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Study of Universities in the United States and France Suggests That Dark Matter May Have Formed in Ultrarelativistic State Just After the Big Bang, During the Post-Inflationary Reheating, Without Hindering the Formation of Galaxies, Contradicting Accepted Models for More Than Four Decades in Modern Cosmology.
A study from the universities of Minnesota Twin Cities and Paris-Saclay suggests that dark matter may have been born extremely hot, almost at the speed of light, during the post-inflationary reheating, challenging decades of cosmological models based on the idea of cold dark matter.
Review of a Central Hypothesis on Dark Matter
Dark matter, the invisible component responsible for shaping the structure of the Universe, is traditionally described as cold and moving slowly since its formation. This view has supported cosmological models for decades and influenced the understanding of the origin of galaxies.
Researchers from the universities of Minnesota Twin Cities and Paris-Saclay questioned this premise by analyzing the formation of dark matter during a specific early period of the cosmos. The work was published in the Physical Review Letters, a journal of the American Physical Society.
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The authors propose that dark matter may have formed in an ultrarelativistic state, extremely hot, without hindering the subsequent formation of large-scale cosmic structures.
Why Dark Matter Was Considered Cold
For many years, it was believed that dark matter needed to be cold at the moment it decoupled from the radiation of the young Universe.
This process is known as freeze-out and occurs when particles stop interacting intensely with other forms of energy.
The hypothesis was based on the argument that particles too fast would smooth out density fluctuations, preventing galaxies from forming. This understanding guided the rejection of candidates considered hot for dark matter.
To test this idea, the new study analyzed the behavior of dark matter during the post-inflationary reheating, a phase following cosmic inflation that marked the rapid creation of energy and particles in the primitive Universe.
Lessons from Neutrinos and Early Theories
Keith Olive, a professor in the School of Physics and Astronomy, recalls that low-mass neutrinos were dismissed as candidates for dark matter more than 40 years ago. According to him, particles of this type would destroy galactic structures instead of favoring their formation.
Neutrinos became the main example of what was called hot dark matter, reinforcing cosmology’s reliance on models of cold dark matter. This distinction shaped theories and experiments over decades.
The new work, however, shows that particles produced under specific conditions may cool over time. If generated during reheating, they would have enough space to lose energy as the Universe expanded.
How Hot Dark Matter Can Become Functional
The study indicates that dark matter may separate from other forms of matter while still in an ultrarelativistic state. Nevertheless, this material would manage to cool before the onset of galaxy formation.
According to the authors, the decisive factor is the timing of the production of these particles. The post-inflationary reheating provides a broad time window for dark matter to gradually reduce its kinetic energy.
Stephen Henrich, the principal author of the paper, states that the assumption that dark matter needs to be born cold has dominated research for about four decades. The results show that this requirement is not necessary for the formation of cosmic structures.
Implications and Next Steps in Research
The team intends to advance in identifying ways to detect this type of dark matter. Strategies include direct experiments with particle accelerators and indirect methods based on cosmological observations.
Yann Mambrini, a professor at the University of Paris-Saclay and co-author of the study, emphasizes that the new conclusions allow for the investigation of a period in the history of the Universe very close to the Big Bang, expanding the reach of experimental cosmology.
The research was funded by the Horizon 2020 program of the European Union, under the Marie Sklodowska-Curie grant. The reference article, titled “Ultrarelativistic Freeze-Out: A Bridge from WIMPs to FIMPs,” was published on November 24, 2025, with DOI 10.1103/zk9k-nbpj, consolidating a new line of investigation into the origin of dark matter and the primitive Universe.
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