5.3 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Black hole | 12/13 | https://en.wikipedia.org/wiki/Black_hole | reference | science, encyclopedia | 2026-05-05T13:31:52.244381+00:00 | kb-cron |
Astronomers use the term active galaxy to describe galaxies with unusual characteristics, such as unusual spectral line emission and very strong radio emission. Theoretical and observational studies have shown that the high levels of activity in the centers of these galaxies, regions called active galactic nuclei (AGN), may be explained by accretion onto supermassive black holes. These AGN consist of a central black hole that may be millions or billions of times more massive than the Sun, a disk of interstellar gas and dust called an accretion disk, and two jets perpendicular to the accretion disk. The black holes in quiescent galaxies accrete matter more slowly or radiate less efficiently. Although supermassive black holes are expected to be found in most AGN, only some galaxies' nuclei have been more carefully studied in attempts to both identify and measure the actual masses of the central supermassive black hole candidates. Some of the most notable galaxies with supermassive black hole candidates include the Andromeda Galaxy, Messier 32, Messier 87, the Sombrero Galaxy, and the Milky Way itself.
=== Microlensing ===
Black holes can be detected by gravitational lensing: the deflection of light rays by the deformation of spacetime around a massive object. A distant star behind a source of gravity may produce multiple images of that star but if the images cannot be resolved, the phenomenon is called microlensing. In microlensing, astronomers see the star magnified by an amount which changes as the source star, lens, and observer move. Astronomers must monitor the star image over a few years and match its light curve to models of the gravitational effect. As a tool for astrophysics, microlensing is uniquely sensitive to dark objects like isolated black holes not paired in a binary object. However comparison of the predicted light curve to observations yields multiple indistinguishable solutions, requiring expensive follow-up measurements to select and confirm candidate black holes. Over 10,000 microlensing events yielded 23 black hole candidates but only one object has been confirmed as an isolated black hole using additional measurements from the Hubble Space Telescope.
== Areas of investigation ==
=== Information loss paradox ===
According to the no-hair theorem, a black hole is defined by only three parameters: its mass, charge, and angular momentum. This seems to mean that all other information about the matter that went into forming the black hole is lost, as there is no way to determine anything about the black hole from outside other than those three parameters. When black holes were thought to persist forever, this information loss was not problematic, as the information can be thought of as existing inside the black hole. However, black holes slowly evaporate by emitting Hawking radiation. This radiation does not appear to carry any additional information about the matter that formed the black hole, meaning that this information is seemingly gone forever. This is called the black hole information paradox. Theoretical studies analysing the paradox have led to both further paradoxes and new ideas about the intersection of quantum mechanics and general relativity. While there is no consensus on the resolution of the paradox, work on the problem is expected to be important for a theory of quantum gravity.
=== Supermassive black holes in the early universe ===
Observations of faraway galaxies have found that ultraluminous quasars, powered by supermassive black holes, existed in the early universe as far back as redshift
z
≥
7
,
{\displaystyle z\geq 7,}
less than a billion years after the Big Bang. These black holes have been assumed to be the products of the gravitational collapse of large population III stars. However, these stellar remnants were not massive enough to produce the quasars observed at early times without accreting beyond the Eddington limit, the theoretical maximum rate of black hole accretion. Physicists have suggested a variety of different mechanisms by which these supermassive black holes may have formed. It has been proposed that smaller black holes may have also undergone mergers to produce the observed supermassive black holes. It is also possible that they were seeded by direct-collapse black holes, in which a large cloud of hot gas avoids fragmentation that would lead to multiple stars, due to low angular momentum or heating from a nearby galaxy. Given the right circumstances, a single supermassive star forms and collapses directly into a black hole without undergoing typical stellar evolution. Additionally, these supermassive black holes in the early universe may be high-mass primordial black holes, which could have accreted further matter in the centers of galaxies. Finally, certain mechanisms allow black holes to grow faster than the theoretical Eddington limit, such as dense gas in the accretion disk limiting outward radiation pressure that prevents the black hole from accreting. However, the formation of bipolar jets prevent super-Eddington rates.
=== Alternatives to black holes ===