An Effective Bias Expansion for 21 cm Cosmology in Redshift Space

TL;DR

This paper develops an effective field theory approach for 21cm cosmology during reionization, incorporating bias and redshift space distortions, validated against simulations, and highlighting the potential for astrophysics-free cosmological measurements.

Contribution

It introduces a refined EFT framework for 21cm brightness temperature analysis in redshift space, including non-linear effects and validation with diverse simulations.

Findings

EFT fits match simulations within a few percent for relevant scales.

The method captures non-linear and astrophysical variations in reionization.

Redshift space measurements contain direct cosmological information independent of astrophysics.

Abstract

A near-future detection of the 21cm signal from the epoch of reionization will provide unique opportunities to probe the underlying cosmology, provided that such cosmological information can be extracted with precision. To this end, we further develop effective field theory (EFT) inspired techniques for the 21cm brightness temperature field during the epoch of reionization, incorporating renormalized bias and a treatment of redshift space distortions. Notably, we confirm that in redshift space, measures of the 21cm brightness, e.g the power spectrum, should have irreducible contributions that lack a bias coefficient and therefore contain direct, astrophysics-free information about the cosmological density field; in this work, we study this effect beyond linear order. To validate our theoretical treatment, we fit the predicted EFT Fourier-space shapes to the THESAN suite of hydrodynamical simulations of reionization at the field level, where the considerable number of modes prevents overfitting. We find agreement at the level of a few percent between the 21cm power spectrum from the EFT fits and simulations over the wavenumber range $k \lesssim 0.8$ h/Mpc and neutral fraction $x_\mathrm{HI} \gtrsim 0.4$, which is imminently measurable by the Hydrogen Epoch of Reionization Array (HERA) and future experiments. The ability of the EFT to describe the 21cm signal extends to simulations that have different astrophysical prescriptions for reionization as well as simulations with interacting dark matter.