Dark matter may not have been “cold” in the first moments after the Big Bang, as has long been thought; Instead, new research from the University of Minnesota Twin Cities and Université Paris-Saclay suggests that dark matter particles could be incredibly hot, traveling at close to the speed of light in the primordial universe, before cooling in time to form galaxies and large-scale structure.
For decades, physicists have classified dark matter according to how fast its constituent particles move, with cold dark matter slow enough to clump together under gravity and help form galaxies and galaxy clusters.
This model has been fundamental to the standard cosmological framework, explaining the web-like structure of the universe.
But the new findings suggest that dark matter could have separated from hot plasma in the early universe while it was still highly relativistic — essentially at very high speeds — and then cooled enough before cosmic structures formed.
This precise picture expands the range of possible behaviors of dark matter particles, expanding the range of candidate particles that physicists might pursue in astronomical experiments and observations.
The study is based on a period in the early universe known as reheating, which followed the explosive expansion of the universe called inflation.
During reheating after inflation, the energy-driven expansion turned into a hot soup of particles and radiation.
The results suggest that, under certain conditions, the dark matter produced at that time could initiate life at speeds close to light and yet still be compatible with the large-scale universe we see today.
If this is true, it could have profound implications for ongoing efforts to detect dark matter, whether through particle colliders, underground detectors, or astrophysical observations.
It also raises new theoretical questions about the fundamental properties of dark matter and its role in cosmic evolution.
“Dark matter is incredibly mysterious,” said Steven Heinrich, a graduate student at the University of Minnesota.
“One of the few things we know about them is that they need to be cold.”
“As a result, for the past four decades, most researchers have believed that dark matter must have been cold when it was born in the primordial universe.”
“Our latest results show that this is not the case; in fact, dark matter can be red hot when it is born, but still has time to cool before galaxies begin to form.”
“The simplest dark matter candidate – the low-mass neutrino – was ruled out more than 40 years ago because it would have erased galaxy-volume structures rather than seeding them,” said Professor Keith Olive from the University of Minnesota.
“The neutrino has become the prime example of hot dark matter, as structure formation depends on cold dark matter.”
“Surprisingly, a similar candidate, if produced at the same time as the hot Big Bang universe, would have cooled to the point that it would actually have been cold dark matter.”
“With our new findings, we may be able to reach a period in the history of the universe very close to the Big Bang,” said Professor Yann Mambrini, a physicist at the University of Paris-Saclay.
team a job Appears in the magazine Physical review letters.
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Stephen E. Henrich et al. 2025. Relative Superfreezing: A Bridge from WIMPs to FIMPs. Phys. Rev. Lit 135, 221002; doi: 10.1103/zk9k-nbpj