Is dark matter black holes
Pro Research Analysisby 
Searched over 200M research papers
Primordial Black Holes as Dark Matter: A Comprehensive Overview
Introduction to Dark Matter and Primordial Black Holes
Dark matter (DM) is a mysterious component of the universe, making up about 27% of its mass-energy content. One intriguing candidate for dark matter is primordial black holes (PBHs), which are black holes formed in the early universe due to high-density fluctuations during the inflationary period .
Formation and Characteristics of Primordial Black Holes
Formation Mechanisms
Primordial black holes are believed to form from the collapse of large density perturbations generated during inflation. These perturbations re-enter the horizon during the radiation-dominated era, leading to the formation of PBHs . The mass of these PBHs can vary widely, depending on the specifics of the inflationary model and the scale of the perturbations.
Observable Signatures
PBHs can be detected through various signatures, such as the emission of particles due to Hawking evaporation and the accretion of surrounding matter. These processes can impact the evolution of the universe and the formation of cosmic structures. Additionally, PBHs can generate gravitational waves (GWs) detectable by missions like LISA, especially if they have masses around (10^{-12} M_{\odot}) .
Constraints on Primordial Black Holes as Dark Matter
Observational Constraints
Several observational constraints limit the mass ranges in which PBHs can constitute a significant fraction of dark matter. For instance, the detection of gravitational waves from black hole mergers by LIGO/VIRGO has provided insights into the possible mass distribution of PBHs . Additionally, the non-detection of ultra-faint dwarf satellite galaxies and the correlation between X-ray and infrared cosmic backgrounds also provide constraints.
Specific Constraints from Gamma-Ray Observations
The observation of the Galactic Center 511 keV gamma-ray line imposes stringent constraints on low-mass PBHs. PBHs with masses (\leq 10^{15}) kg would produce positrons through Hawking evaporation, which would then annihilate to produce gamma rays. These observations suggest that PBHs contribute to less than 1% of the dark matter density.
Alternative Theories and Models
2-2-Holes as Dark Matter
An alternative to PBHs is the concept of 2-2-holes, which are ultracompact objects arising 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 unique thermodynamic behavior and could potentially explain certain dark matter phenomena.
Nonthermal Production of Dark Matter
Another scenario involves the nonthermal production of dark matter from the evaporation of PBHs. In this model, a period of early matter domination leads to the formation of PBHs, which then evaporate before big bang nucleosynthesis, producing the correct relic abundance of dark matter.
Conclusion
Primordial black holes remain a compelling candidate for dark matter, with various formation mechanisms and observable signatures supporting this hypothesis. However, stringent observational constraints limit the mass ranges in which PBHs can constitute a significant fraction of dark matter. Alternative models, such as 2-2-holes and nonthermal production, provide additional avenues for exploration. Future observations, particularly from gravitational wave detectors like LISA, will be crucial in testing these theories and potentially confirming the role of PBHs in the dark matter puzzle.
Sources and full results
Most relevant research papers on this topic