Scientists from the University of Minnesota Twin Cities and Université Paris-Saclay are calling into question a long-standing idea about dark matter. Their latest findings suggest that this elusive substance may have been “incredibly hot”-moving at nearly the speed of light-when it first formed, rather than cold and slow as researchers have long assumed.
The work was published in Physical Review Letters, the flagship journal of the American Physical Society. By reexamining how dark matter may have emerged in the early Universe, the study expands the range of possible explanations for where dark matter came from and how it may interact with other forms of matter.
Challenging the Cold Dark Matter Assumption
For decades, scientists believed dark matter had to be cold when it separated from the intense radiation filling the young Universe, a process known as freezing out. Cold dark matter moves slowly, a property thought to be essential for forming galaxies and large-scale cosmic structures. To revisit this assumption, the research team focused on a critical but less explored phase of cosmic history called post-inflationary reheating.
During reheating, the Universe was rapidly filling with particles following the end of cosmic inflation. The researchers examined how dark matter could have been produced during this energetic period and what that would mean for its later behavior.
Why Hot Dark Matter Was Once Rejected
“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.”
In the past, fast-moving particles like neutrinos were dismissed because their high speeds would have smoothed out matter in the early Universe, preventing galaxies from forming. This made cold dark matter the preferred explanation for decades.
Cooling Down in Time to Build Galaxies
The new study shows that dark matter does not necessarily need to start out cold. The researchers demonstrated that dark matter particles could separate from other matter while still ultrarelativistic-or extremely hot-and still slow down enough before galaxies began to form. The reason this works is tied directly to reheating, which provides enough time for the particles to cool as the Universe expands.
“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.”
Looking Ahead to Detection and the Early Universe
The team plans to build on these results by exploring how such dark matter particles might be detected. Possible approaches include direct searches using particle colliders or scattering experiments, as well as indirect detection through astronomical 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 Université Paris-Saclay in France and co-author on the paper.
The research was supported by funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement.
