Dark Matter May Have Started Hot and Cooled Later

Scientists at the University of Minnesota have found that dark matter could have started out moving very fast, close to the speed of light, just after the Big Bang. This challenges the long-held idea that it had to be slow and cold from the start to help form galaxies. The work looks at a brief period right after cosmic inflation ended, when the universe was filling up with particles.

Background

Dark matter makes up about 85 percent of the matter in the universe, but no one has seen it directly. It holds galaxies together through gravity, yet it does not interact with light or normal matter in obvious ways. For over 40 years, researchers thought dark matter particles had to be cold, meaning they moved slowly, when they first separated from the hot soup of particles in the young universe. This separation is called freeze-out.

If dark matter was hot and fast like neutrinos, it would have spread out too much. That would have stopped small clumps of matter from forming into stars and galaxies. Neutrinos, once considered a possible dark matter candidate, got ruled out for this reason. They moved too quickly and smoothed out the universe's structure.

Cosmic inflation sets the stage for all this. It was a super-fast expansion of the universe a tiny fraction of a second after the Big Bang. When inflation stopped, the energy from that phase did not turn into particles right away. Instead, it went through a stage called reheating. During reheating, an inflaton field broke down, filling space with particles and heat. This period lasted longer than many models assumed, and it is key to the new findings.

Post-inflationary reheating is chaotic. The universe goes from empty and cold to packed with energy. Temperatures swing high, and particles bounce around. Earlier studies often skipped over details of this phase or treated it simply. The new research digs into how dark matter could form right there.

Key Details

The team modeled dark matter production during reheating. They found particles could freeze out while still ultra-relativistic, meaning very hot and fast. These particles move near light speed at first. But as the universe expands, they lose energy and slow down.

Reheating gives them the time they need. Unlike later stages when the universe is full of radiation, reheating stretches out the process. Dark matter decouples early, before full radiation dominance. This extra window lets fast particles cool before structure formation kicks in, around when the universe cools to about one electron volt.

Ultra-Relativistic Freeze-Out

Researchers call this ultra-relativistic freeze-out, or UFO. It sits between two older ideas: freeze-out where particles are already slow, and freeze-in where they never really heat up much. UFO works well with heavy mediators, like a Z' particle, that help dark matter interact.

For dark matter masses from keV to TeV, UFO fits if the mediator weighs between 1 TeV and 1 PeV. Even if the mediator is heavier than reheating temperatures, production still happens. The models match what we see in galaxy surveys and cosmic microwave background data.

The study checks different inflaton potentials, the math describing the inflation field. Production depends on the peak temperature after inflation and the shape of that potential. Steeper potentials lead to higher max temperatures, boosting dark matter yield.

"Dark matter is famously enigmatic. One of the few things we know about it is that it needs to be cold," said Stephen Henrich, a graduate student in physics at the University of Minnesota and lead author. "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."

Henrich worked with professors Keith Olive and Yann Mambrini from Université Paris-Saclay. Their paper appeared in Physical Review Letters, a top physics journal.

Past work on reheating showed dark matter sensitivity to these early conditions. But this is the first full look at UFO in Z' portal models during reheating. It connects WIMP freeze-out, FIMP freeze-in, and UFO smoothly.

What This Means

This opens up new ways to think about dark matter. Models once dismissed because they made hot particles now have a path. Physicists can test more ideas in accelerators like the Large Hadron Collider or future ones. Indirect searches, looking at gamma rays or galaxy dynamics, get fresh targets too.

Cosmology models might need tweaks. Structure formation simulations could include hot-start dark matter that cools in time. This fits current data from Planck satellite maps and galaxy clusters without changes.

The work also ties into bigger questions about inflation. Different reheating scenarios predict different dark matter amounts. Future gravitational wave detectors might spot reheating signals, confirming or ruling out these pictures.

For dark matter hunters, stronger interactions become possible. UFO allows bigger couplings or lighter mediators than pure freeze-in. This raises chances of detection in labs or space telescopes.

The findings do not solve dark matter's identity. They just widen the search. Teams now plan to model more particle types, like fermionic dark matter, and check against upcoming data from Euclid telescope or Rubin Observatory.

Reheating details matter more than ever. Short reheating might favor cold starts, but longer ones allow hot ones. Observations of primordial black holes or stochastic gravitational waves could pin down reheating length.

This shifts focus back to the universe's first moments. What happened in that brief window shapes everything we see billions of years later.