Dark matter may hold even darker secrets

Telescopes can’t image it and it’s far from fully understood, but dark matter is everywhere.

The deepest mysteries of dark matter relate to its properties and behavior. The popular idea about dark matter is the cold dark matter theory (CDM), which proposes that dark matter consists of low-speed particles that do not interact with each other. This idea has sparked debate and will spark controversy again. A research team at the University of California, Riverside, led by astrophysicist Haibo Yu, has proposed an alternative idea that could explain the two extreme cases where cold dark matter doesn’t play well.

Galaxies and galaxy clusters are thought to be surrounded by halos of dark matter. One end of the argument is that galactic dark matter haloes are too dense to be consistent with CDM, while the other end is that galactic dark matter haloes are too diffuse to be understood by CDM.Yu and his colleagues instead suggested some dark forces (sorry, Star Wars fans don’t this force) causing dark matter particles to collide with each other. This is self-interacting dark matter (SIDM).

The idea of ​​invisible particles interacting, either pushing each other farther and outward in a diffuse halo, or pulling each other closer and inward in a dense halo, might be what we’re looking for in the dark. But first, why is dark matter thought to be overwhelmingly cold?

lost in darkness

Dark matter is dark because its interactions with visible matter and electromagnetic radiation are either weak or non-existent. No light can illuminate it because it cannot interact significantly with any type of electromagnetic radiation. The reason dark matter is considered cold, at least according to cold dark matter theory, is because slow particles are thought to move much slower than the speed of light.

The CDM remains the standard model of dark matter because it is suitable for building and maintaining cosmic structures such as galaxies. If dark matter was cold, it would clump and clump more easily than it would if it were traveling through the vacuum of space, and if all dark matter was hot or made up of lighter particles moving at high speeds, it would clump and clump more easily. The hot dark matter particle moves too fast to form structures for long periods of time, and it flattens any existing structures it encounters. Warm dark matter is 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 believe there is more than one kind.

While the CDM doesn’t mean that this invisible matter is completely stationary, it doesn’t allow large numbers of dark matter particles to collide. This is where SIDM comes into play.

shine more

While hot and warm dark matter were beyond the scope of their study, Yu and his team tested whether SIDM could account for an aspect of cold dark matter that actually doesn’t work: It has trouble explaining extremely dense and extremely diffuse dark matter haloes.

inside [diffuse scenario]the interaction transfers heat from the outer halo region to the inner halo region, reducing the central density; inside [dense scenario]The direction of the heat flow reversed and the inner halo became denser than its CDM counterpart, they said in a recent study published in the Astrophysical Journal Letters.

Ultradiffuse galaxies (UDGs) are a particularly faint class of dwarf galaxies because their stars are spread far apart from each other. The star-forming gas is spread too thinly to create many new stars. The dark matter halos of ultra-diffuse galaxies are farther away than those of ordinary dwarf galaxies, and this should not happen because the collisionless particles proposed by CDM particles do not interact, they come closer and create a denser halo, But the range is shorter. SIDM allows particles to collide and transfer heat, and the resulting expansion of dark matter could explain why these halos are so spread out.

The second scenario involves dark matter halos that are so dense that they interfere with gravitational lensing. These dense halos have enough dark matter to bend space-time, so light passing through that region of space also bends. Due to these perturbations, objects behind the lensing galaxy are magnified, albeit often with some distortion. SIDM also supports dark matter concentrations high enough to affect gravitational lensing, since colliding particles should be able to move inward as well as outward. The particles collide closer to each other, increasing the density of the halo, and a swarm of them can form a giant blob of dark matter, creating a gravitational lensing effect.

There are many ways in which we still know nothing about dark matter. Until we can find ways to detect it directly, we have to rely on theoretical work to figure out whether ideas like SIDM are feasible. Perhaps one day we will have a direct method to detect it, finally unraveling the mysteries of dark matter.

Astrophysical Journal Letters, 2023. DOI: 10.3847/2041-8213/ad0e09