Black holes and dark matter
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Black Holes as Dark Matter Candidates
The idea that black holes could make up dark matter has gained significant attention. Primordial black holes (PBHs), which are black holes formed in the early universe, are considered a natural candidate for dark matter. Several studies suggest that PBHs could account for all or part of the dark matter, depending on their mass and abundance. Observational constraints indicate that PBHs could make up all dark matter if their masses fall within specific ranges, such as around 10^20 grams or in the window between 10^17 and 10^23 grams, and possibly in the range of tens of solar masses, as suggested by gravitational wave detections from LIGO/Virgo 4578+2 MORE.
Primordial Black Holes: Formation, Stability, and Observational Signatures
PBHs are thought to form from density fluctuations in the early universe. Their stability and ability to persist as dark matter depend on their mass and the physics of their evaporation. Some PBHs, especially those that are nearly extremal (close to the maximum charge or spin allowed), could be cosmologically stable and avoid rapid evaporation, making them viable dark matter candidates. These black holes may also interact with hypothetical dark sector particles, such as dark electrons, which can further suppress their decay . The end states of evaporating black holes, possibly stable relics, have also been proposed as a form of dark matter 16.
Black Holes and Dark Matter Interactions
The presence of black holes can influence the distribution of dark matter around them, leading to features like dark matter cusps, spikes, and mounds. The interaction between black holes and dark matter can also affect the evolution of black hole binaries, with energy transfer occurring through dynamical friction. These interactions may leave observable signatures, such as gravitational waves, which could help detect and characterize dark matter overdensities around black holes .
Theoretical Extensions: Dark Black Holes and Dark Quarks
Some theoretical models extend the concept of black holes and dark matter by introducing dark sector analogs, such as dark quarks and dark black holes. These models propose that dark matter could consist of particles and compact objects similar to those in the visible sector but interacting primarily through forces not yet observed. The distinction between "bright" (ordinary) and "dark" black holes is based on their charge and color charge properties, offering new avenues for understanding dark matter's nature and its astrophysical signatures .
Gravitational Waves and Astrophysical Probes
Recent gravitational wave observations have renewed interest in PBHs as dark matter. The detection of a stochastic gravitational wave background by NANOGrav and the potential for future observations by instruments like LISA provide new ways to test the PBH dark matter hypothesis. These signals could be linked to the formation of PBHs and their role as dark matter, especially if the mass distribution of PBHs matches the observed gravitational wave background .
Constraints and Open Questions
While PBHs remain a compelling dark matter candidate, their abundance and mass range are tightly constrained by various observations, including gamma-ray backgrounds and gravitational wave data. The possibility that PBHs could escape into extra dimensions or interact with a "dark dimension" further complicates the picture and affects the allowed mass ranges for PBHs as dark matter . Despite these constraints, PBHs could still play a significant role in cosmic structure formation and the seeding of supermassive black holes in galactic centers .
Conclusion
Black holes, especially primordial black holes, are a leading alternative to particle-based dark matter candidates. Their formation, stability, and interactions with both visible and dark sector particles are active areas of research. Observational advances in gravitational waves and gamma-ray astronomy continue to refine the possible role of black holes in explaining dark matter, keeping this hypothesis at the forefront of cosmological studies 1234+6 MORE.
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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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