Correlations between cosmic strings and extra relativistic 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.

11 pages, 3 figures; Included new models in the analysis, matches the published version

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• The Atacama Cosmology Telescope: Cosmological parameters from three seasons of data
• The number of cosmic string loops
• Planck 2013 results. XXV. Searches for cosmic strings and other topological defects
• Exact Scale-Invariant Background of Gravitational Waves from Cosmic Defects
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• Irreducible background of gravitational waves from a cosmic defect network: update and comparison of numerical techniques
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• Is there evidence for additional neutrino species from cosmology?
• An Étude on the Regularization and Renormalization of Divergences in Primordial Observables
• Fitting BICEP2 with defects, primordial gravitational waves and dust