Cosmic String Power Spectrum, Bispectrum and Trispectrum
D.M. Regan, E.P.S. Shellard

TL;DR
This paper analytically calculates the gravitational effects of cosmic strings on the CMB, focusing on the power spectrum, bispectrum, and trispectrum, especially for upcoming experiments like Planck, revealing distinctive non-Gaussian signatures.
Contribution
It extends previous work by integrating string contributions from last scattering to present, providing new estimates for the bispectrum and trispectrum relevant for CMB observations.
Findings
The bispectrum shape is well-behaved and distinguishable from inflationary signals.
The bispectrum nonlinearity parameter is approximately f_NL -20.
The trispectrum for cosmic strings is large, with tau_NL 10^5.
Abstract
We use analytic calculations of the post-recombination gravitational effects of cosmic strings to estimate the resulting CMB power spectrum, bispectrum and trispectrum. We place a particular emphasis on multipole regimes relevant for forthcoming CMB experiments, notably the Planck satellite. These calculations use a flat sky approximation, generalising previous work by integrating string contributions from last scattering to the present day, finding the dominant contributions to the correlators for multipoles l > 50. We find a well-behaved shape for the string bispectrum (without divergences) which is easily distinguishable from the inflationary bispectra which possess significant acoustic peaks. We estimate that the nonlinearity parameter characterising the bispectrum is approximately f_NL \sim -20 (given present string constraints from the CMB power spectrum. We also apply these…
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