Constraints on the Nambu-Goto cosmic string contribution to the CMB power spectrum in light of new temperature and polarisation data

Abstract

Cosmic strings generate vector and tensor modes in the B-channel of polarization, as well as the usual temperature power spectrum and E-mode polarization spectrum. We use the power spectrum obtained from high-resolution Nambu-Goto cosmic string simulations together with the Planck and BICEP2 likelihoods to explore the degeneracies appearing between cosmic strings and other cosmological parameters in different inflationary scenarios, as well as the constraints that can be imposed on cosmic strings in each of these situations. In standard $Λ$CDM, the Planck likelihood yields an upper limit $Gμ<1.49 \times 10^{-7}$ (95% confidence). We also analyse the possibility of explaining the BB power spectrum signal recently detected by the BICEP2 probe. We find that cosmic strings alone are able to explain only part of the B-mode polarization signal. Apart from the standard $Λ$CDM model, we look at the following non-minimal parameters: the running of the spectral index, non-zero tensor-to-scalar ratio, additional degrees of freedom ($N_{eff}$) and sterile neutrinos. We find that in both Planck and BICEP2 scenarios adding $N_{eff}$ induces degeneracies between cosmic strings and $N_{eff}$ and other $Λ$CDM parameters. With $N_{eff}$ a larger contribution from cosmic strings is allowed, even favoured, but after combining with large-scale structure data, such as BAOs, strings remain strongly constrained.