Dark matter and black holes
Pro Research Analysisby 
Searched over 200M research papers
Dark Matter and Black Holes: A Comprehensive Overview
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 in the universe. These black holes are thought to form in the early universe from the collapse of large density perturbations generated during inflation 12. PBHs can have a wide range of masses, and their formation is accompanied by the generation of gravitational waves (GWs) 47. The detection of GWs from mergers of solar-mass black hole binaries has further fueled interest in PBHs as a dark matter candidate .
Constraints on PBH Abundance
Current observational constraints significantly limit the mass ranges in which PBHs can constitute a significant fraction of dark matter. For instance, PBHs with masses around (10^{-12} M_{\odot}) are still considered viable candidates 47. However, PBHs with masses (\leq 10^{15}) kg are severely constrained by the observation of the Galactic Center 511 keV (\gamma)-ray line, as their evaporation would produce positrons that annihilate with ambient electrons, contributing to this (\gamma)-ray line .
Detection and Signatures
The emission of particles due to Hawking evaporation and the accretion of surrounding matter are key signatures of PBHs. These processes can impact the evolution of the universe and the formation of structures . Additionally, the LISA mission is expected to detect the GW power spectrum associated with PBHs, providing a testable prediction for PBH dark matter 47.
Black Holes and Dark Matter Interactions
Black Hole Shadows and Dark Matter
The presence of dark matter around black holes can alter the observable properties of black holes, such as their shadows. A model considering a Schwarzschild black hole surrounded by dark matter shows that the radius of the black hole's shadow changes significantly when the effective radius of the dark matter halo is comparable to (\sqrt{3 M \Delta M}) . This effect is more pronounced when dark matter is concentrated near the black hole.
Nonthermal Production of Dark Matter
Primordial black holes can also contribute to the nonthermal production of dark matter. The evaporation of PBHs before big bang nucleosynthesis can produce dark matter particles, providing a mechanism for generating the correct relic abundance of dark matter 89. This process does not rely on non-gravitational interactions between the dark sector and the Standard Model, making it a promising avenue for dark matter production.
Alternative Black Hole Candidates for Dark Matter
2-2-Holes in Quadratic Gravity
An alternative to PBHs as dark matter candidates are 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, such as a Hawking-like temperature and an entropy-area law. The evaporation of 2-2-holes slows down at a late stage, leading to stable remnants that could constitute dark matter .
Conclusion
Primordial black holes remain a compelling candidate for dark matter, with various formation mechanisms, constraints, and detection methods being actively studied. The interaction between black holes and dark matter, as well as alternative candidates like 2-2-holes, provide rich avenues for future research. Observational missions like LISA are expected to play a crucial role in testing these theories and advancing our understanding of dark matter and black holes.
Sources and full results
Most relevant research papers on this topic