Dark matter is everywhere, even though it is impossible to image with telescopes and is far from completely understood.
The deepest mysteries about dark matter concern its nature and behavior. A common idea about dark matter is the cold dark matter theory (CDM), which posits that dark matter is made up of non-interacting slow particles. This idea is controversial and again debatable. A team of researchers at the University of California, Riverside, led by astrophysicist Haibo Yu, has devised another idea to explain two extreme situations in which cold dark matter fails.
Galaxies and galaxy clusters are thought to be surrounded by a halo of dark matter. On one end of the debate we have galactic dark matter halos that are too dense to match the CDM, and on the other end we have galactic dark matter halos that are too diffuse for the CDM to understand. . Yu and his colleagues instead suggest the existence of dark forces (sorry, Star Wars fans, that’s not the case). of force) causes dark matter particles to collide with each other. This is self-interacting dark matter (SIDM).
The idea that invisible particles interact, pushing each other away and outward in a diffuse halo, and pulling each other closer and inward in a dense halo is what we were looking for in the dark. It may be something. First of all, why is dark matter overwhelmingly thought to be cold?
lost in the dark
Dark matter is “dark” because its interactions with visible matter and electromagnetic radiation are weak or non-existent. Light cannot illuminate it because it has no significant interaction with electromagnetic radiation of any kind. The reason dark matter is said to be “cold,” at least according to cold dark matter theory, is that dull particles are thought to travel much slower than the speed of light.
The CDM remains the standard model for dark matter because it functions in building and maintaining cosmic structures such as galaxies. When dark matter is cold, it can clump and clump together more easily than when it travels through the vacuum of space. That would be the case if all dark matter was “hot” or made of fast-moving, light particles. Hot dark matter particles are too fast to form structures over long periods of time, flattening existing structures they collide with. Warm dark matter lies somewhere between cold and hot.
It is unknown how many types of dark matter actually exist. Some scientists argue that all dark matter is cold, while others argue that there is more than one type.
CDM doesn’t say this invisible matter is completely immobile, but it doesn’t allow many dark matter particles to collide. This is where SIDM comes into play.
shed more light
Although hot and warm dark matter were beyond the scope of their research, Yu and his team tested whether SIDM could explain an aspect of cold dark matter that doesn’t really work. That is, we are struggling to explain the very dense and very diffuse halo of dark matter.
“into [diffuse scenario], the interaction transports heat from the outer halo region to the inner halo region, reducing the central density.inside [dense scenario]the direction of heat flow is reversed and the inner halo becomes denser than its CDM counterpart,” they say in their paper. study Recently published in The Astrophysical Journal Letters.
superdiffuse galaxy (UDG) is a particularly faint dwarf galaxy because its stars are scattered far apart from each other. The star-forming gas is too thin and spread out to form many new stars. The dark matter halos of superdiffuse galaxies reach much farther than those of regular dwarf galaxies, but this should not happen with the collisionless particles proposed by CDM. Particles that do not interact move closer together, creating a denser halo with a shorter reach. SIDM allows particle collisions and heat transfer, and the resulting expansion of dark matter may explain why these halos are so diffuse.
The second scenario deals with a halo of dark matter dense enough to affect gravitational lensing. These dense halos contain enough dark matter to bend space-time, so that light passing through that region of space also bends. Because of these perturbations, objects behind the lens galaxy are magnified, but often with a somewhat distorted shape. Since colliding particles should be able to travel inward as well as outward, SIDM also supports dark matter concentrations sufficient to influence gravitational lensing. As particles collide closer together, the density of the halo increases, and the swarm can form huge clumps of dark matter that cause gravitational lensing.
Dark matter is still a mystery in many ways. Until we have a way to detect it directly, we’ll have to rely on theoretical research to determine whether ideas like SIDM apply to something. Perhaps one day we will have a direct detection method that will finally reveal the mysteries of dark matter.
Astrophysics Journal Letter, 2023. DOI: 10.3847/2041-8213/ad0e09