Primordial Black Holes - Perspectives in Gravitational Wave Astronomy - (1801.05235v1)

Abstract: This is a review article on the primordial black holes (PBHs), with particular focus on the massive ones ($\gtrsim 10^{15}{\rm g}$) which have not evaporated by the present epoch by the Hawking radiation. By the detections of gravitational waves by LIGO, we have gained a completely novel tool to observationally search for PBHs complementary to the electromagnetic waves. Based on the perspective that gravitational-wave astronomy will make a significant progress in the next decades, a purpose of this article is to give a comprehensive review covering a wide range of topics on PBHs. After discussing PBH formation as well as several inflation models leading to PBH production, we summarize various existing and future observational constraints. We then present topics on formation of PBH binaries, gravitational waves from PBH binaries, various observational tests of PBHs by using gravitational waves.

• The paper demonstrates gravitational wave detection as an innovative method to identify and analyze primordial black holes via their merger event signatures.
• It investigates PBH formation, using inflationary models to constrain their mass distribution and potential contribution to dark matter.
• It highlights future prospects with next-generation observatories to refine models of early universe conditions and PBH dynamics.

Primordial Black Holes: Perspectives in Gravitational Wave Astronomy

The paper "Primordial Black Holes - Perspectives in Gravitational Wave Astronomy" authored by S. Sasaki et al. offers a comprehensive assessment of the role of primordial black holes (PBHs) in the context of gravitational wave (GW) astronomy. The advent of gravitational wave detection technologies like LIGO has opened up new opportunities to investigate PBHs, especially those with masses above $10^{15}$ g, which have not evaporated due to Hawking radiation by the present epoch.

Overview of Primordial Black Holes

Primordial black holes are hypothesized to form in the early universe following density fluctuations which can collapse under their gravity. Unlike supermassive or stellar black holes formed through astrophysical processes like the collapse of massive stars, PBHs feature a wider range of possible masses and could have formed seconds after the Big Bang. This paper explores mechanisms of PBH formation, various inflationary models that predict PBH creation, and observational constraints currently in place.

Gravitational Wave Detection and PBHs

The detection of gravitational waves has revolutionized the means by which PBHs can be studied. Gravitational wave observations provide a direct and complementary methodology to electromagnetic means for probing these elusive objects. This paper highlights how GW astronomy could significantly augment our understanding of PBHs by helping to identify potential PBHs through their GW signatures.

Constraints and Observational Challenges

The paper reviews existing and future observational constraints on PBHs using gravitational wave data. Current electromagnetic observations and microlensing surveys have already constrained PBHs' mass and abundance within the dark matter paradigm. Gravitational wave astronomy, with its increased sensitivity and observational reach, will further narrow down the parameter space for PBH candidates. This involves assessing candidate GW signals for characteristics typical of PBHs, such as certain merger event rates or spectral features.

Binary Formation and Gravitational Wave Emissions

The formation of PBH binaries and their gravitational wave characteristics are extensively discussed. The paper suggests that PBH binaries could form either in the early universe through decoupling from cosmic expansion or by dynamical processes in dense environments at later cosmic times. These binaries are significant sources of gravitational waves, and their event rate estimations provide insights into the abundance of PBHs. The implications here are twofold: understanding GW signals from such binaries allows for the probing of PBH existence and provides insight into the conditions prevalent in the early universe.

Implications and Future Prospects

Theoretical and practical implications of this field of paper are profound. Gravitational wave observations of PBH mergers could provide not only evidence of PBHs' contribution to dark matter but also serve as probes for high-energy physics taking place in the early universe. The paper proposes strategies for discerning PBH contributions from those of astrophysical black holes, contemplating how observational data and theoretical models can jointly refine our understanding of cosmic history.

Conclusion

In conclusion, this review highlights the potential combinatory power of gravitational wave astronomy with traditional methods to probe the existence and characteristics of primordial black holes. It sets a framework for future developments in the field by connecting theoretical modeling with observational opportunities provided by advanced GW detectors. As GW astronomy progresses, we will better understand the cosmos's initial conditions and dark matter's constituent parts, with PBHs playing a crucial role in both regards. The paper invites ongoing discussion and research into integrating GW observations into the broader cosmological context to explore the universe's formative epochs.

Future Directions

Looking forward, the paper speculates about the role that next-generation space-based observatories and advanced terrestrial facilities will play in distinguishing among competing models of PBH formation and evolution. It emphasizes the significance of analyzing future GW data streams in tandem with electromagnetic signals to construct a more comprehensive picture of PBH dynamics and their cosmic roles. In summary, the synthesis of GW data with the paper of PBHs stands as a gateway to deepening our grasp of both primordial and contemporary cosmic energy densities and structures.