The observable 21cm signal from reionization may be perturbative

TL;DR

This paper develops a perturbative bias expansion model for the 21cm signal from reionization, showing it accurately describes the signal over relevant scales and offers insights into its shape and underlying physics.

Contribution

It introduces an effective perturbation theory for the 21cm reionization signal, linking it to source biases and ionized region properties, validated by large-scale simulations.

Findings

Bias expansion describes 21cm modes over much of the probed wavenumber range.

The signal can be characterized with 2-3 bias coefficients.

Bias coefficients relate to source biases, neutral fraction, and ionized region size.

Abstract

We develop an effective perturbation theory (and, equivalently, a bias expansion) for the inhomogeneous 21cm radiation field from reionization. Using large-scale simulations of cosmological reionization, we find that this expansion describes the modes in the simulated 21cm signal over much of the wavenumber range probed by upcoming 21cm arrays. This result provides an understanding of the potential signal shapes that compliments the nonlinear numerical modeling that has been the focus of most previous work. We find that the observable signal often can be described with 2-3 bias coefficients that can be interpreted in terms of the source biases, the average neutral fraction, the characteristic size of ionized regions, and the patchiness of reionization. The 21cm signal serves as a wacky example of a bias expansion in cosmology, with our approach synthesizing key results.