 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 2 sigma level, paying the price of a higher extra radiation component. CMB data also give a moderate preference for a model with n(s) = 1. The inclusion of non-CMB data lowers the preference both for strings and for the extra relativistic species. DOI: 10.1103/PhysRevD.86.123014