New insights challenge the long-held belief that dark matter was “cold” in the immediate aftermath of the Big Bang. A groundbreaking study from the University of Minnesota Twin Cities and the University of Paris-Saclay reveals that dark matter particles might have been extraordinarily hot and traveling at near-light speeds in the primordial universe, before cooling down during the formative epochs of galaxies and large-scale structures.
For decades, physicists have categorized dark matter based on the velocity of its constituent particles. Cold dark matter is slow enough to clump under gravitational forces, contributing to the formation of galaxies and galaxy clusters.
This categorization is a cornerstone of the standard cosmological model, explaining the universe’s intricate web-like structure.
However, the recent findings indicate that dark matter may have emerged from the hot plasma of the early universe in an ultrarelativistic state—essentially moving at ultra-high speeds—before cooling adequately during the formation of cosmic structures.
This refined perspective broadens the potential behaviors of dark matter particles and expands the pool of candidate particles physicists can investigate through experiments and astronomical observations.
The study concentrates on a critical phase in the early universe known as reheating, which followed an explosive inflationary expansion.
During the reheating phase, the energy fueling the universe’s expansion transformed into a dense hot mixture of particles and radiation.
This discovery suggests that under certain conditions, dark matter produced during this period could exist at speeds approaching that of light while still aligning with the vast universe we observe today.
If validated, these findings could significantly impact ongoing dark matter detection initiatives, including particle colliders, underground detectors, and astrophysical studies.
Moreover, they pose new theoretical challenges regarding the fundamental nature of dark matter and its role in the universe’s evolution.
“Dark matter remains one of the biggest mysteries in physics,” explains Stephen Henrik, a graduate student at the University of Minnesota.
“Historically, one consistent assumption has been that dark matter must be cold at its inception in the primordial universe.”
“Our findings reveal a different narrative. In fact, dark matter may start off as red-hot, but has ample time to cool before galaxies commence formation.”
“The simplest dark matter candidate, low-mass neutrinos, was deemed incompatible decades ago because they could annihilate galaxy-sized structures instead of facilitating them,” states Keith Olive, a professor at the University of Minnesota.
“Neutrinos serve as a prime example of hot dark matter, whose structural formation relies on cold dark matter.”
“If a similar candidate arose during the hot Big Bang, it’s remarkable that it could cool sufficiently to behave as cold dark matter.”
“This new discovery allows us to explore a period in the universe’s history that is very close to the Big Bang,” adds Professor Yann Mambrini, a physicist at the University of Paris-Saclay.
The team’s research has been published in the journal Physical Review Letters.
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Stephen E. Henrik et al. 2025. Ultra-relativistic freezeout: Bridge from WIMP to FIMP. Physics Review Letters 135, 221002; doi: 10.1103/zk9k-nbpj
Source: www.sci.news
