Forecasted 21 cm constraints on compensated isocurvature perturbations

TL;DR

This paper explores how future 21 cm absorption observations could detect compensated isocurvature perturbations generated by curvaton models, requiring large radio arrays for significant detection.

Contribution

It provides the first estimate of the large collecting area needed for detecting curvaton-induced compensated isocurvature perturbations via 21 cm observations.

Findings

A 20 km² Fourier transform telescope could detect these perturbations at over 5 sigma significance.

Compensated isocurvature perturbations have no effect on CMB or galaxy surveys, but affect 21 cm signals.

Detection requires very large, advanced radio arrays.

Abstract

A "compensated" isocurvature perturbation consists of an overdensity (or underdensity) in the cold dark matter which is completely cancelled out by a corresponding underdensity (or overdensity) in the baryons. Such a configuration may be generated by a curvaton model of inflation if the cold dark matter is created before curvaton decay and the baryon number is created by the curvaton decay (or vice-versa). Compensated isocurvature perturbations, at the level producible by the curvaton model, have no observable effect on cosmic microwave background anisotropies or on galaxy surveys. They can be detected through their effect on the distribution of neutral hydrogen between redshifts 30 to 300 using 21 cm absorption observations. However, to obtain a good signal to noise ratio, very large observing arrays are needed. We estimate that a fast Fourier transform telescope would need a total collecting area of about 20 square kilometers to detect a curvaton generated compensated isocurvature perturbation at more than 5 sigma significance.