Primordial Black Holes: Do They Exist and Are They Useful?

TL;DR

This review examines primordial black holes (PBHs) as potential relics from the early Universe and as probes of fundamental physics. It connects PBH formation to primordial density fluctuations, inflationary dynamics, and phase-transition physics, and it discusses how PBH evaporation would imprint gamma-ray and cosmic-ray signatures that constrain their abundance. It also explores PBHs as dark matter candidates, the role of critical phenomena in PBH formation, and how quantum gravity scenarios—such as Planck-mass relics, brane cosmology, and TeV-scale gravity—could be tested via PBH observations or collider phenomena. The work highlights that PBHs offer a unique intersection between cosmology, gravitational collapse, high-energy physics, and quantum gravity, yielding both constraining observations and potentially transformative experimental opportunities.

Abstract

Recent developments in the study of primordial black holes (PBHs) are reviewed, with particular emphasis on their formation and evaporation. It is still not clear whether PBHs formed but, if they did, they could provide a unique probe of the early Universe, gravitational collapse, high energy physics and quantum gravity. Indeed their study may place interesting constraints on the physics relevant to these areas even if they never existed.

Figures (6)

• Figure 1.: Constraints on $\beta(M)$
• Figure 2.: Constraints on $\epsilon(M)$
• Figure 3.: Constraints on spectral index $n$ in terms of reheat time $t_1$
• Figure 4.: Instantaneous emission from a 1 GeV black hole
• Figure 5.: Spectrum of particles from uniformly distributed PBHs