Observational Evidence for Primordial Black Holes: A Positivist Perspective (2306.03903v2)

Abstract: We review numerous arguments for primordial black holes (PBHs) based on observational evidence from a variety of lensing, dynamical, accretion and gravitational-wave effects. This represents a shift from the usual emphasis on PBH constraints and provides what we term a positivist perspective. Microlensing observations of stars and quasars suggest that PBHs of around $1\,M_{\odot}$ could provide much of the dark matter in galactic halos, this being allowed by the Large Magellanic Cloud microlensing observations if the PBHs have an extended mass function. More generally, providing the mass and dark matter fraction of the PBHs is large enough, the associated Poisson fluctuations could generate the first bound objects at a much earlier epoch than in the standard cosmological scenario. This simultaneously explains the recent detection of high-redshift dwarf galaxies, puzzling correlations of the source-subtracted infrared and X-ray cosmic backgrounds, the size and the mass-to-light ratios of ultra-faint-dwarf galaxies, the dynamical heating of the Galactic disk, and the binary coalescences observed by LIGO/Virgo/KAGRA in a mass range not usually associated with stellar remnants. Even if PBHs provide only a small fraction of the dark matter, they could explain various other observational conundra, and sufficiently large ones could seed the supermassive black holes in galactic nuclei or even early galaxies themselves. We argue that PBHs would naturally have formed around the electroweak, quantum chromodynamics and electron-positron annihilation epochs, when the sound-speed inevitably dips. This leads to an extended PBH mass function with a number of distinct bumps, the most prominent one being at around $1\,M_{\odot}$, and this would allow PBHs to explain many of the observations in a unified way.

• The paper presents comprehensive observational evidence supporting the influence of primordial black holes on dark matter composition and cosmic structure formation.
• The paper utilizes methodologies such as gravitational lensing, galactic dynamics, and CMB analyses to identify key signatures of PBHs.
• The paper outlines future research opportunities through advanced gravitational wave detectors and next-generation surveys to refine PBH cosmological roles.

Observational Evidence for Primordial Black Holes: A Comprehensive Review

The manuscript discusses the compelling observational evidence in favor of primordial black holes (PBHs), significantly deviating from the traditional focus on constraints against their existence. This provides a broader and more affirmative perspective on the possibility that PBHs contribute to cosmological phenomena, including dark matter composition, early galaxy formation, and gravitational wave (GW) events.

Overview of Observational Evidence

1. Gravitational Lensing: The document details various observations of gravitational lensing that suggest the presence of PBHs. Traditional gravitational lensing techniques, along with recent microlensing observations (e.g., from MACHO and OGLE projects), suggest that PBHs can account for a significant fraction of compact dark matter in galactic halos. The positive identification of stellar objects micro-lensed by unseen massive bodies aligns with the hypothesis that these bodies could be PBHs in the mass range of a few solar masses.
2. Galactic Dynamics and Accretion: PBHs are considered in explaining the kinematic peculiarities observed in galactic dynamics. They are posited to provide dynamical heating of stellar disks and influence the structural formation of ultra-faint dwarf galaxies (UFDGs). Furthermore, PBHs could explain the dynamically inferred paucity of high-velocity stars if they disrupt star trajectories through gravitational interactions.
3. Cosmic Microwave Background (CMB) and Large-Scale Structures: The document speculates that PBHs could have influenced early cosmological epochs, particularly the thermal history of the universe, by accreting baryonic matter and contributing to cosmic backgrounds. Accretion scenarios are considered that may align with fluctuations observed in X-ray and infrared cosmic backgrounds, suggesting a non-negligible role of PBHs in shaping LSS.
4. Gravitational Wave Phenomena: The analysis explores the potential primordial nature of some components in GW events detected by LIGO/Virgo, particularly in mass ranges and configurations atypical for stellar remnants. This includes interpreting events within supposed "mass gaps" as possibly primordial in origin, rather than resultant from stellar evolution.
5. New Astrophysical Phenomena: The implications of PBHs in explaining various cosmic observations remain significant, including their potential role in the early formation of supermassive black holes (SMBHs) and intriguing links to intriguing phenomena such as fast radio bursts (FRBs) and the missing pulsar problem near galactic centers.

Theoretical and Future Implications

The presence of PBHs across different mass scales necessitates reevaluating traditional cosmological models, particularly concerning non-baryonic dark matter candidates. The document sets a plausible theoretical framework linking PBHs with the primordial density fluctuations characterized by distinct mass windows from particle physics epochs like the QCD transition.

Additionally, future developments in astrophysical surveys (e.g., those conducted by the JWST and LSST) may provide further scrutiny and potential evidence of PBHs through detailed analysis of early cosmic structures and faint X-ray/infrared sources. With enhancements in GW detectors, there's growing potential to discern primordial characteristics more explicitly in GW signatures.

Conclusion

The paper argues for a paradigmatic shift from dismissive constraints to a qualified endorsement of primordial black holes' contributions to cosmic phenomena. This extends beyond dark matter themselves, reshaping cosmological narratives around origins of large-scale structures and the interplay of baryonic and non-baryonic components in the universe's evolution. This holistic approach underlines a unified model that potentially integrates PBHs into the broader tapestry of cosmic history, suggesting numerous avenues for future research to further specify their cosmological roles.