New Cosmological Model Proposes Ultra-Relativistic Dark Matter Decoupling in the Early Universe

In January 2026, a groundbreaking study published in the journal Physical Review Letters challenged long-standing cosmological assumptions regarding the genesis of dark matter. A collaborative team of researchers from the University of Minnesota Twin Cities and the University Paris-Saclay introduced a sophisticated model suggesting that dark matter may have separated from the early universe in an ultra-relativistic, or "hot," state. Despite this energetic beginning, the model demonstrates that the substance could still cool sufficiently to reach the temperatures required for the eventual formation of cold dark matter.

This research fundamentally questions the established postulate that dark matter must inherently be cold at the moment of its "freeze-out" during the post-inflationary reheating epoch. Historically, the scientific community has insisted on the necessity of cold dark matter because hot dark matter, such as low-mass neutrinos, tends to suppress the development of the universe's large-scale structures. Professor Keith Olive of the University of Minnesota has frequently highlighted this specific constraint, noting how hot dark matter would otherwise inhibit the natural evolution of cosmic architecture.

Stephen Henrich, a doctoral student at the University of Minnesota’s School of Physics and Astronomy and the study’s lead author, clarified that while dark matter must be cold to facilitate gravitational structure-building, it does not necessarily need to be in a cold phase during its initial separation in the primordial universe. By analyzing dark matter production mechanisms during the high-energy period following inflation, the team demonstrated that ultra-relativistic decoupling provides an adequate timeframe for the matter to cool before cosmic structures begin to take shape. This finding ensures the new model remains consistent with existing observational constraints.

Professor Yann Mambrini of the University Paris-Saclay, a co-author of the paper, emphasized that this work provides a rare glimpse into a period of cosmic history extremely close to the Big Bang by linking dark matter properties directly to the physics of reheating. The theoretical calculations indicate that dark matter with a mass exceeding several thousand electronvolts would successfully cool to approximately one electronvolt by the time cosmic structures begin to grow. This trajectory aligns with the rigorous data obtained from galactic surveys and cosmic microwave background measurements, effectively expanding the viable parameter space for various dark matter models, including Weakly Interacting Massive Particles (WIMPs) and Feebly Interacting Massive Particles (FIMPs).

This theoretical advancement, which received support from the European Union’s Horizon 2020 program through a Marie Skłodowska-Curie grant, now awaits empirical validation. The research team is looking toward future experiments involving high-energy particle accelerators, underground scattering tests, and astrophysical probes to verify the hypothesis of ultra-relativistic decoupling. Ultimately, the study advocates for a more flexible understanding of the early universe, suggesting that the chaotic reheating period plays a far more significant role in determining the final characteristics of dark matter than previously understood by the scientific community.

The implications of this study reach far beyond theoretical physics, as it provides a new framework for interpreting the earliest moments of our universe. By redefining the thermal history of dark matter, researchers can now explore a wider range of particle masses and interaction strengths that were previously dismissed as incompatible with the observed universe. This shift in perspective could eventually lead to a breakthrough in our understanding of the invisible mass that constitutes the majority of the cosmos.