Primordial Black Hole Scenario for the Gravitational-Wave Event GW150914 (1603.08338v3)

Abstract: We point out that the gravitational-wave event GW150914 observed by the LIGO detectors can be explained by the coalescence of primordial black holes (PBHs). It is found that the expected PBH merger rate would exceed the rate estimated by the LIGO scientific Collaboration and Virgo Collaboration if PBHs were the dominant component of dark matter, while it can be made compatible if PBHs constitute a fraction of dark matter. Intriguingly, the abundance of PBHs required to explain the suggested lower bound on the event rate, $> 2$ events ${\rm Gpc}^{-3} {\rm yr}^{-1}$, roughly coincides with the existing upper limit set by the nondetection of the cosmic microwave background spectral distortion. This implies that the proposed PBH scenario may be tested in the not-too-distant future.

• The paper presents a primordial black hole model to explain the GW150914 event detected by LIGO.
• It employs an event rate analysis that reconciles PBH merger frequencies with upper limits from cosmic microwave background data.
• The study suggests that future gravitational wave and CMB observations can validate the PBH scenario as a component of dark matter.

Primordial Black Hole Scenario for the Gravitational-Wave Event GW150914: An Analytical Review

The gravitational-wave event GW150914, observed by LIGO, initiated an intriguing discourse on the nature of black holes, particularly those implicated in the merger process. Among several theories posited to explain this phenomenon, the paper in question explores the potentiality of primordial black holes (PBHs) in accounting for the observations made by LIGO and Virgo.

The underlying proposition of the paper is that the GW150914 event might have been produced by the merger of a primordial black hole binary. Primordial black holes are hypothetical black holes that are assumed to have formed in the early Universe, potentially prior to the formation of any other astronomical objects. Their formation is speculated to have occurred through the gravitational collapse of high-density regions during cosmic inflation, leading to entities with masses comparable to the horizon mass at the epoch of their formation.

Key Analytical Insights

1. Event Rate and PBH Contribution: The authors explore the event rate of PBH mergers. Traditional calculations of event rates assumed the PBHs to be a dominant component of dark matter with significant mass. However, their paper allows for PBHs to be a fraction of dark matter, providing a more realistic frequency of merger events. The derived event rate surpasses the LIGO inferred estimates if PBHs are the predominant form of dark matter but aligns within limits if they constitute a minor fraction.
2. Conformity with Observational Data: The inclination of the paper aligns the PBH merger rate with the upper limits of cosmic microwave background spectral distortions not detected due to the accretion of gas onto PBHs. This congruity suggests that the PBH model for GW150914 is not only plausible but also verifiable with future experimental advancements.
3. Formation Mechanism Differences: The paper contrasts its approach with previous studies, particularly concerning the mechanics of binary formation and the requisite fraction of PBHs within dark matter. It diverges from the mechanism proposed in prior literature, wherein binary formation was attributed to gravitational interactions during significant sidelong BH encounters.

Implications and Broader Perspectives

The theoretical implications of this research touch expansively on our understanding of dark matter's composition. Experimentally, the premise that PBH fractions nearly saturate the upper bounds from the cosmic microwave background nondetection can be put to the test as experimental sensitivity improves.

• CMB Spectral Distortion Tests: The potential alignment of PBH density limits obtained from this paper with upcoming CMB spectral distortion measurements, such as those envisaged by the PIXIE experiment, could substantiate or invalidate the PBH scenario.
• Mass Distribution Analysis: Further analysis and increased detection frequency of gravitational waves from BH mergers can offer insight into the BH mass distribution, potentially distinguishing between PBH origin scenarios and other astrophysical explanations like Population III star binaries.
• Gravitational Wave Background: Another avenue of investigation is the low-frequency gravitational wave background potentially emitted by long-lived PBH binaries. Understanding this background can yield further insights into the PBH hypothesis.

In conclusion, this paper offers a substantive contribution to the discourse on primordial black holes, proposing a scenario that accounts for GW150914 within a viable framework. While direct evidence remains to be conclusively obtained, the alignment with existing observational upper bounds and the theoretical rigor employed herein presents an intriguing scientific possibility for further exploration.