Intensity mapping of the 21cm emission: lensing

TL;DR

This paper investigates the effects of gravitational lensing on 21cm intensity mapping, deriving higher-order corrections and identifying a new lensing term absent in CMB studies, with implications for future surveys.

Contribution

It derives a third-order perturbation theory formula for 21cm lensing, revealing a new lensing term and assessing its significance for upcoming surveys.

Findings

Lensing effects are negligible for large-scale surveys (lmax < 700).

A new lensing term unique to intensity mapping is identified.

Signal-to-noise ratio for 21cm lensing in SKA2 is about 10.

Abstract

In this paper, we study lensing of 21cm intensity mapping (IM). Like in the cosmic microwave background (CMB), there is no first order lensing in intensity mapping. The first effects in the power spectrum are therefore of second and third order. Despite this, lensing of the CMB power spectrum is an important effect that needs to be taken into account, which motivates the study of the impact of lensing on the IM power spectrum. We derive a general formula up to third order in perturbation theory including all the terms with two derivatives of the gravitational potential, i.e. the dominant terms on sub-Hubble scales. We then show that in intensity mapping there is a new lensing term which is not present in the CMB. We obtain that the signal-to-noise of 21 cm lensing for futuristic surveys like SKA2 is about 10. We find that surveys probing only large scales, lmax < 700, can safely neglect the lensing of the intensity mapping power spectrum, but that otherwise this effect should be included.