Cosmic shear bispectrum from second-order perturbations in General Relativity

TL;DR

This paper calculates the cosmic shear bispectrum including relativistic effects for future large-scale surveys, revealing potential contamination of primordial non-Gaussianity signals, especially at low redshifts.

Contribution

It provides the first comprehensive computation of the second-order relativistic corrections to the cosmic shear bispectrum and compares their impact to primordial non-Gaussianity.

Findings

Relativistic effects are smaller than standard non-linear couplings but become significant at low redshifts.

In the squeezed limit, relativistic effects can mimic local primordial non-Gaussianity for z<0.5.

Relativistic corrections can contaminate the measurement of f_NL>10 in future surveys.

Abstract

Future lensing surveys will be nearly full-sky and reach an unprecedented depth, probing scales closer and closer to the Hubble radius. This motivates the study of the cosmic shear beyond the small-angle approximation and including general relativistic corrections that are usually suppressed on sub-Hubble scales. The complete expression of the reduced cosmic shear at second order including all relativistic effects was derived in [1]. In the present paper we compute the resulting cosmic shear bispectrum when all these effects are properly taken into account and we compare it to primordial non-Gaussianity of the local type. The new general relativistic effects are generically smaller than the standard non-linear couplings. However, their relative importance increases at small multipoles and for small redshifts of the sources. The dominant effect among these non standard corrections is due to the inhomogeneity of the source redshift. In the squeezed limit, its amplitude can become of the order of the standard couplings when the redshift of the sources is below 0.5. Moreover, while the standard non-linear couplings depend on the angle between the short and long mode, the relativistic corrections do not and overlap almost totally with local type non-Gaussianity. We find that they can contaminate the search for a primordial local signal by f_NL>10.