New dark matter research casts doubt on decades-old theories and sheds light on the early beginnings of the universe.
Researchers from the University of Minnesota, Twin Cities and Paris-Saclay University have discovered that dark matter may have been “incredibly hot” and moving at nearly the speed of light when it first formed.
This research provides new clues about the origin of the universe and opens up broader possibilities for how dark matter interacts with other matter.
Until now, dark matter was thought to be cold and slow-moving.
For decades, researchers believed that dark matter must be cold or slow-moving when it “freezes out” from the early universe’s radiation bath.
The research team studied the production of dark matter during a period in the history of the universe known as post-inflationary reheating.
“The simplest dark matter candidates (low-mass neutrinos) were ruled out more than 40 years ago because they could kill galaxy-sized structures rather than seed them,” says Keith Olive, a professor in the Department of Physics and Astronomy.
“Neutrinos have become a prime example of hot dark matter whose structure formation depends on cold dark matter. It is surprising that if a similar candidate had been created at the very time the hot Big Bang universe was being formed, it could have been cooled to the point where it actually acted as cold dark matter.”
Cooling before galaxy formation
The researchers showed that dark matter can separate even in ultra-relativistic (very hot) conditions and that galaxies still have time to cool down before they begin to form into what we know today.
The key feature that makes this possible is that dark matter is created during an epoch known as reheating early in the history of the universe.
“Dark matter is notoriously mysterious,” said Stephen Henrich, a graduate student in the Department of Physics and Astronomy who led the study. “One of the few things we know about dark matter is that it must be cold.
“As a result, for the past 40 years, most researchers have believed that dark matter must have been cold when it was born in the primordial universe.”
He added: “Our recent findings show that this is not the case. In fact, dark matter can be bright red at birth, but it still has time to cool before galaxies begin to form.”
Discovering the best way to detect particles
Research will continue by determining the best way to detect these particles, either directly using collider and scattering experiments, or indirectly through astrophysical observations.
“With our new discovery, we may be able to access an era in the history of the universe very close to the Big Bang,” concluded Yann Mambrini, a professor at the University of Paris-Saclay in France and co-author of the study.
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