Early universe dark matter born red hot before cooling

by Clarence Oxford
Los Angeles CA (SPX) Jan 14, 2026

Researchers at the University of Minnesota Twin Cities and Universite Paris-Saclay report that dark matter in the early universe may have started out moving at nearly the speed of light, challenging the long held assumption that it had to be born cold and slow. Their work suggests that the universe's dominant unseen matter component could have been incredibly hot at birth yet still cooled sufficiently before galaxies began to assemble.

The study, published in Physical Review Letters, examines how dark matter might have been produced during a period in cosmic history known as post inflationary reheating, when the universe was transitioning from a hot, dense radiation bath into a state where particles such as dark matter could decouple. For decades, cosmologists have argued that dark matter must be cold when it freezes out of this radiation background so that it can seed the growth of structure rather than erase it.

Keith Olive, a professor in the School of Physics and Astronomy at the University of Minnesota, notes that the simplest dark matter candidate, a low mass neutrino, was ruled out more than 40 years ago because it behaved as hot dark matter and would have washed out galactic scale structure instead of helping form it. In this context the neutrino became the prototype for hot dark matter, reinforcing the view that viable dark matter must be cold for structure formation models to work.

The new work revisits this picture by analyzing dark matter production during reheating, when the universe was still extremely energetic and particles could be highly relativistic. The team shows that under these conditions dark matter can decouple while ultrarelativistic, effectively red hot, and yet have enough time to cool before the onset of galaxy formation so that it behaves observationally like cold dark matter.

Lead author and graduate student Stephen Henrich from the School of Physics and Astronomy explains that dark matter is famously enigmatic and that one of the few robust requirements has been that it be cold by the time it shapes cosmic structure. He emphasizes that the prevailing assumption for four decades has been that dark matter must also be cold at birth in the primordial universe, but the new results demonstrate that it can instead be born red hot and still cool down in time.

Co author Yann Mambrini of Universite Paris-Saclay points out that the findings open a window onto an epoch in cosmic history very close to the big bang by linking dark matter properties to the physics of reheating. By allowing for an ultrarelativistic freeze out, the work broadens the viable parameter space for dark matter models, providing a conceptual bridge between scenarios such as weakly interacting massive particles and feebly interacting massive particles.

The researchers now plan to explore how best to test these ideas through both particle physics and astrophysical measurements. Possible strategies include direct searches for dark matter candidates at colliders, underground scattering experiments designed to detect rare interactions with ordinary matter, and indirect probes based on astrophysical observations that trace how dark matter influences the formation and behavior of cosmic structures.

Research Report:Ultrarelativistic Freeze-Out: A Bridge from WIMPs to FIMPs