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The connection between black holes and the Big Bang: Why can the collapse of a black hole change our understanding of the universe?
In the early universe, direct-collapse black holes (DCBHs) may have been a key origin of supermassive black holes. These high-mass black hole seeds are formed by the direct collapse of large amounts of matter and are speculated to have formed in the redshift range z = 15 to 30, when the universe was about 100 to 250 million years old. Unlike black hole seeds that form from the first generation of stars (the so-called population-three generation of stars), these straight-collapse black holes form via direct general relativistic instabilities. Their typical mass can reach about 105 M☉. This class of black hole seeds was first proposed theoretically to solve the challenge of the existence of supermassive black holes at redshift z~7, which has been confirmed by many observations. .
Vertical-collapse black holes are formed through a unique combination of environmental physical conditions that are rarely met simultaneously.
Formation mechanism
The formation of a direct-collapse black hole involves several key environmental conditions that promote the direct collapse of gas rather than the formation of a cluster of stars. First, the gas must be free of metal (i.e. contain only hydrogen and helium), secondly, it must have an atomic cooling effect, and finally, there must be a Lyman-Wierth photon stream of sufficient intensity to destroy the hydrogen molecules, which are Very effective gas coolant. If the cooling process of these gases cannot be interrupted, the gas cloud will undergo gravitational collapse and reach an extremely high mass density of about 107 g/cm³. At these densities, objects will experience general relativistic instabilities, leading to the formation of black holes with masses typically up to 105 M☉, or even as high as 1 million M☉.
These objects collapse directly from primordial gas clouds, without going through an intermediate stellar stage, and are therefore called direct-collapse black holes.
A recent computer simulation reports that powerful, cold accretion flows in rare concentrations can form these black hole seeds without the need for an ultraviolet background or supersonic flows, or even atomic cooling. In this simulation, it was not until the baryon mass grew to 40 million solar masses that gravity finally overcame the turbulence and the star simply collapsed to form two supermassive stars, which eventually became straight-collapse black holes with masses of 31,000 and 2,000, respectively. and 40,000 M☉.
Number of vertical collapse black holes
Despite the important theoretical value of straight-collapse black holes, scientists generally believe that they are relatively rare in the high-redshift universe because it is very challenging to simultaneously meet the three basic conditions required for their generation. Current cosmological simulations predict that at redshift 15 there is, on average, only about one direct-collapse black hole per cubic gigapascal. Depending on different assumptions, the number of straight-collapse black holes predicted could be as high as 107 per cubic gigapascal, but this would require an extremely high Lyman-Wierth photon flux.
This also leads to expectations for future observations, especially observations by the Zhang Xianwang Space Telescope will be key.
Detection and Discovery
In 2016, a team led by Harvard University astrophysicist Fabio Pacucci used data from the Hubble Space Telescope and the Chandra X-ray Observatory to identify two vertical collapses for the first time. Black hole candidates, the redshift z values of these two candidates are both greater than 6, and are consistent with the spectral characteristics of this type of celestial body. These sources are predicted to have a significant excess of infrared emission when compared to other sources at higher redshifts. Future observations will be key to determining the properties of these black hole candidates.
Differences from other black holes
It should be noted that primordial black holes are formed due to a direct sudden drop in energy, ionized matter, or both, while direct collapse black holes are formed due to the direct collapse of dense and large gas clouds. Furthermore, black holes formed by population-three generation stars are not classified as "direct" collapse black holes.
The discovery of vertical-collapse black holes undoubtedly provides a new perspective for studying the galaxy structure and black hole formation in the early universe, which will lead to deeper thinking: What other unknowns are hidden in the universe during the formation of vertical-collapse black holes? How to solve the mystery?The existence of vertical collapse black holes not only explains the formation of black holes in the early universe, but also questions our understanding of the evolution of the universe.
