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Dark matter is one of the most mysterious components in the universe. it occupies most of the matter in the universe, but it has never been directly observed. The existence of dark matter can only be inferred by its gravitational action on visible matter, such as affecting the formation and motion of galaxies. What exactly is dark matter made of? Can it collapse to form a black hole under the action of gravity, like ordinary matter?

A black hole is an extreme celestial body whose density and gravitational field are so large that even light cannot escape. According to general relativity, any object with sufficient density and mass can form a black hole. In order to form a black hole, the radius of a celestial body must reach a critical value, called the Schwarzschild radius, which is proportional to the mass of the celestial body. For example, if the sun is to become a black hole, it must be compressed to a size of less than 3 kilometers.

Image source Pixabay, can dark matter form black holes in a similar way? Can it achieve such a density? The answer is uncertain. Because we don't know what dark matter is made of, and we don't know if there are any interactions between them other than gravity. At present, the most popular assumption is that dark matter consists of high-mass weakly interacting particles (WIMPs), which can only exchange information with other particles through gravity and weak nuclear forces. If this assumption is correct, then dark matter is very difficult to form black holes, and there are two main reasons.

The first reason is that there is no electromagnetic interaction between WIMPs, which means that they cannot release excess energy and angular momentum through radiation or friction. And these energy and angular momentum will keep them in high orbit all the time, will not move closer to the center, and the density will not increase. Another reason is the weak interaction between WIMPs, which means that when the density of dark matter increases, WIMPs will decay or annihilate and produce other types of particles (such as neutrinos), thus reducing the density of dark matter itself.

Therefore, without a mechanism to release angular momentum and counteract the effects of weak nuclear forces, dark matter will never reach the density needed to produce the event horizon. Of course, it does not rule out the existence of dark matter particles of other types or properties.

In addition, there is another mechanism for the formation of black holes, which believes that primitive black holes were formed at the beginning of the birth of the universe. A primordial black hole is a black hole formed shortly after the Big Bang, not by the collapse of stars. As early as the 1970s, famous physicists Stephen Hawking and Bernard Carr put forward such a theory: at the moment after the Big Bang, the density distribution in the universe was not uniform, and some places were more "uneven" than others. with more mass. These "bump" regions collapse into small primordial black holes.

Studies have shown that if most primitive black holes are "born" with about 1.4 times the mass of the sun, then they can explain all dark matter. What is the evidence for this model? There are no direct observations, but some indirect clues may suggest a link between primitive black holes and dark matter. In 2016, a NASA scientist found some unusual fluctuations in background radiation at different wavelengths in the universe. These ups and downs may have been caused by primitive black holes, which are one of the candidates to make up dark matter.

Another indirect clue to the image source Pixabay comes from the black hole merging event observed by gravitational wave detectors LIGO and Virgo. Since 2015, LIGO and Virgo have detected dozens of such events and found that some of them involve black holes with very large masses (tens of times the mass of the sun) or very small masses (several times the mass of the sun). These black holes are difficult to explain in the traditional way, because black holes formed by collapsing stars are usually no more than 20 times the mass of the sun, nor less than 3 times the mass of the sun. But if they are primitive black holes, they can be of any size and can meet and merge in the early universe.

Of course, none of these clues are enough to prove that primitive black holes are dark matter, nor can they rule out other possibilities. In order to verify this model, we need more accurate and comprehensive observations.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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