Probing memory-burdened Primordial Black Holes with global 21 cm signal

TL;DR

This study explores how memory-burdened primordial black holes could influence the global 21 cm signal during cosmic dawn, providing new constraints on their abundance and mass range based on their extended evaporation effects.

Contribution

It introduces the impact of the memory burden effect on PBH evaporation and derives novel constraints on low-mass PBHs from the global 21 cm signal observations.

Findings

PBHs in the mass range 10^8-10^13 g are constrained at f_PBH > 10^-8 for certain transition scenarios.

Memory burden effect extends PBH lifetimes, affecting early universe energy injection.

Constraints vary significantly with the transition dynamics of the memory-burden phase.

Abstract

We investigate the imprints of memory-burdened primordial black holes (PBH) on the global 21 cm signal during the cosmic dawn. Recent studies reopened the possibility of a mass window of PBHs as a compelling candidate for dark matter, particularly in low-mass regimes ($M_{\text {PBH}}< 10^{15}$ g) where conventional constraints from evaporation are being revisited in light of quantum gravitational effects. One such effect, the \textit{memory burden effect}, slows down black hole evaporation by incorporating the backreaction of radiation on the black hole microstates, substantially extending the lifetime of light PBHs and thus modifying their late-time emission spectra. This prolonged emission can dramatically alter the energy injection history in the early universe. By computing the modified energy injection rates into the intergalactic medium and incorporating them into the thermal and ionization evolution of neutral hydrogen, we obtain projected constraints on the fraction of dark matter. The bounds are obtained from the fact that these low mass PBHs, which were thought otherwise evaporated, can modify the absorption amplitude in the global 21 cm signal at redshift $z\approx17$. Considering the two viable scenarios of transition to the memory-burden phase: fast (or instantaneous) and slow (transition with a finite width), we show how the 21 cm bounds are sensitive to different mass ranges. For a broad transition with $\delta=10^{-2}$ we find that PBHs in the mass range $M_{\rm PBH}\simeq10^{8}$-$10^{13}$ g are excluded at the level of $f_{\rm PBH}\gtrsim10^{-8}$. In contrast, for a fast-transition case ($k=1$), the evaporation is suppressed so efficiently that no meaningful 21 cm constraint remains for $M_{\rm PBH}\gtrsim10^{7}$ g.