The CMB temperature bispectrum induced by cosmic strings

Abstract

The Cosmic Microwave Background (CMB) bispectrum of the temperature anisotropies induced by a network of cosmic strings is derived for small angular scales, under the assumption that the principal cause of temperature fluctuations is the Gott-Kaiser-Stebbins (GKS) effect. We provide analytical expressions for all isosceles triangle configurations in Fourier space. Their overall amplitude is amplified as the inverse cube of the angle and diverges for flat triangles. The isosceles configurations generically lead to a negative bispectrum with a power law decay l^(-6) for large multipole l. However, collapsed triangles are found to be associated with a positive bispectrum whereas the squeezed triangles still exhibit negative values. We then compare our analytical estimates to a direct computation of the bispectrum from a set of 300 statistically independent temperature maps obtained from Nambu-Goto cosmic string simulations in a Friedmann-Lemaitre-Robertson-Walker (FLRW) universe. We find good agreement for the overall amplitude, the power law behaviour and angle dependency of the various triangle configurations. At l~500 the cosmic string GKS effect contributes approximately the same equilateral CMB bispectrum amplitude as an inflationary model with |fNL|~10^3, if the strings contribute about 10% of the temperature power spectrum at l=10. Current bounds on fNL are not derived using cosmic string bispectrum templates, and so our fNL estimate cannot be used to derive bounds on strings. However it does suggest that string bispectrum templates should be included in the search of CMB non-Gaussianities.

15 pages, 12 figures, uses RevTex. References and physical discussion added. Matches published version

References in corpus (11)

• Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for Cosmology
• Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation
• Extended Limber Approximation
• The bispectrum of galaxies from high-redshift galaxy surveys: primordial non-Gaussianity and non-linear galaxy bias
• CMB power spectrum contribution from cosmic strings using field-evolution simulations of the Abelian Higgs model
• Optimal limits on f_{NL}^{local} from WMAP 5-year data
• Fitting CMB data with cosmic strings and inflation
• Constraints on Supersymmetric Models of Hybrid Inflation
• Small-Angle CMB Temperature Anisotropies Induced by Cosmic Strings
• CMB polarization power spectra contributions from a network of cosmic strings
• B-modes from Cosmic Strings