How dense are black holes
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Understanding the Density of Black Holes
Introduction to Black Hole Density
Black holes are often perceived as extremely dense objects, a notion that is reinforced by their comparison to white dwarfs and neutron stars. However, the density of black holes can vary significantly depending on their mass. While smaller black holes indeed possess tremendous densities, supermassive black holes, such as those found at the centers of galaxies, can have surprisingly low densities1.
Density Variations in Black Holes
Small vs. Supermassive Black Holes
The density of a black hole is inversely proportional to its mass. This means that as the mass of a black hole increases, its density decreases. For instance, smaller black holes, which are formed from the collapse of massive stars, have extremely high densities. In contrast, supermassive black holes, which can be millions to billions of times the mass of the Sun, have much lower densities. This is because the volume of a black hole increases faster than its mass as it grows larger1.
Primordial Black Holes
Primordial black holes (PBHs), which are hypothesized to have formed in the early universe, also exhibit a range of densities. Recent studies using X-ray data have set upper limits on the abundance and density of these PBHs. The interactions of PBHs with the interstellar medium should result in significant X-ray emissions, which help in constraining their number density in various mass ranges5.
Black Hole Mass Density in the Universe
Local Black Hole Mass Density
Recent revisions in black hole scaling relations suggest that the local mass density of black holes is significantly higher than previously estimated. This increase in local black hole mass density has implications for the accretion properties and demographics of Active Galactic Nuclei (AGN). It is proposed that a substantial portion of black hole growth occurs through radiatively inefficient channels, such as super-Eddington accretion2.
Growth and Absorption
The mass density of massive black holes observed locally is consistent with the hard X-ray background, provided that most of the radiation produced during their growth was absorbed by surrounding gas. This absorption is attributed to cold dusty clouds in young spheroidal galaxies, which play a crucial role in the formation of galaxy bulges and central black holes3 4.
Implications of Black Hole Density
Gravitational Wave Observations
The density of black holes also has implications for gravitational wave observations. Binary black holes, especially those in dense environments like galactic centers or globular clusters, exhibit unique gravitational wave signatures. These signatures can provide direct information about the density and environment of the black holes7.
Theoretical Models and Observations
Theoretical models suggest that black holes formed in dense star clusters can undergo repeated mergers, leading to second-generation black holes with distinct mass and spin properties. These second-generation mergers contribute significantly to the population of black holes detectable by observatories like LIGO/Virgo9.
Conclusion
The density of black holes is a complex and variable characteristic that depends on their mass and formation history. While smaller black holes are incredibly dense, supermassive black holes can have much lower densities. The local mass density of black holes is higher than previously thought, influenced by factors such as radiatively inefficient accretion and absorption by surrounding gas. Understanding these variations in density is crucial for interpreting gravitational wave data and the overall growth and evolution of black holes in the universe.
Sources and full results
Most relevant research papers on this topic
How dense is a black hole
Supermassive black holes can have a very low density, challenging the assumption that black holes must be extremely dense to form.
Mass without radiation: Heavily obscured AGNs, the X-ray background, and the black hole mass density
The local mass density in black holes can be significantly higher than previously determined, allowing for more heavily buried, Compton-thick AGNs without violating the limit imposed by the hard X-ray and mid-infrared backgrounds.
THE OBSCURED GROWTH OF MASSIVE BLACK HOLES
Massive black holes form galaxy bulges and central black holes through the emission of a slow wind and a quasar-like spectrum, with the wind exerting the most force on the gas, terminating growth both black hole and galaxy when the Thomson depth drops to about unity.
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Massive black holes form galaxy bulges and central black holes through the emission of a slow wind and a quasar-like spectrum, with the wind exerting the most force on the gas and terminating growth when the black hole reaches a critical mass.
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Primordial black hole density is limited to a few M to 2 107 M, with a marginal allowance for the origin of black holes detected by LIGO.
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A black hole's three-dimensional volume is large, potentially impacting the information paradox discussion.
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Future space gravitational wave detectors like LISA could potentially detect time-dependent Doppler shifts in gravitational wave signals, providing information about binary black hole environments and their surrounding mass.
Not quite a black hole
The 2-2-hole, a novel horizonless configuration, may be the nearly black end point of gravitational collapse, with a shrinking volume and timelike curvature singularity.
Highly CitedBlack holes: The next generation—repeated mergers in dense star clusters and their gravitational-wave properties
More than 10% of merging binary black holes in dense star clusters have components formed from previous mergers, accounting for more than 20% of the mergers detected by LIGO/Virgo.
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