Solving the case of the universe’s missing antimatter may help us pin down one other thing we can’t seem to find: dark matter. The solution involves a twist in the tale of gravitational waves.
Matter and antimatter should have been produced in equal amounts after the big bang. But antimatter is nowhere to be found.
Physicists think that primordial processes produced a tiny excess of matter over antimatter. When matter meets antimatter, it annihilates. So, the leftover excess of matter would have given us stars and galaxies. But how did the mismatch come about?
Advertisement
One previous answer involved particles called neutrinos, which have a property called chirality — a term that refers to the way they twist, or their handedness. In principle, particles can be either left-handed or right-handed, but neutrinos are only left-handed.
Taking inspiration from this approach, of Brown University at Providence, Rhode Island, wondered what would happen if this chiral asymmetry wasn’t limited to particles. He found that if the infant universe had gravitational waves – which are ripples in the fabric of space time – with a preferred chirality, that could also create an excess of matter over antimatter. “There are many working mechanisms for generating chiral gravitational waves, all of which will lead to a matter-antimatter asymmetry,” says .
Dark quarks
That led McDonough, along with at Brown University, and at Princeton University to wonder whether the same physical mechanism could also account for dark matter. Dark matter is the universe’s unseen mass that we think is out there because we can detect its gravitational influence on normal matter.
For decades now, physicists have thought that dark matter is made of so-called weakly interacting massive particles (WIMPs) — an outcome of the assumption that the universe is supersymmetric, meaning all the known particles have heavier partner particles. But all searches for supersymmetry or WIMPs have so far come to nought.
To see if they could get at dark matter differently, the team built a model in which the primordial universe had particles called dark matter quarks, which aren’t the same as the dark matter that’s around today.  If these particles had a chirality like that of neutrinos, they would then interact with the chiral gravitational waves to produce the kind of dark matter in the current universe.
“It’s a cool idea,” says at Stanford University in Palo Alto, California. “Right now, dark matter is completely open. Anything you can do that brings in a new idea into this area, it opens a door. And then you have to walk down that corridor and see whether there are interesting things there that suggest new experiments. This opens another door.”
Wimpier than WIMPs
Just as with matter and antimatter, you get an asymmetry between dark quarks and anti dark quarks, and eventually the universe is left with a small excess of dark quarks. As the cosmos cools down, the dark matter quarks condense into a weird state of matter called a superfluid. This would form a background field that would still exist today. In the same way we think of photons as excitations of an electromagnetic field, excitations of this field would be akin to dark matter particles.
Their calculations show that this dark matter particle would be much lighter than a WIMP and wouldn’t interact with normal matter. “It’s much wimpier than WIMPs,” says Spergel.
Given that, it won’t be possible to see these particles directly. But they will behave differently to WIMPs on cosmological scales. For example, such dark matter would be distributed more evenly throughout a galaxy than WIMPs—and unlike WIMPs, it won’t be found clustering on small scales. Also, the ratio of dark matter to normal matter may not be constant throughout the universe. So the composition of the universe could vary from place to place.
These characteristics could give us a way to spot dark matter, Spergel says. For example, the uniform distribution of dark matter would create a tell-tale signature in the cosmic microwave background, which is the radiation leftover from the big bang, or influence the formation of large-scale structures like clusters of galaxies.
Reference: arXiv,Â
Read more: There could be entire stars and planets made out of dark matter
