Primordial Black Holes (0801.0116v1)

Abstract: Primordial black holes (PBHs) are a profound signature of primordial cosmological structures and provide a theoretical tool to study nontrivial physics of the early Universe. The mechanisms of PBH formation are discussed and observational constraints on the PBH spectrum, or effects of PBH evaporation, are shown to restrict a wide range of particle physics models, predicting an enhancement of the ultraviolet part of the spectrum of density perturbations, early dust-like stages, first order phase transitions and stages of superheavy metastable particle dominance in the early Universe. The mechanism of closed wall contraction can lead, in the inflationary Universe, to a new approach to galaxy formation, involving primordial clouds of massive BHs created around the intermediate mass or supermassive BH and playing the role of galactic seeds.

• The paper demonstrates how mechanisms like density perturbations and phase transitions can trigger primordial black hole formation in the early universe.
• It employs observational constraints, such as gamma-ray backgrounds and gravitational wave signals, to limit PBH abundance and probe high-energy physics.
• The study implicates PBHs in galaxy formation and dark matter models, offering refined insights into inflationary dynamics and early cosmological conditions.

Primordial Black Holes: Insights and Implications

The discussed paper by Maxim Yu. Khlopov explores the concept of primordial black holes (PBHs) and their potential implications for both cosmology and particle physics. PBHs are formed in the early universe and could serve as a probe into the high-energy physical processes that might have taken place at that time. This essay provides an overview of the paper's key arguments and findings, with a focus on the formation mechanisms, constraints, and potential implications of PBHs.

Mechanisms of PBH Formation

The paper outlines several mechanisms by which PBHs could form in the early universe:

1. Density Perturbations: Initial density inhomogeneities and non-linear metric perturbations could lead to PBH formation if these perturbations collapse gravitationally.
2. Inflationary Dynamics: Features like a blue spectrum with enhanced power on small scales or spikes from phase transitions could increase the probability of PBH formation.
3. First-Order Phase Transitions: As the universe cools, first-order phase transitions could generate bubbles of true vacuum that collide and potentially result in PBH formation.
4. Topological Defects: Phase transitions could also produce topological defects such as cosmic strings or closed vacuum walls, which may collapse into PBHs under certain conditions.

Observational Constraints and Theoretical Implications

PBHs, particularly those evaporating due to Hawking radiation, provide valuable constraints on particle physics models. For example, the paper discusses how constraints on PBH abundance and evaporation can limit models predicting enhanced early universe inhomogeneity or particle states beyond the standard model.

Observational constraints are derived from several phenomena:

• PBH evaporation can produce secondary particles, such as gamma rays, which contribute to the cosmic background and must be consistent with observations.
• The mass distribution of PBHs reflects conditions in the early universe, thus offering a unique window into high-energy physics inaccessible by other means.
• Gravitational wave signals from events like closed wall collapse or PBH mergers provide another observational avenue.

Implications for Cosmology

The work implies significant cosmological impacts:

• Galaxy Formation Theories: The paper speculates on the role of PBHs in galaxy formation, suggesting that massive PBH clusters could act as seeds for early galaxies.
• Dark Matter Considerations: PBHs could constitute a portion of dark matter, an idea depending on their abundance and mass spectrum over cosmic history.
• Cosmological Models of Inflation: The constraints on PBH formation shed light on inflationary models by limiting the possible types and scales of density perturbations that could arise during inflation.

Conclusion and Future Directions

The paper of PBHs tackles fundamental questions about the universe's origin and evolution. By analyzing the formation and constraints surrounding PBHs, the paper contributes to the broader understanding of cosmological and particle physics. Future research could explore the implications of PBHs for dark matter, refine observational techniques for detecting PBHs, and further explore their role in explaining cosmic structural formation.

Primordial black holes remain a pivotal topic in cosmology, offering insights into the complex interplay of microphysics and the evolution of the macro universe. Through improved observational data, targeted research might either confirm the presence of PBHs or further constrain theoretical scenarios of the early universe.