It’s impossible for a telescope to image far from perfect, but dark matter is everywhere.
The deepest mysteries about dark matter are related to its nature and behavior. The prevailing idea about dark matter is the cold dark matter theory (CDM), which states that dark matter is composed of low-velocity particles that do not interact with each other. This thinking has been debated and it is up for debate again. Led by astrophysicist Hai-Bo Yu, a team of researchers from the University of California, Riverside has developed an alternative idea that explains the two extremes at which cold black matter is not ideal.
Galaxies and galaxy clusters are thought to be surrounded almost exclusively by dark matter. At one end of the controversy is galactic dark matter that is almost too dense to be consistent with CDM, and at the other galactic dark matter that is almost too faint to be understood by CDM. Yu and his colleagues instead suggest that there is a dark force (sorry, Star Wars fans not THE Force) causes dark matter particles to smash into each other. This is Self-Interacting Dark Matter (SIDM).
The idea of invisible particles interacting, either pushing each other apart and out of a diffuse halo, or pulling each other closer together and into a dense one, may be what we’re looking for. in the dark. But first, why is dark matter considered cool?
Lost in the dark
Dark matter is dark because its interactions with visible matter and electromagnetic radiation are weak or absent. No light can illuminate it due to its inability to have a significant interaction with any type of electromagnetic radiation. The reason dark matter is said to be cold, at least according to the cold dark matter theory, is because slow particles are believed to move much slower than the speed of light.
CDM is still the standard model for dark matter because it works for the construction and maintenance of cosmic structures such as galaxies. If dark matter is cold, then it can clump and coalesce more quickly than if it were zooming through the vacuum of space that would happen if all dark matter were hot or made of lighter particles traveling by at high speed. Hot dark matter particles become too fast to form structures over long periods of time, and they flatten any structures they contain. Hot black matter falls somewhere between cold and hot.
How many types of dark matter exist is unknown. Some scientists insist that all dark matter is cold, while others argue that there is more than one type.
While CDM does not say that this invisible matter is completely motionless, it does not allow many dark matter particles to collide. This is where SIDM comes in.
Give more light
While hot and hot dark matter was not in the scope of their study, Yu and his team tested whether SIDM could explain one aspect of cold dark matter that doesn’t work at all: it struggles to radiate. -look at the denser and more diffuse forms of dark matter.
In the [diffuse scenario], the interactions transport heat from the outer to the inner regions of the halo, lowering the central density; on [dense scenario]the direction of heat flow reverses and the inner halo becomes denser than its CDM counterpart, they said in a study recently published in The Astrophysical Journal Letters.
Ultradiffuse galaxies (UDGs) are dwarf galaxies that are particularly dim because their stars are spread far apart from each other. The star-forming gas is too thin to produce many new stars. An ultradiffuse galaxy’s dark matter halo extends further than a regular dwarf galaxy, which should not occur with collisionless particles as suggested by CDM particles that do not interact may be closer. and creates a denser halo with shorter reach. SIDM allows particles to collide and transfer heat, and the expansion of dark matter results in explaining why it is nearly diffuse.
The second scenario deals with dark matter almost too dense to affect gravitational lensing. These are dense enough that there is enough dark matter to bend spacetime so that light passing through that region of space also bends. Because of these perturbations, an object behind the lensing galaxy is magnified, although usually in a slightly distorted way. SIDM also supports concentrations of dark matter high enough to affect gravitational lensing because colliding particles must be going in as much as they are going out. Particles that collide with each other increase the density of the halo, and a swarm of them becomes a large dark matter blob that produces gravitational lensing.
We are still in the dark about dark matter in many ways. Until there is a way to directly detect it, we will have to rely on theoretical work to determine whether ideas like SIDM can work on something. Perhaps in the future, we will have a method of direct detection that will finally bring the mysteries of dark matter to light.
Astrophysical Journal Letters, 2023. DOI: 10.3847/2041-8213/ad0e09
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Image Source : arstechnica.com