Constraining primordial black holes as dark matter using the global 21-cm signal with X-ray heating and excess radio background

TL;DR

This paper uses the global 21-cm signal data, considering X-ray heating and excess radio background, to place new constraints on primordial black holes as dark matter candidates in the mass range 10^{15}-10^{17} g.

Contribution

It introduces improved constraints on primordial black hole dark matter by incorporating the full shape of the 21-cm absorption feature and effects of X-ray heating and excess radio background.

Findings

Excludes PBHs with fraction > 10^{-9.7} for 10^{15} g mass at 95% CL.

Constraints weaken as PBH mass increases, following an approximate M^4 relation.

Provides bounds on the small-scale curvature power spectrum.

Abstract

Using the global 21-cm signal measurement by the EDGES collaboration, we derive constraints on the fraction of the dark matter that is in the form of primordial black holes (PBHs) with masses in the range $10^{15}$-$10^{17}\,$g. Improving upon previous analyses, we consider the effect of the X-ray heating of the intergalactic medium on these constraints, and also use the full shape of the 21-cm absorption feature in our inference. In order to account for the anomalously deep absorption amplitude, we also consider an excess radio background motivated by LWA1 and ARCADE2 observations. Because the heating rate induced by PBH evaporation evolves slowly, the data favour a scenario in which PBH-induced heating is accompanied by X-ray heating. Also, for the same reason, using the full measurement across the EDGES observation band yields much stronger constraints on PBHs than just the redshift of absorption. We find that 21-cm observations exclude $f_{\mathrm{PBH}} \gtrsim 10^{-9.7}$ at 95% CL for $M_{\mathrm{PBH}}=10^{15}\,$g. This limit weakens approximately as $M_{\mathrm{PBH}}^4$ towards higher masses, thus providing the strongest constraints on ultralight evaporating PBHs as dark matter over the entire mass range $10^{15}$-$10^{17}\,$g. Under the assumption of a simple spherical gravitational collapse based on the Press-Schechter formalism, we also derive bounds on the curvature power spectrum at extremely small scales ($k\sim 10^{15}\,$Mpc$^{-1}$). This highlights the usefulness of global 21-cm measurements, including non-detections, across wide frequency bands for probing exotic physical processes.