New research challenges the cold dark matter assumption

New research challenges the cold dark matter assumption Dark matter may have started out blazing hot, then quietly cooled down to shape the Universe we see today. Date: January 15, 2026 Source: University of Minnesota Summary: Dark matter, one of the Universe’s greatest mysteries, may have been born blazing hot instead of cold and sluggish as scientists long believed. New research shows that dark matter particles could have been moving near the speed of light shortly after the Big Bang, only to cool down later and still help form galaxies. By focusing on a chaotic early era known as post-inflationary reheating, researchers reveal that “red-hot” dark matter could survive long enough to become the calm, structure-building force we see today. Share: 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...

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