For decades, the prevailing theory held that dark matter – the invisible substance making up roughly 85% of the universe’s mass – was “cold” in the immediate aftermath of the Big Bang. Meaning, its constituent particles moved slowly enough to clump together via gravity, forming the galaxies and larger cosmic structures we observe today. New research challenges this assumption, suggesting dark matter may have initially been incredibly hot, traveling at near-light speed before cooling sufficiently to seed galactic formation.
The Shift in Understanding
The study, conducted by researchers at the University of Minnesota Twin Cities and the Universit’e Paris-Saclay, proposes that dark matter could have “decoupled” from the early universe’s high-energy plasma while still ultrarelativistic – moving at extreme speeds. This scenario expands the range of possible behaviors for dark matter particles, potentially opening doors to new experimental and observational avenues.
The key lies in the period of reheating following the universe’s rapid expansion (inflation). During reheating, energy from inflation converted into a hot mix of particles and radiation. Under specific conditions, the researchers demonstrate that dark matter created at this time could have been born at near-light speeds but still cooled down enough to match the large-scale structure of the universe.
Why This Matters
The “cold dark matter” model has long been central to our understanding of cosmology. If these new findings hold, they could reshape our search for dark matter itself. Current detection efforts rely on various methods—particle colliders, underground detectors, and astrophysical observations—all predicated on the idea that dark matter particles are relatively slow-moving.
This nuance also prompts deeper theoretical questions about the fundamental properties of dark matter and its role in cosmic evolution.
“Dark matter is famously enigmatic,” said Stephen Henrich, a graduate student at the University of Minnesota. “One of the few things we know about it is that it needs to be cold… 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.”
The Paradox of “Hot” Dark Matter
Historically, the idea of “hot dark matter” – particles moving too fast to efficiently form structures – was dismissed. The low-mass neutrino, once a prime candidate, was ruled out decades ago due to its tendency to wipe out galactic structures rather than seed them.
However, the new study suggests that if dark matter were produced during the chaotic reheating phase of the Big Bang, it could have cooled sufficiently to act as “cold dark matter.” This reopens possibilities for previously discarded particle candidates.
Future Implications
The team’s findings, published in Physical Review Letters, provide a new lens through which to view the universe’s earliest moments. If confirmed, they could unlock a deeper understanding of the conditions immediately after the Big Bang and refine our search for the elusive substance that shapes the cosmos.
The research team’s work could allow scientists to access the history of the universe very close to the Big Bang.
The study is titled: Ultrarelativistic Freeze-Out: A Bridge from WIMPs to FIMPs. Phys. Rev. Lett 135, 221002; doi: 10.1103/zk9k-nbpj
