How small can black holes be
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The Smallest Black Holes: Insights from Recent Research
Introduction to Small Black Holes
Black holes are often associated with massive stellar remnants, but research indicates that black holes can exist at much smaller scales. These small black holes, which can be as light as (10^{-5}) grams, present unique characteristics and challenges for detection and study.
Primordial Black Holes and Their Formation
Primordial black holes (PBHs) are hypothesized to have formed during the early universe due to density fluctuations. These black holes can have masses ranging from (10^{-5}) grams to much larger scales4 8. The formation of such small black holes is linked to the chaotic conditions of the early universe, where high-density regions could collapse into black holes3.
Constraints and Detection of Small Black Holes
Evaporation and Hawking Radiation
Black holes with masses smaller than approximately (10^{15}) grams would have evaporated by now due to Hawking radiation. This evaporation process constrains their abundance at formation, as the particles emitted during evaporation would have observable effects on big bang nucleosynthesis, the cosmic microwave background, and cosmic ray backgrounds4.
Gamma-Ray Bursts and Femtolensing
Recent studies have used gamma-ray bursts to search for femtolensing effects caused by small black holes. The lack of femtolensing detection in these data suggests that primordial black holes in the mass range (10^{17}) to (10^{20}) grams do not constitute a major fraction of dark matter7.
Theoretical Models and Higher-Dimensional Effects
String Theory and Higher-Curvature Corrections
In string theory, small black holes are characterized by a classically singular horizon with vanishing Bekenstein-Hawking entropy. Higher-curvature corrections have been proposed to resolve these singularities, but recent studies question this resolution, suggesting that the introduction of solitonic 5-branes and Kaluza-Klein monopoles may be necessary1.
Extra Dimensions and Modified Black Hole Properties
Theories involving extra spatial dimensions propose that small black holes could have different properties compared to traditional (3+1)-dimensional black holes. These black holes could be larger, colder, and longer-lived, primarily decaying into bulk graviton modes rather than standard-model particles9.
Experimental and Observational Constraints
Density and Distribution
The abundance and distribution of small black holes are constrained by various observational data, including gravitational lensing, dynamical effects, and the influence on large-scale structure. These constraints are subject to uncertainties but provide important limits on the possible contribution of small black holes to dark matter and cosmic structure4.
Late-Forming Black Holes
Microstructure black holes (MSBHs) can form from the collapse of small-scale dark matter structures at any time from the early universe to the present day. These black holes, if small enough, would evaporate via Hawking radiation, and their abundance is constrained by observations of diffuse extragalactic gamma- and x-ray sources10.
Conclusion
The study of small black holes, particularly primordial black holes, offers valuable insights into the early universe and the nature of dark matter. While theoretical models and observational constraints continue to evolve, the detection and characterization of these elusive objects remain a significant challenge in modern astrophysics. Understanding the smallest black holes not only sheds light on fundamental physics but also provides a unique probe into the conditions of the early universe.
Sources and full results
Most relevant research papers on this topic
The small black hole illusion
Higher-curvature corrections do not resolve the singular horizon of small black holes, and the resolution involves the introduction of solitonic 5-branes and a Kaluza-Klein monopole, not quadratic curvature corrections.
Precision Counting of Small Black Holes
This study tests the hypothesis that supersymmetric higher-derivative interactions in = 2 supergravity can accurately probe black-hole micro-states, potentially revealing new insights into string theory.
Highly CitedConstraints on primordial black holes
Primordial black holes may play an important cosmological role and provide a unique probe of the early Universe, even if they contribute only a small amount of dark matter.
Highly CitedSmall Black Holes on Cylinders
Small black holes on cylinders have a non-zero metric and deviations in thermodynamics and binding energy from the $(d+1)$-dimensional Schwarzschild black hole, impacting our understanding of black holes.
Absorption Cross-Section of Small Black Holes
The absorption cross-section of small nonrotating black holes is 1/8 of the cross-section of massive scalar particles, with the latter being exactly 1/8 of the former for all energies in the limit where the wavelength is larger than the Schwarzschild radius.
Highly CitedNew constraints on primordial black holes abundance from femtolensing of gamma-ray bursts
Primordial black holes with a mass range of 1017-1020 g do not constitute a major fraction of dark matter, as they do not detect femtolensing effects in gamma-ray bursts.
Highly CitedCosmological effects of primordial black holes
Small black holes, formed from density fluctuations in the early universe, may provide valuable information on early universe conditions.
Highly CitedBlack holes and sub-millimeter dimensions
Small black holes in new sub-millimeter dimensions are bigger, colder, and longer-lived than usual, potentially serving as dark matter seeds and seeds for early galaxy and QSO formation.
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