Global 21 cm signal: a promising probe of primordial features

TL;DR

This paper investigates how bump-like primordial features influence the global 21 cm signal, revealing their potential to probe inflationary physics and establishing constraints on their amplitude using current CMB data.

Contribution

It provides a detailed analysis of primordial bump-like features' effects on the 21 cm signal and derives observational constraints on their amplitude.

Findings

Primordial features significantly alter the ionization history and 21 cm profile.

A special scale exists where the effect of primordial features vanishes.

Upper limits on feature amplitude are derived from CMB optical depth data.

Abstract

Inflationary models that involve bursts of particle production generate bump-like features in the primordial power spectrum of density perturbations. These features influence the evolution of density fluctuations, leaving their unique signatures in cosmological observations. A detailed investigation of such signatures would help constrain physical processes during inflation. With this motivation, the goal of this paper is two-fold. First, we conduct a detailed analysis of the effects of bump-like primordial features on the sky-averaged 21 cm signal. Using semi-numerical simulations, we demonstrate that the primordial features can significantly alter the ionization history and the global 21 cm profile, making them a promising probe of inflationary models. We found a special scale (namely, the turnover wavenumber, $k^{\rm turn}$) at which the effect of primordial bump-like features on the global 21 cm profile vanishes. Also, we found that the behaviour of the primordial features on the global profile and ionization history are quite opposite for $k > k^{\rm turn}$ and $k < k^{\rm turn}$. We trace the root cause of these behaviours to the effects of primordial features on the halo mass function at high redshifts. Furthermore, we discuss the degeneracy between the astrophysical parameters and the primordial features in detail. Secondly, for a fixed set of astrophysical parameters, we derive upper limits on the amplitude of bump-like features in the range $10^{-1} < k\,[{\rm Mpc}^{-1}] < 10^2$ using current limits on optical depth to reionization from CMB data by Planck.