How the universe's expansion may have stopped dark matter taking over

There's a lot we still don't know about dark matter – that mysterious, invisible mass that could make up as much as 85 percent of everything around us – but a new paper outlines a rather unusual hypothesis about the very creation of the stuff.

In short: dark matter creates dark matter. The idea is that at some point in the early stages of the Universe, dark matter particles were able to create more dark matter particles out of particles of regular matter, which would go some way to explaining why there's now so much of the stuff about.

The new research builds on earlier proposals of a 'thermal bath', where regular matter in the form of plasma produced the first bits of dark matter – initial particles which could then have had the power to transform heat bath particles into more dark matter.

"This leads to an exponential growth of the dark matter number density in close analogy to other familiar exponential growth processes in nature," the international team of physicists, led by Torsten Bringmann from the University of Oslo in Norway, write in their newly published paper.

There are some unanswered questions about this new hypothesis, as is normal for anything to do with dark matter, but importantly it fits with the observations of dark matter we have today via the cosmic microwave background (CMB).

While we can't actually see dark matter directly, the behavior of the Universe, together with the electromagnetic radiation that makes up the CMB, strongly suggests that dark matter is out there somewhere – and in seriously large amounts.

There's a variety of scenarios attempting to explain conditions that could constrain the proportions of dark matter we see. One, called a freeze-in scenario, proposes that however dark matter might have appeared in the hot bath of early plasma, nothing cancelled it out. As the Universe expanded, its gradual generation simply ceased, forever locking in a certain amount.

By contrast, a freeze-out model suggests dark matter appeared as rapidly as normal matter, but reached an equilibrium once antiparticles cancelled some out. Once again, the cooling of the expanding Universe chilled its generation but also its ability to quickly annihilate, leaving us with a set amount.

This new study proposes yet another possibility – more or less in between the two extremes. If it's right, it would mean the amount of dark matter grew very quickly as the Universe expanded, with this growth slowing and eventually stopping as the expansion of the Universe has slowed down.

With regular matter and dark matter becoming more spaced out from one another over time, this dark matter production line has petered out. What's more, according to the researchers, somewhere out there in the CMB there should be proof that this theory is correct, so the next job is to find it.

We have hugely sensitive dark matter detectors monitoring the cosmos, so it might not be too long before we hear more about this new approach to understanding dark matter production – in turn, teaching us more about the creation and the growth of the Universe.

"Our mechanism complements both freeze-in and freeze-out thermal production scenarios in a generic way," write the researchers. "Further, and detailed, exploration of this new way of producing dark matter from the thermal bath thus appears highly warranted."

Reference:

Dark Matter from Exponential Growth by Torsten Bringmann, Paul Frederik Depta, Marco Hufnagel, Joshua T. Ruderman, and Kai Schmidt-Hoberg Phys. Rev. Lett. 127, 191802 – Published 3 November 2021 DOI: 10.1103/PhysRevLett.127.191802