Ultrarelativistic Dark Matter Could Explain Galaxy Formation

New research suggests dark matter might have begun as a searingly hot substance before cooling to shape the cosmos. A study published in Physical Review Letters proposes that dark matter could have originated in a hot state, transitioning to a cold form through cosmic expansion mechanisms.

Researchers from the University of Minnesota and Université Paris-Saclay argue this model addresses longstanding questions about galaxy formation.

Stephen Henrich, a co-author of the study, described the challenge: "Dark matter is famously enigmatic. One of the few things we know about it is that it needs to be cold."

However, the team’s findings contradict this assumption. "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 stated.

The mechanism hinges on post-inflationary reheating, a phase in the early universe where energy from the inflationary period was converted into particles. Ultrarelativistic particles—moving near light-speed—could have emerged during this phase.

As the universe expanded, these particles lost energy, slowing to velocities characteristic of cold dark matter. This transition aligns with observations of large-scale cosmic structures but remains unconfirmed by direct detection methods.

Funded by the European Union’s Horizon 2020 program via a Marie Sklodowska-Curie grant, the study is peer-reviewed but emphasizes that experimental verification is speculative. The authors caution that while the model fits theoretical constraints, further research is needed to test its predictions.