You're getting warmer! Hot dark matter could refine cosmic game of hide and seek

New research suggests that dark matter, the universe's most mysterious "stuff," may actually have been born "hot." If this is the case, the best current model we have of cosmic evolution, the standard model of cosmology, also known as the Lambda Cold Dark Matter (LCDM), may need serious revision or overwriting altogether, altering the rules of the epic game of hide and seek that has been ongoing between dark matter and scientists for decades.

Dark matter is a headache for researchers because it doesn't interact with electromagnetic radiation, light, in layman's terms. This not only makes dark matter effectively invisible, but it also means that scientists know it can't be made of the electrons, protons, and neutrons that compose the atoms making up everything from the most massive stars down to the tiniest bacteria, because they do interact with light. Couple this with the fact that dark matter outweighs ordinary matter in the universe by a ratio of five to one.

This mystery has sparked a search for candidate particles for dark matter beyond the standard model of particle physics. Thus far, this search has favored "cold" dark matter, which doesn't refer to temperature but instead references the speed at which the particles move (cold meaning much slower than light, hot meaning moving at speeds approaching light). In the standard picture, cold dark matter emerges from the hot and dense soup of energy that filled the early universe.

The new research suggests an alternative origin. Dark matter could have instead been born extremely hot, opening up alternative possibilities of how it interacts with everyday matter.

The team proposes that incredibly hot dark matter moving at near-light speeds could have been born in the universe during a period called post-inflationary reheating. This refers to the point at which the inflation field driving the rapid initial expansion of the universe decayed and transformed into a hot and incredibly dense "soup" of radiation and particles.

"Dark matter is famously enigmatic. One of the few things we know about it is that it needs to be cold," research leader Stephen Henrich, of the University of Minnesota's School of Physics and Astronomy, said in a statement. "As a result, for the past four decades, most researchers have believed that dark matter must be cold when it is born in the primordial universe.

"Our recent results show that this is not the case; in fact, dark matter can be red hot when it is born but still have time to cool down before galaxies begin to form."

Henrich and his colleagues demonstrated that dark matter could stop significantly interacting with ordinary matter and electromagnetic radiation while still very hot and thus moving at speeds approaching that of light, a process called "decoupling." If produced during post-inflationary reheating, this would give dark matter plenty of time to cool off and start acting like cold dark matter, assisting in the formation of the first galaxies by forming gravitational waves into which ordinary matter clusters.

The concept could resurrect one of the earliest and simplest candidates for dark matter, low-mass neutrinos, which were ruled out around four decades ago because it was thought they would have wiped out galactic-scale structures rather than promoting them.

"The neutrino became the prime example of hot dark matter, where structure formation relies on cold dark matter," team member Keith Olive, also of the University of Minnesota's School of Physics and Astronomy, said. "It is amazing that a similar candidate, if produced just as the hot Big Bang universe was being created, could have cooled to the point where it would, in fact, act as cold dark matter."

The team will now attempt to produce and observe these particles using experiments on Earth, including tests conducted with powerful particle accelerators, as well as detecting them in the early universe. This investigation could not only reveal the true nature of dark matter, but it could also help scientists build a clearer picture of one of the most crucial, yet mysterious, periods of cosmic evolution.

"With our new findings, we may be able to access a period in the history of the universe very close to the Big Bang," team member Yann Mambrini of the Université Paris-Saclay in France said.

The team's research was published in November in Physical Review Letters.