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Primordial Black Holes - Recent Developments

B. J. Carr

TL;DR

Primordial black holes (PBHs) offer a unique cross-disciplinary probe of the early Universe, gravitational collapse, high-energy physics, and quantum gravity by linking formation physics to evaporative signatures. The paper surveys formation mechanisms—driven by primordial inhomogeneities, phase transitions, and inflationary perturbations—as well as the rich phenomenology of Hawking evaporation, including possible Planck-mass relics and dark-matter implications. It also discusses observational constraints from gamma-ray backgrounds and cosmic-ray spectra, and explores high-energy signatures in accelerators and brane-world cosmologies, where TeV-scale gravity could enable PBH-like production and altered evaporation. The work emphasizes that PBHs can constrain inflationary dynamics, critical collapse, and new physics, while offering potential observational tests across cosmology, astrophysics, and particle physics.

Abstract

Recent developments in the study of primordial black holes (PBHs) will be reviewed, with particular emphasis on their formation and evaporation. PBHs 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 formed. In the "early Universe" context, particularly useful constraints can be placed on inflationary scenarios, especially if evaporating PBHs leave stable Planck-mass relicts. In the "gravitational collapse" context, the existence of PBHs could provide a unique test of the sort of critical phenomena discovered in recent numerical calculations. In the "high energy physics" context, information may come from gamma-ray bursts (if a subset of these are generated by PBH explosions) or from cosmic rays (if some of these derive from evaporating PBHs). In the "quantum gravity" context, the formation and evaporation of small black holes could lead to observable signatures in cosmic ray events and accelerator experiments, providing there are extra dimensions and providing the quantum gravity scale is around a TeV.

Primordial Black Holes - Recent Developments

TL;DR

Primordial black holes (PBHs) offer a unique cross-disciplinary probe of the early Universe, gravitational collapse, high-energy physics, and quantum gravity by linking formation physics to evaporative signatures. The paper surveys formation mechanisms—driven by primordial inhomogeneities, phase transitions, and inflationary perturbations—as well as the rich phenomenology of Hawking evaporation, including possible Planck-mass relics and dark-matter implications. It also discusses observational constraints from gamma-ray backgrounds and cosmic-ray spectra, and explores high-energy signatures in accelerators and brane-world cosmologies, where TeV-scale gravity could enable PBH-like production and altered evaporation. The work emphasizes that PBHs can constrain inflationary dynamics, critical collapse, and new physics, while offering potential observational tests across cosmology, astrophysics, and particle physics.

Abstract

Recent developments in the study of primordial black holes (PBHs) will be reviewed, with particular emphasis on their formation and evaporation. PBHs 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 formed. In the "early Universe" context, particularly useful constraints can be placed on inflationary scenarios, especially if evaporating PBHs leave stable Planck-mass relicts. In the "gravitational collapse" context, the existence of PBHs could provide a unique test of the sort of critical phenomena discovered in recent numerical calculations. In the "high energy physics" context, information may come from gamma-ray bursts (if a subset of these are generated by PBH explosions) or from cosmic rays (if some of these derive from evaporating PBHs). In the "quantum gravity" context, the formation and evaporation of small black holes could lead to observable signatures in cosmic ray events and accelerator experiments, providing there are extra dimensions and providing the quantum gravity scale is around a TeV.

Paper Structure

This paper contains 23 sections, 28 equations, 6 figures.

Table of Contents

  1. INTRODUCTION
  2. PBHS AS A PROBE OF PRIMORDIAL INHOMOGENEITIES
  3. Simplistic Analysis
  4. Refinements of Simplistic Analysis
  5. PBHs and Critical Collapse
  6. PBHS AS A PROBE OF THE EARLY UNIVERSE
  7. Soft Equation of State
  8. Collapse of Cosmic Loops
  9. Bubble Collisions
  10. PBHs and Inflation
  11. Varying Gravitational Constant
  12. PBHS AND DARK MATTER
  13. PBH Formation at Phase Transitions
  14. PBHs and Supermassive Black Holes
  15. Planck Mass Relics
  16. ...and 8 more sections

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
  • ...and 1 more figures