Ultralight Primordial Black Holes (2405.00546v1)

Abstract: The fate of ultralight black holes depends on whether or not evaporation stops at or around the Planck scale. If evaporation stops, the general expectation is that a population of Planck-scale will be left over, possibly including a significant fraction of electrically charged relics. If evaporation does not stop, a runaway "explosion" would occur, with significant and potentially detectable high-energy emission. Here, I review both possibilities, with an emphasis on current status and future detection prospects.

• The paper investigates the evaporation dynamics of ultralight PBHs, evaluating scenarios where evaporation halts at the Planck scale or continues to produce high-energy emissions.
• It studies the possibility of stable Planck-scale relics as dark matter candidates, focusing on their physical properties and the challenges in detecting uncharged remnants.
• The research outlines detection prospects using advanced setups like paleo-detectors, neutrino observatories, and large-scale dark matter detectors to capture PBH signals.

Overview of Ultralight Primordial Black Holes

The paper "Ultralight Primordial Black Holes" by S. Profumo presents a detailed investigation into the end-stage processes of primordial black holes (PBHs) with sub-Planckian mass scales. The primary focus is on whether PBH evaporation halts at or around the Planck scale, potentially resulting in stable Planck-scale relics, or whether it continues uninhibited, leading to complete disintegration and a burst of high-energy emissions. The paper analyzes these scenarios and discusses prospects for detection, theoretical implications, and future research directions.

Key Discussion Points

1. PBH Evaporation Dynamics: The paper carefully explores the fate of ultralight PBHs, particularly how the process of Hawking evaporation might lead to varying outcomes as the black hole's mass approaches the Planck scale. Three possibilities are considered: the formation of stable Planck-scale relics, the creation of naked singularities, or continued evaporation down to zero mass. This latter scenario may result in a detectable runaway high-energy emission.
2. Planck-Scale Relics: If PBHs leave stable remnants, these could conceivably contribute to or even constitute the dark matter in the universe. The paper explores the physical characteristics of such relics, including their potential to carry electromagnetic or even exotic charges. Experimental detection would be challenging unless these remnants are charged, as uncharged ones are effectively undetectable. The impulse-momentum characteristics of these remnants are described, considering their formation history and possible velocities.
3. Detection Prospects: The paper evaluates the detection possibilities for Planck-scale relics. Traditional high-energy physics detectors are inefficient for detecting such massive stable particles. Instead, the potential for paleo-detectors, neutrino observatories, and large-scale direct dark matter detectors like XENON1T and the proposed ICARUS, holds more promise. These detectors can potentially capture signatures left by charged relics.
4. Runaway PBH Explosions: The paper addresses the possibility that PBH could finish evaporating in contemporary times, resulting in what Hawking termed "explosions". The discussion includes evaluating the observational signatures of such events, likening them to gamma-ray bursts but with distinct characteristics like proper motion. Detection mechanisms and sensitivity analyses are provided for various current and future gamma-ray observatories, with a conclusion that unless PBH explosions occur exceptionally close by, detection remains challenging.
5. Cosmological Implications and Constraints: The paper extends significantly into the field of cosmology, touching upon the matter of how evaporating PBHs prior to big bang nucleosynthesis could contribute to the reheating of the universe, the production of dark matter and baryon asymmetry, or even a high-frequency gravitational wave background. Furthermore, the paper discusses the vacuum stability implications connected with black hole evaporation in the early universe.

Implications and Future Directions

This examination into the end processes of ultralight PBHs illuminates several experimental and theoretical avenues. The existence and detection of Planck-scale relics would open a new frontier in understanding dark matter and quantum gravity effects at minuscule scales. Recent and upcoming high-energy observatories, combined with innovative detection strategies like the use of paleo-detectors, place the exploration of PBHs on a promising trajectory.

Furthermore, new insights into cosmological histories pre-BBN provide depth to existing theoretical models and potentially expand our understanding of the dark sector and early universe conditions. Future work might explore experimental setups capable of fine-tuning detection thresholds, alongside more detailed modeling of PBH-induced phenomena within the cosmic timeline. As such, this domain promises to contribute substantially to foundational physics questions related to dark matter constituents, quantum gravity, and beyond-standard-model physics.