Are black holes dark matter
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Are Black Holes Dark Matter?
Primordial Black Holes as Dark Matter Candidates
Formation and Characteristics of Primordial Black Holes (PBHs)
Primordial black holes (PBHs) are considered a natural candidate for dark matter (DM) in the universe. These black holes are thought to form in the early universe from the collapse of large density perturbations generated by inflation Villanueva-Domingo2021Green2020. PBHs can emit particles due to Hawking evaporation and accrete surrounding matter, which could impact the evolution of the universe and the formation of structures .
Observational Constraints and Detection
The detection of gravitational waves from mergers of tens of solar mass black hole binaries has increased interest in PBHs as a dark matter candidate . Current observational constraints leave only a few PBH mass ranges as viable for dark matter, such as around (10^{-12} M_{\odot}) . The LISA mission could potentially detect the gravitational wave power spectrum associated with these PBHs, providing a testable prediction .
Constraints from Galactic Observations
Observations of the Galactic Center 511 keV (\gamma)-ray line provide strong constraints on the fraction of dark matter that can be composed of low mass PBHs. PBHs with masses (\leq 10^{15}) kg will evaporate to produce positrons, which then annihilate with ambient electrons, producing detectable (\gamma)-rays. These constraints suggest that PBHs contribute to less than 1% of the dark matter density .
Evidence Supporting PBHs as Dark Matter
Several observations support the existence of PBHs as a significant fraction of dark matter. These include the mass and spin of black holes detected by LIGO/VIRGO, microlensing events of distant quasars and stars, and the existence of super-massive black holes early in the universe's history. These hints are compatible with a broad PBH mass distribution peaking around (3 M_{\odot}) .
Alternative Theories and Models
2-2-Holes as Dark Matter
An alternative to PBHs is the concept of 2-2-holes, which arise in quadratic gravity. These objects possess a black hole exterior without an event horizon and may serve as the endpoint of gravitational collapse. They exhibit intriguing thermodynamic behavior and could provide a resolution for the information loss problem. The cosmological and astrophysical implications of 2-2-hole remnants as dark matter are also being explored .
Nonthermal Production of Dark Matter
Another scenario involves the nonthermal production of dark matter from the evaporation of primordial black holes. This process could produce all the dark matter in the universe if the scalar power spectrum at small scales is significantly enhanced .
Conclusion
While black holes, particularly primordial black holes, are a compelling candidate for dark matter, there are significant constraints and ongoing debates in the scientific community. Observational evidence and theoretical models continue to evolve, providing new insights and challenges in understanding the true nature of dark matter.
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A Brief Review on Primordial Black Holes as Dark Matter
Primordial black holes are a potential candidate for dark matter, with their formation, abundance, and signatures potentially impacting the evolution of the Universe and the formation of structures.
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