The observed galaxy bispectrum from single-field inflation in the squeezed limit

TL;DR

This paper derives the observed galaxy bispectrum in the squeezed limit from single-field inflation using the consistency relation, emphasizing the importance of gauge mode removal and estimating relativistic effects that could mimic primordial non-Gaussianity.

Contribution

It provides a novel derivation of the galaxy bispectrum in the squeezed limit from single-field inflation, highlighting gauge issues and relativistic lightcone effects.

Findings

Gauge modes must be carefully removed in second-order perturbations.

Relativistic lightcone effects can mimic primordial non-Gaussianity.

Derived the bispectrum using the consistency relation in Fourier space.

Abstract

Using the consistency relation in Fourier space, we derive the observed galaxy bispectrum from single-field inflation in the squeezed limit, in which one of the three modes has a wavelength much longer than the other two. This provides a non-trivial check of the full computation of the bispectrum based on second-order cosmological perturbation theory in this limit. We show that gauge modes need to be carefully removed in the second-order cosmological perturbations in order to calculate the observed galaxy bispectrum in the squeezed limit. We then give an estimate of the effective non-Gaussianity due to general relativistic lightcone effects that could mimic a primordial non-Gaussian signal.