The bispectrum of cosmic string temperature fluctuations including recombination effects

TL;DR

This paper computes the cosmic microwave background temperature bispectrum from cosmic strings, including recombination effects, and finds that these effects have minimal impact on string tension constraints compared to the power spectrum.

Contribution

It introduces the first calculation of the CMB bispectrum from cosmic strings including last scattering surface effects, validated against full sky maps and Boltzmann code results.

Findings

Including last scattering effects has minimal impact on string tension limits.

The bispectrum is unlikely to outperform the power spectrum in constraining cosmic strings.

Validated approximation methods against full sky maps and Boltzmann code results.

Abstract

We calculate the cosmic microwave background temperature bispectrum from cosmic strings, for the first time including the contributions from the last scattering surface, using a well-established Gaussian model for the string energy-momentum correlation functions, and a simplified model for the cosmic fluid. We check our approximation for the integrated Sachs-Wolfe (ISW) contribution against the bispectrum obtained from the full sky map of the cosmic string ISW signal used by the Planck team, obtaining good agreement. We validate our model for the last scattering surface contribution by comparing the predicted temperature power spectrum with that obtained from a full Boltzmann code treatment applied to the Unconnected Segment Model of a string network. We find that including the last scattering contribution has only a small impact on the upper limit on the string tension resulting from the bispectrum at Planck resolutions, and argue that the bispectrum is unlikely to be competitive with the power spectrum at any resolution.