Primordial black holes and their gravitational-wave signatures (2310.19857v1)

Abstract: In the recent years, primordial black holes (PBHs) have emerged as one of the most interesting and hotly debated topics in cosmology. Among other possibilities, PBHs could explain both some of the signals from binary black hole mergers observed in gravitational wave detectors and an important component of the dark matter in the Universe. Significant progress has been achieved both on the theory side and from the point of view of observations, including new models and more accurate calculations of PBH formation, evolution, clustering, merger rates, as well as new astrophysical and cosmological probes. In this work, we review, analyse and combine the latest developments in order to perform end-to-end calculations of the various gravitational wave signatures of PBHs. Different ways to distinguish PBHs from stellar black holes are emphasized. Finally, we discuss their detectability with LISA, the first planned gravitational-wave observatory in space.

Overview of the Paper on Primordial Black Holes and Their Gravitational-Wave Signatures

Primordial black holes (PBHs) represent a fascinating area of research in cosmology, offering potential explanations for phenomena such as dark matter and gravitational wave events. The paper authored by Eleni Bagui et al., titled "Primordial black holes and their gravitational-wave signatures," provides a comprehensive review of recent developments in PBH research, focusing particularly on their theoretical foundations and observational prospects via gravitational-wave (GW) studies.

Theoretical Context and Formation Scenarios

The notion of PBHs arises from the hypothesis that in the early Universe, high-density regions could gravitationally collapse to form black holes. The paper revisits the historical framework established by Zel'dovich, Novikov, and Hawking, among others, emphasizing how these early ideas have evolved. Various formation mechanisms are explored, including gravitational instabilities during inflation, phase transitions, and the collapse of topological defects. Notably, the inflationary models, characterized by scenarios such as single-field, multi-field, and hybrid inflationary potentials, are crucial for understanding the enhancements in the scalar power spectrum necessary for PBH formation.

Primordial Power Spectra and Non-Gaussianities

A significant aspect of PBH formation models involves the power spectrum of primordial curvature perturbations. The authors explore the necessity of power spectrum enhancement on small scales for effective PBH generation. They also address the role of non-Gaussian statistics of perturbations, which can significantly affect PBH abundance predictions. Stochastic inflation and quantum diffusion effects play a crucial role in modifying the curvature perturbation distribution, hinting at potential exponential tails that could increase the likelihood of PBH formation even with moderate power spectrum amplitudes.

Gravitational Waves and PBH Detection

PBHs could manifest through gravitational waves in a variety of ways. Binary mergers of PBHs are prime candidates for producing observable GW signals, potentially matching some detected events by LIGO/Virgo. The paper discusses both early-universe and late-universe PBH binaries and how their dynamics and clustering can influence the GW signals. Additionally, the GW spectrum from secondary mechanisms, notably those induced by high-amplitude scalar perturbations, is predicted to be detectable by future GW observatories like LISA.

Observational Implications and Future Prospects

Current observational constraints on PBH abundances are derived from a wide range of phenomena including microlensing, cosmic microwave background (CMB) anisotropies, and dynamical effects. Importantly, the authors highlight the sensitivity of future GW detectors in probing PBH signatures and providing constraints on models. The potential detection of a stochastic background of GWs, generated by PBH mergers and other mechanisms, provides a promising avenue for confirming PBH scenarios.

Limitations and Uncertainties

While the paper is comprehensive, it delineates several uncertainties in current PBH research, notably in the precise details of PBH formation and merger rates. Discrepancies in the predicted and observed BH mass spectra and merger rates point to the need for more detailed modeling, incorporating variables such as mass distributions, clustering effects, and accretion.

Conclusion

The review by Bagui et al. consolidates the theoretical and observational landscape surrounding PBHs, underscoring their potential to unravel fundamental mysteries in cosmology, such as dark matter composition, the origin of GWs, and the nature of the early Universe. Continued advancements in GW astronomy are expected to play a pivotal role in either constraining or validating PBH models, further enhancing our understanding of cosmic evolution. The paper serves as a pivotal reference point for researchers aiming to bridge the gap between theoretical models and observational data in the quest to elucidate the enigmatic nature of primordial black holes.