Interpreting the Global 21-cm Signal from High Redshifts. I. Model Independent Constraints

TL;DR

This paper develops a model-independent framework to interpret the global 21-cm signal, providing constraints on the intergalactic medium's properties during the early universe without relying on specific astrophysical models.

Contribution

It introduces a simplified, model-independent method to extract physical constraints from the 21-cm signal, focusing on the IGM's properties without detailed source modeling.

Findings

The 21-cm absorption signal constrains the Universe's heating rate density.

The method provides bounds on the Lyman-alpha background intensity.

It allows inference of the ionized fraction and its evolution independently.

Abstract

The sky-averaged (global) 21-cm signal is a powerful probe of the intergalactic medium (IGM) prior to the completion of reionization. However, it has so far been unclear that even in the best case scenario, in which the signal is accurately extracted from the foregrounds, that it will provide more than crude estimates of when the Universe's first stars and black holes form. In contrast to previous work, which has focused on predicting the 21-cm signatures of the first luminous objects, we investigate an arbitrary realization of the signal, and attempt to translate its features to the physical properties of the IGM. Within a simplified global framework, the 21-cm signal yields quantitative constraints on the Lyman-alpha background intensity, net heat deposition, ionized fraction, and their time derivatives, without invoking models for the astrophysical sources themselves. The 21-cm absorption signal is most easily interpreted, setting strong limits on the heating rate density of the Universe with a measurement of its redshift alone, independent of the ionization history or details of the Lyman-alpha background evolution. In a companion paper we extend these results, focusing on the confidence with which one can infer source emissivities from IGM properties.