Scientists propose that dark matter particles were moving near the speed of light shortly after the Big Bang, challenging the long-held view of cold dark matter. This hot origin allows the particles to cool in time to form galaxies. The findings come from researchers at the University of Minnesota and Université Paris-Saclay.
For decades, the prevailing theory has held that dark matter must have been cold and slow-moving when it separated from the early Universe's intense radiation, a process called freezing out. This sluggish behavior is seen as crucial for clumping together to build galaxies and cosmic structures. However, a new study published in Physical Review Letters questions this assumption by examining the post-inflationary reheating phase, when the Universe rapidly filled with particles after cosmic inflation ended.
The researchers, including Stephen Henrich, a graduate student at the University of Minnesota's School of Physics and Astronomy, along with professors Keith Olive from the same institution and Yann Mambrini from Université Paris-Saclay, argue that dark matter could have formed as ultrarelativistic particles—extremely hot and fast. "Dark matter is famously enigmatic. One of the few things we know about it is that it needs to be cold," Henrich said. "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."
Previously, hot dark matter candidates like low-mass neutrinos were rejected because their high speeds would have smoothed out matter distributions, hindering structure formation. "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," Olive explained. The new model shows that reheating provides sufficient time for these particles to slow as the Universe expands, effectively turning hot dark matter into the cold variety needed for galaxy formation.
Looking forward, the team aims to investigate detection methods, such as particle colliders, scattering experiments, and 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," Mambrini noted. This work broadens possibilities for dark matter's origins and interactions.