Priyanka Sarmah and Kingman Cheung, researchers from the University of California, Berkeley, and the University of Kansas, respectively, have delved into the intriguing world of primordial black holes (PBHs) and their potential impact on the early universe. Their work, published in the journal Physical Review D, explores how these enigmatic objects might influence the global 21 cm signal, a key tool for understanding the cosmos’ infancy.
Primordial black holes are hypothetical black holes that could have formed in the early universe, potentially serving as a candidate for dark matter. Recent studies have suggested that PBHs with masses less than 10^15 grams could be viable dark matter candidates, as conventional constraints from evaporation are being revisited in light of quantum gravitational effects. One such effect, dubbed the “memory burden effect,” slows down black hole evaporation by considering the backreaction of radiation on the black hole microstates. This prolongs the lifetime of light PBHs and modifies their late-time emission spectra, which can dramatically alter the energy injection history in the early universe.
Sarmah and Cheung investigated how this prolonged emission from PBHs could affect the global 21 cm signal, which is a measure of the spin-flip transition of neutral hydrogen atoms. This signal is sensitive to the thermal and ionization history of the universe and can provide valuable insights into the early cosmos. By computing the modified energy injection rates into the intergalactic medium and incorporating them into the thermal and ionization evolution of neutral hydrogen, the researchers obtained projected constraints on the fraction of dark matter that could be composed of PBHs.
The researchers considered two scenarios for the transition to the memory-burden phase: a fast (or instantaneous) transition and a slow transition with a finite width. They found that the 21 cm bounds are sensitive to different mass ranges in these scenarios. For a broad transition with a width of 10^-2, PBHs in the mass range of 10^8 to 10^13 grams are excluded at the level of f_PBH greater than or equal to 10^-8. In contrast, for a fast-transition case, the evaporation is suppressed so efficiently that no meaningful 21 cm constraint remains for PBHs with masses greater than or equal to 10^7 grams.
For the energy sector, this research highlights the potential of using cosmological observations to constrain the properties of dark matter candidates. As we continue to explore the universe and its energy dynamics, understanding the nature of dark matter and its interactions with ordinary matter will be crucial for developing new energy technologies and harnessing the cosmos’ vast energy resources. Moreover, the study of primordial black holes and their effects on the early universe can provide valuable insights into the fundamental laws of physics and the behavior of matter and energy under extreme conditions.
This article is based on research available at arXiv.