How are black holes formed
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How Are Black Holes Formed?
Black holes are one of the most fascinating and enigmatic objects in the universe. Their formation can occur through various mechanisms, each contributing to the diversity of black holes observed today. This article explores the primary pathways through which black holes are formed, including stellar collapse, primordial black holes, and direct collapse in the early universe.
Stellar Collapse Black Hole Formation
High-Mass Stellar Collapse
One of the most well-known mechanisms for black hole formation is the collapse of massive stars. When a star with an initial mass greater than 40 solar masses exhausts its nuclear fuel, it can collapse directly into a black hole without a supernova explosion. This process is observed in the x-ray binary Cygnus X-1, where the black hole formed in situ from a progenitor star, suggesting that high-mass stellar black holes may form promptly when massive stars disappear silently .
Gravothermal Collapse in Dark Matter Halos
Another pathway involves the gravothermal collapse of self-interacting dark matter (SIDM) halos. In this scenario, very massive SIDM halos can directly form supermassive black holes (SMBHs) greater than 10^6 solar masses. Smaller halos can produce seed black holes of 100-1000 solar masses, which can merge or accrete to form SMBHs .
Primordial Black Holes
Formation from Cosmological Perturbations
Primordial black holes (PBHs) are hypothesized to form in the early universe from the collapse of large amplitude perturbations in the metric. These perturbations, characterized by a certain threshold, can collapse into black holes during the radiative era of the early universe. The abundance and formation of PBHs depend on the shape of the initial energy density profile and the amplitude of the perturbations .
Electroweak Phase Transition
PBHs can also form via a first-order electroweak phase transition (FOEWPT). This process involves extending the Standard Model with a real singlet scalar, leading to the formation of non-topological solitons called Fermi-balls. These solitons collapse into PBHs due to internal Yukawa attractive forces, providing a potential dark matter candidate .
Inflaton Fragmentation
Another mechanism for PBH formation is the fragmentation of the inflaton field into long-lived spatially localized oscillon configurations. This process can lead to the production of sublunar mass PBHs, which could account for all dark matter. Solar-mass PBHs relevant for gravitational wave astronomy may also form through this mechanism .
Direct Collapse in the Early Universe
Formation of Supermassive Black Holes
Supermassive black holes (SMBHs) with masses up to 10^9 solar masses are found in the centers of galaxies and were present as early as redshift z ≥ 6. One plausible formation pathway is the direct collapse of gas in atomic cooling halos with virial temperatures above 10^4 K. This process, regulated by turbulence, can lead to the formation of massive seed black holes of 10^5-10^6 solar masses, which can grow through accretion and mergers .
Conclusion
Black holes can form through various mechanisms, including the collapse of massive stars, the gravothermal collapse of dark matter halos, and the formation of primordial black holes from early universe perturbations and phase transitions. Each pathway contributes to the rich diversity of black holes observed in the universe, from stellar-mass black holes to supermassive black holes at the centers of galaxies. Understanding these formation processes helps unravel the mysteries of black hole evolution and their role in cosmic history.
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Most relevant research papers on this topic
Primordial black holes from an electroweak phase transition
Primordial black holes can form through a first-order electroweak phase transition, with potential dark matter candidates and experimental signals including gravitational waves and multi-lepton/jet final states.
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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