Non-Gaussianity and the CMB bispectrum: Confusion between primordial and lensing-Rees-Sciama contribution?

Abstract

We revisit the predictions for the expected cosmic microwave background bispectrum signal from the cross-correlation of the primary-lensing-Rees-Sciama signal; we point out that it can be a significant contaminant to the bispectrum signal from primordial non-Gaussianity of the local type. This non-Gaussianity, usually parametrized by the non-Gaussian parameter ${f}_{NL}$, arises, for example, in multifield inflation. In particular both signals are frequency-independent, and are maximized for nearly squeezed configurations. While their detailed scale-dependence and harmonic imprints are different for generic bispectrum shapes, we show that, if not included in the modeling, the primary-lensing-Rees-Sciama contribution yields an effective ${f}_{NL}$ of 10 when using a bispectrum estimator optimized for local non-Gaussianity. Considering that expected $1\mathrm{\text{\ensuremath{-}}}\ensuremath{\sigma}$ errors on ${f}_{NL}$ are $<10$ from forthcoming experiments, we conclude that the contribution from this signal must be included in future constraints on ${f}_{NL}$ from the cosmic microwave background bispectrum.