Researchers from the University of Minnesota and Université Paris-Saclay have unveiled groundbreaking findings that question the long-standing assumption regarding dark matter. Published in Physical Review Letters, their study suggests that dark matter may have originated as “red-hot,” moving at nearly the speed of light shortly after the Big Bang, rather than the cold and sluggish form previously assumed.
The researchers focused on a pivotal yet less explored phase of cosmic history known as post-inflationary reheating. This period, which follows the rapid expansion of the Universe, involved an increase in particle activity that could contribute to the formation of dark matter. The implications of this study broaden the understanding of how dark matter interacts with other matter and ultimately influences the structure of galaxies.
Revisiting the Cold Dark Matter Model
For decades, scientists believed that dark matter had to be cold when it separated from the intense radiation of the early Universe, a process termed “freezing out.” This notion supported the idea that cold dark matter, which moves slowly, was essential for the formation of galaxies and large-scale structures in the cosmos.
However, the latest research challenges this view. The study indicates that dark matter particles could have emerged from an ultrarelativistic state during the reheating phase, suggesting that they did not need to be cold from the outset. Keith Olive, a professor in the School of Physics and Astronomy at the University of Minnesota, explained that the previously favored cold dark matter model relied on the assumption that fast-moving particles, like neutrinos, would disrupt galaxy formation. He stated, “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.”
Cooling Down to Form Galaxies
The research demonstrates that dark matter does not necessarily need to begin cold. It is possible for dark matter particles to remain ultrarelativistic when they separate from other matter and subsequently slow down enough to facilitate galaxy formation. This phenomenon is directly linked to the reheating phase, which allows sufficient time for particles to cool as the Universe expands.
Stephen Henrich, a graduate student and lead author of the paper, emphasized the significance of these findings, stating, “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.”
Future Directions and Detection Methods
Looking ahead, the team aims to explore potential methods for detecting these hot dark matter particles. Possible strategies include direct searches using particle colliders or scattering experiments, as well as indirect detection through astronomical observations. Yann Mambrini, a professor at Université Paris-Saclay and co-author of the study, noted, “With our new findings, we may be able to access a period in the history of the Universe very close to the Big Bang.”
This research was supported by funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement. The findings pave the way for further investigations into dark matter’s role in the cosmos, potentially reshaping our understanding of the Universe’s formation.
