9 February 2021
We don’t know what darkish matter is. We do know what it isn’t, and that’s an issue. Matter is product of elementary particles, from the quarks and electrons that make up atoms and molecules, to primordial neutrinos unfold all through the cosmos. However not one of the identified elementary particles can comprise darkish matter, so what’s it?
A number of concepts have been proposed, from Weakly Interacting Large Particles (WIMPs), to hypothetical particles often known as axions, to tiny black holes. However thus far, the proof doesn’t actually help these concepts. So astronomers have seemed to a different theoretical particle often known as sterile neutrinos.
Neutrinos are a type of darkish matter, as a result of they’ve mass, and weakly work together with gentle. However neutrinos have such a small mass and excessive vitality that they transfer by means of the universe at almost the velocity of sunshine. Because of this, they’re often known as scorching darkish matter. We all know from observations that darkish matter is generally chilly, which means that darkish matter particles should transfer comparatively slowly. So whereas neutrinos make up a small a part of darkish matter, most of it should be one thing else.
There are three identified sorts or flavors of neutrinos, and so they have some fairly unusual properties. One among these unusual properties is their helicity. Elementary particles have spin, and once they journey the spin is both oriented alongside their path of movement (right-handed helicity) or reverse to their movement (left-handed helicity). Most particles can have both helicity relying on the interplay, however the helicity of neutrinos is all the time left-handed. We aren’t fully positive why, however we do know that if right-handed neutrinos exist they wouldn’t work together with common matter by means of the electroweak power. They’d solely work together with matter gravitationally, so they’re often known as sterile neutrinos.
If sterile neutrinos exist, and they’re simply common neutrinos with right-handed helicity, then they’d be scorching darkish matter and never the chilly darkish matter we’re searching for. However there are some theories the place sterile neutrinos are way more huge than common neutrinos. These heavy sterile neutrinos may comprise darkish matter. That’s in the event that they exist.
If there are heavy sterile neutrinos on the market, they might be found by their radioactive decay. Heavy particles can decay into lighter particles over time, so it’s doable that sterile neutrinos can decay to their lighter counterparts, emitting x-ray photons within the course of. In an effort to find these x-ray emissions, a group combed by means of knowledge from the Chandra X-Ray Observatory. They didn’t discover any proof of sterile neutrinos. Their outcomes weren’t sturdy sufficient to thoroughly rule out the concept, but it surely does slender down the theoretical candidates a bit. Particularly, the examine locations a tough restrict on how sterile neutrinos can decay in the event that they exist.
We nonetheless don’t know what darkish matter is. Research like this might sound disappointing, however they play an vital position. By narrowing down our choices, they power us to concentrate on extra viable darkish matter candidates. We’ve realized one thing extra, however for now, we’re nonetheless at midnight.