HI and Cosmology: What We Need To Know

TL;DR

This paper reviews the potential of 21 cm line observations of neutral hydrogen across different cosmic epochs to inform cosmology, emphasizing the technical and observational challenges involved.

Contribution

It provides a comprehensive overview of the regimes, challenges, and future prospects for using 21 cm observations to advance cosmological understanding.

Findings

Identification of three key observational regimes: inflationary physics, Dark Ages and reionization, galaxy evolution and Dark Energy.

Discussion of technical and observational challenges in each regime.

Outline of upcoming pathfinder projects and their role in preparing for SKA.

Abstract

There are three distinct regimes in which radio observations of the redshifted 21 cm line of HI can contribute directly to cosmology in unique ways. The regimes are naturally divided by redshift, from high to low, into: inflationary physics, the Dark Ages and reionization, and galaxy evolution and Dark Energy. Each measurement presents its own set of technical, theoretical, and observational challenges, making "what we need to know" not so much an astrophysical question at this early stage as a comprehensive experimental question. A wave of new pathfinder projects are exploring the fundamental aspects of what we need to know (and what we should expect to learn in the coming years) in order to achieve the goals of the Square Kilometer Array (SKA) and beyond.