Researchers at the University of Minnesota Twin Cities and Universit´e Paris-Saclay have challenged a decades-old dark matter theory. Their new research shows that the Universe’s most mysterious material could have been “incredibly hot”–moving at nearly the speed of light–when it was first born. 

The study was recently published in Physical Review Letters, the premier journal of the American Physical Society. The research gives new clues about the origins of our Universe and opens up a broader range of possibilities for dark matter and how it interacts with other matter.

Previously, researchers believed for decades that dark matter must be cold–or slow moving–when it “freezes out” from the radiation bath in the early Universe. The team studied dark matter production during an era in the Universe’s history known as post-inflationary reheating.

“The simplest dark matter candidate (a low mass neutrino) was ruled out over 40 years ago since it would have wiped out galactic size structures instead of seeding it,” said Keith Olive, professor in the School of Physics and Astronomy.  “The neutrino became the prime example of hot dark matter, where structure formation relies on cold dark matter. 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.” 

Researchers showed that dark matter can decouple while ultrarelativistic–or very hot–and still have time to cool before galaxies begin to form into what we know today. The key feature which enables this to be possible is that dark matter is produced during an era in the early Universe’s history known as reheating.

“Dark matter is famously enigmatic. One of the few things we know about it is that it needs to be cold,” said Stephen Henrich, graduate student in the School of Physics and Astronomy and lead author of the paper. “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.”

The research will continue by determining the best methods to detect these particles either directly using colliders or scattering experiments, or indirectly via astrophysical observations.

“With our new findings, we may be able to access a period in the history of the Universe very close to the Big Bang,” said Yann Mambrini, professor from the Universit´e Paris-Saclay in France and co-author on the paper. 

This research was funded by the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement.