CMB lensing and primordial squeezed non-Gaussianity

TL;DR

This paper introduces a new non-perturbative method to accurately account for gravitational lensing effects on squeezed primordial non-Gaussianity in the CMB, with implications for constraining early-universe physics.

Contribution

A simple, non-perturbative prescription for calculating lensing effects on any squeezed primordial bispectrum shape, including polarization and trispectrum considerations.

Findings

Lensing smoothing does not significantly bias local non-Gaussianity estimators.

Tau_NL trispectrum estimators can be approximated by temperature modulation estimators, with potential 10-30% bias.

Including dipole sky modulations can reduce tau_NL error bars by half.

Abstract

Squeezed primordial non-Gaussianity can strongly constrain early-universe physics, but it can only be observed on the CMB after it has been gravitationally lensed. We give a new simple non-perturbative prescription for accurately calculating the effect of lensing on any squeezed primordial bispectrum shape, and test it with simulations. We give the generalization to polarization bispectra, and discuss the effect of lensing on the trispectrum. We explain why neglecting the lensing smoothing effect does not significantly bias estimators of local primordial non-Gaussianity, even though the change in shape can be >~10%. We also show how tau_NL trispectrum estimators can be well approximated by much simpler CMB temperature modulation estimators, and hence that there is potentially a ~10-30% bias due to very large-scale lensing modes, depending on the range of modulation scales included. Including dipole sky modulations can halve the tau_NL error bar if kinematic effects can be subtracted using known properties of the CMB temperature dipole. Lensing effects on the g_NL trispectrum are small compared to the error bar. In appendices we give the general result for lensing of any primordial bispectrum, and show how any full-sky squeezed bispectrum can be decomposed into orthogonal modes of distinct angular dependence.