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

TL;DR

This work probes memory-burden–modified Hawking evaporation of light primordial black holes (PBHs) and their imprint on the global 21 cm signal during cosmic dawn. By computing MB-altered energy injection into the intergalactic medium and evolving the gas temperature with the DarkHistory framework, the authors derive constraints on the PBH dark matter fraction $f_{ m PBH}$ under fast (instantaneous) and slow (finite-width) MB transitions. They find that broad slow transitions with $\delta \approx 10^{-2}$ exclude PBHs in the mass range $M_{ m PBH} \simeq 10^{8}$--$10^{13}$ g down to $f_{ m PBH} \lesssim 10^{-8}$, while fast transitions with $k \gtrsim 1.5$ suppress evaporation so strongly that no 21 cm bound remains for $M_{ m PBH} \gtrsim 10^{7}$ g. Overall, the 21 cm cosmological probe provides a powerful, complementary constraint to CMB and BBN bounds on memory-burdened PBHs in the sub-10^{15} g mass window, highlighting the potential of upcoming 21 cm observations to test quantum-gravity–driven modifications of black hole evaporation.

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 $δ=10^{-2}$ 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.

Figures (6)

• Figure 1: Suppression factor $\mathcal{K}$ as a function of PBH mass for the fast and slow memory-burden transitions. Left: Fast-transition limit, where the evaporation rate changes instantaneously. In this regime, the suppression follows the entropy scaling $S\!\propto\! M^{2}$, giving $\mathcal{K}\!\propto\! M^{-2k}$. As a result, the curves (shown for $k=1,2,3$) become increasingly steep with larger $k$, and higher-mass PBHs are suppressed much more strongly. Right: Slow-transition regime ($\delta>0$), where the evaporation evolves smoothly over a finite interval. Here, the heavier PBHs experience less suppression because their longer semiclassical lifetime $\tau_{\rm sc}\propto M^{3}$. Curves for different $\delta$ values show that a larger $\delta$ produces broader and more gradual suppression.
• Figure 2: Redshift evolution of the matter temperature $T_m$ for a benchmark PBH mass $M_{\rm PBH}=3.7\times10^{6}\,\mathrm{g}$ and $f_{\rm PBH}=1$, comparing fast (left) and slow (right) transition to memory burden regime. A fast transition ($k=2$) produces an abrupt suppression of Hawking radiation through $S^{-k}$ scaling, leading to only modest heating and $T_m$ stays below $T_{\rm CMB}$ for all relevant redshifts. For the slow-transition case ($\delta = 10^{-2}$), the suppression turns on gradually, and the PBH continues to evaporate at an appreciable fraction of its semiclassical rate. This prolonged emission injects substantial energy into the IGM, raising $T_m$ above $T_{\rm CMB}$ around $1+z\approx25$.
• Figure 3: 21 cm sensitivity for PBHs in fast and slow transition memory burden phase at $z=17$, shown for fixed abundance $f_{\rm PBH}=1$ . The shaded areas correspond to $T_m(z=17) > T_{\max}\,(=9.41\, {\rm K})$, with $T_m$ the predicted matter temperature. Left: Fast-transition case (controlled by $k$), where the evaporation rate changes sharply. The suppression factor scales as $\mathcal{K}\propto M^{-2k}$, leading to strong suppression of Hawking emission for large $M_{\rm PBH}$. Hence, only lighter PBHs and small values of $k$ produce sufficient heating to be excluded by 21 cm data. Right: Slow transition case with a width $\delta$ of transition, where the evaporation rate evolves gradually over a finite interval. The suppression becomes weaker for larger PBH masses- due to the longer semiclassical lifetime $\tau_{\rm sc}\propto M^{3}$- high mass PBHs retain more luminosity and hence can heat the IGM efficiently. The corresponding sensitivity region, therefore, extends to larger masses.
• Figure 4: Bounds on the PBH-dark matter fraction from the 21 cm signal for fast memory-burden transitions. Each panel shows the upper limit on $f_{\rm PBH}$ as a function of PBH mass $M_{\rm PBH}$ obtained by requiring the matter temperature $T_m(z=17) \leq T_{\max}$. The curves correspond to fast-transition memory-burden models with different suppression exponent $k$ (left: $k=1.0$, right: $k=1.5$). Larger values of $k$ induces steeper suppression $\mathcal{K}\!\propto\! M^{-2k}$, weakening the constraints at high masses.
• Figure 5: Bounds on the PBH-dark matter fraction from the 21 cm signal for slow memory-burden transitions. Each panel shows the upper limit on $f_{\rm PBH}$ as a function of PBH mass $M_{\rm PBH}$ obtained by requiring the matter temperature $T_m(z=17) \leq T_{\max}$. The two panels correspond to slow-transitions with different transition widths $\delta$ (left: $\delta=10^{-2}$, right: $\delta=10^{-10}$). Smaller $\delta$ produces a sharper memory-burden suppression, reducing the emission spectra more strongly and weakening the 21 cm constraint at high masses. Larger $\delta$ yields a more gradual transition, allowing PBHs to remain luminous for longer and tightening the resulting limits on $f_{\rm PBH}$ across a wider mass range.