to the curved sky. These include visibility-based estimators for the power spectrum, treatments of interloper lines, and the “foreground wedge" signature of spectrally smooth foregrounds.'
author:
- 'Adrian Liu$^{\dagger}$, Yunfan Zhang, Aaron R. Parsons'
bibliography:
- 'biblio.bib'
title: Spherical Harmonic Analyses of Intensity Mapping Power Spectra
---

Introduction {#sec:Intro}
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[[^1]]{} In recent years, intensity mapping has been hailed as a promising method for conducting cosmological surveys of unprecedented volume. In an intensity mapping survey, the brightness temperature of an optically thin spectral line is mapped over a three-dimensional volume, with radial distance information provided by the observed frequency (and thus redshift) of the line. By observing brightness temperature fluctuations on cosmologically relevant scales (without resolving individual sources responsible for the emission or absorption), intensity mapping provides a relatively cheap way to survey our Universe. In addition, with an appropriate choice of spectral line and a suitably designed instrument, the volume accessible to an intensity mapping survey is enormous. This allows measurements to be made over a large number of independent cosmological modes, providing highly precise constraints on both astrophysical and cosmological models. For example, intensity mapping experiments tracing the $21\,\textrm{cm}$ hyperfine transition of hydrogen can easily access $\sim 10^9$ 