How Dark Matter's Hot Beginning Shaped Our Universe

For a long time, dark matter was believed to be cold after separating from the intense radiation that filled the young Universe. This process is known as freezing out. The slow movement of cold dark matter is believed to be essential for the formation of galaxies and large-scale cosmic structures. But a new team of researchers decided to rewind the cosmic movie to an earlier, wilder chapter: post-inflationary reheating. This was the Universe’s after-party, when inflation ended and space was suddenly flooded with a storm of particles. Instead of assuming dark matter only emerged from the calm freeze-out, they asked: What if it was born in the chaos of reheating? If so, its behavior later on, whether slow, fast, or something in between, could change the way we understand how the Universe built its grand structures. Researchers from the University of Minnesota Twin Cities and Université Paris-Saclay now think dark matter may have been born blazing hot, racing close to the speed of light, rather than cold and sluggish as long believed. This fresh idea widens the possibilities for how dark matter first appeared and how it might connect with the ordinary matter that makes up stars, planets, and us. “The simplest dark matter candidate (a low mass neutrino) was ruled out over 40 years ago since it would have wiped out galactic-sized structures instead of seeding them,” 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.” The universe’s first type of molecule is found at last For years, scientists ruled out fast-moving particles, such as neutrinos, as dark matter candidates. Their speed would have acted like a cosmic blender, smoothing out the early Universe so galaxies could never take shape. That’s why the “cold dark matter” idea, slow, heavy particles, dominated for decades. But the new study flips that assumption. It shows dark matter didn’t have to start cold. Instead, particles could have broken away while still blazing hot, zipping near light speed, and later cooled down enough to let galaxies grow. The secret lies in reheating, a phase when the Universe was expanding and filled with energy, giving dark matter plenty of time to slow down and settle into the role of cosmic scaffolding. “Dark matter is famously enigmatic. One of the few things we know about it is that it needs to be cold,” said Stephen Henrich, graduate student in the School of Physics and Astronomy and lead author of the paper. “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 has time to cool down before galaxies begin to form.” Exotic black holes: A byproduct of dark matter The researchers now want to figure out how to detect these dark matter particles through strategies including direct searches using particle colliders or detectors that look for tiny scattering signals and indirect searches, studying the sky for hints of dark matter in cosmic patterns and light. “With our new findings, we may be able to access a period in the history of the Universe very close to the Big Bang,” said Yann Mambrini, professor from the Université Paris-Saclay in France and co-author on the paper. Journal Reference: