Dark matter accounts for most of the matter in the Universe, yet its basic properties remain uncertain. In a recent theoretical study led by Prof. Keith Olive at the University of Minnesota Twin Cities, researchers are revisiting one of the most widely accepted assumptions in cosmology: that dark matter must have been cold from the moment it formed. Working with collaborators including Stephen Henrich and Yann Mambrini, the team argues that dark matter may instead have been born extremely hot, moving at nearly the speed of light, before cooling sufficiently to shape the large-scale structure of the Universe.
Henrich, S. E., Mambrini, Y., & Olive, K. A. (2025). Ultrarelativistic Freeze-Out: A Bridge from WIMPs to FIMPs. Physical Review Letters, 135(22), 221002. https://doi.org/10.1103/zk9k-nbpj
For several decades, the cold dark matter model has provided a consistent framework for explaining how galaxies and clusters formed. In this picture, dark matter particles move slowly enough that they clump together early, providing the gravitational scaffolding for ordinary matter. Faster, “hot” dark matter candidates were largely ruled out in the late twentieth century, most notably light neutrinos, because their rapid motion would have smoothed out density fluctuations and prevented galaxies from forming.
Prof. Keith Olive at the University of Minnesota Twin Cities stated,
“With our new findings, we may be able to access a period in the history of the Universe very close to the Big Bang.”
The new work revisits this conclusion by focusing on a specific and often overlooked phase in the early Universe known as post-inflationary reheating. This period followed cosmic inflation, when the Universe rapidly expanded and was then repopulated with particles and radiation. The researchers examined how dark matter produced during this stage would evolve as space expanded and temperatures fell, paying particular attention to the moment when dark matter decoupled, or “froze out,” from the surrounding plasma.
Their calculations show that dark matter does not need to be slow-moving at freeze-out to be compatible with the observed Universe. Instead, particles can decouple while still ultrarelativistic and then cool significantly as the Universe expands. By the time galaxies begin to form hundreds of millions of years later, these particles would behave in practice like cold dark matter, despite their hot origins.
This idea draws an interesting parallel with earlier debates about neutrinos. Neutrinos were once considered promising dark matter candidates but were dismissed because their high velocities suppressed the formation of small-scale structures. The new study shows that timing matters. If similar particles are produced earlier, during reheating rather than later in the hot Big Bang phase, they have more time to lose energy through cosmic expansion and can avoid the problems that ruled neutrinos out.
The results broaden the range of viable dark matter models. They connect traditional weakly interacting massive particle scenarios with alternative frameworks involving feebly interacting particles, suggesting that the boundary between these categories may be less rigid than previously thought. From a theoretical standpoint, this provides a more continuous picture of how dark matter properties could emerge from early-Universe physics.
Beyond cosmology, the work has practical implications for ongoing and future searches for dark matter. If dark matter was born hot, its interactions and mass range could differ from what many experiments are optimized to detect. This motivates a wider approach to detection strategies, including collider experiments, precision scattering measurements, and astrophysical observations that probe the earliest stages of cosmic history.
Perhaps most importantly, the study highlights how assumptions can quietly shape an entire field. Cold dark matter has been successful, but it may not tell the whole story about the origin of the invisible matter that dominates the cosmos. By reopening the question of how dark matter was born, researchers are gaining a clearer view of the narrow window of time just after the Big Bang, when the fundamental properties of the Universe were still being set.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
