How small can a black hole be
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The Smallest Possible Black Holes: Insights from Recent Research
Primordial Black Holes and Their Minimum Mass
Formation and Constraints of Primordial Black Holes
Primordial black holes (PBHs) are theorized to have formed from density fluctuations in the early universe. These black holes can have masses significantly smaller than those formed from stellar collapse. Research indicates that PBHs could have masses as low as (10^{-5}) grams, but those smaller than approximately (10^{15}) grams would have evaporated by now due to Hawking radiation Carr2020Blair1974Chapline1975. The constraints on the abundance of these small PBHs are derived from their potential effects on big bang nucleosynthesis, the cosmic microwave background (CMB), and various cosmic ray backgrounds .
Evaporation and Detection of Small Black Holes
Black holes with masses below (10^{15}) grams would have completely evaporated, releasing particles that could influence the CMB and other cosmic phenomena. This evaporation process places stringent limits on the initial abundance of such small black holes Carr2020Blair1974. Detecting these black holes or setting upper limits on their number density involves looking for the remnants of their evaporation or their gravitational effects on surrounding matter .
Theoretical Models and Microscopic Properties
String Theory and Small Black Holes
In string theory, small black holes are characterized by a classically singular horizon with vanishing Bekenstein-Hawking entropy. Higher-curvature corrections are proposed to resolve these singularities, aligning the macroscopic properties of black holes with their microscopic states. However, some studies question the effectiveness of these corrections, suggesting that the resolution might involve additional elements like solitonic 5-branes and Kaluza-Klein monopoles .
Supersymmetric Black Holes and Entropy Calculations
Supersymmetric higher-derivative interactions in supergravity theories provide a framework for calculating the microscopic entropy of small black holes with high precision. These calculations help in understanding the microstates of black holes and their thermodynamic properties, offering insights into the fundamental nature of black holes in string theory .
Observational and Experimental Approaches
Detecting Small Black Holes
Experimental approaches to detect small black holes involve looking for their gravitational effects or the remnants of their Hawking radiation. Proposed experiments aim to set meaningful upper limits on the number density of small black holes by observing their potential influence on cosmic phenomena .
Cosmological Implications
The presence of small black holes in the universe could provide valuable information about the conditions in the early universe. Observing these black holes, especially those with masses smaller than typical stellar black holes, could offer insights into the density fluctuations and other cosmological parameters from the early stages of the universe .
Conclusion
The smallest possible black holes, particularly primordial black holes, offer a unique window into the early universe and the fundamental laws of physics. While theoretical models and observational constraints provide a framework for understanding these enigmatic objects, ongoing research and experimental efforts continue to refine our knowledge of their properties and implications. The study of small black holes not only enhances our understanding of black hole physics but also sheds light on the conditions of the early universe and the nature of dark matter.
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