New findings from researchers at the University of Minnesota Twin Cities and Université Paris-Saclay are upending the long-standing cold dark matter assumption, proposing that this elusive substance may have been incredibly hot and fast-moving in the early universe. This groundbreaking work suggests dark matter could have emerged moving near the speed of light shortly after the Big Bang, later cooling down to form the cosmic structures we observe today.
For decades, the standard cosmological model has relied on dark matter being “cold” – meaning it moved slowly – at its formation. This slow movement was considered essential for gravitational clumping, allowing galaxies and large-scale structures to form in the early universe. Without this property, it was believed, the universe would appear vastly different, lacking the intricate cosmic web we see.
The new study, published in Physical Review Letters, re-examines a critical but less-explored phase of cosmic history: post-inflationary reheating. This era, immediately following cosmic inflation, saw the universe rapidly filling with particles. By focusing on how dark matter could have been produced during this energetic period, the research expands our understanding of its potential origins and interactions.
Rethinking dark matter’s initial state
The prevailing view held that if dark matter particles were fast-moving, like neutrinos, their high speeds would have smoothed out matter distribution, preventing the formation of galaxies. This led to the dismissal of “hot dark matter” candidates in favor of the cold dark matter assumption. However, the new research challenges this, demonstrating a mechanism where even ultrarelativistic dark matter can cool sufficiently.
According to ScienceDaily.com, the team found that dark matter particles could separate from other matter while still incredibly hot and still slow down enough before galaxies began to form. This cooling process is directly linked to the expansion of the universe during the reheating phase, providing ample time for these particles to lose energy and become the structure-building force necessary for cosmic evolution.
Stephen Henrich, a graduate student in the School of Physics and Astronomy and lead author, emphasized this shift. “One of the few things we know about [dark matter] is that it needs to be cold,” Henrich stated. “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.” This insight fundamentally rewrites decades of cosmological thought.
Implications for detection and early universe physics
This revised understanding of dark matter’s origins opens new avenues for future research and detection efforts. If dark matter can originate from a “hot” state, the range of particle candidates that could constitute dark matter significantly broadens. Scientists can now explore different types of particles and interaction mechanisms that were previously discounted.
The research team plans to build on these findings by exploring how such “red-hot” dark matter particles might be detected. Possible approaches include direct searches using particle colliders or scattering experiments, as well as indirect detection through astronomical observations. This could involve looking for subtle gravitational effects or decay products that align with the new model.
Yann Mambrini, professor from Université Paris-Saclay and co-author, noted the broader impact: “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 work pushes the boundaries of our understanding, potentially unlocking secrets about the universe’s earliest moments and the fundamental forces at play.
The challenge to the established cold dark matter assumption represents a significant pivot in theoretical cosmology. By demonstrating that dark matter does not necessarily need to start cold, but can cool down effectively to shape galaxies, researchers have expanded the landscape of possibilities for one of the universe’s most enduring puzzles. The path ahead involves rigorous experimental verification and further theoretical refinement, potentially revealing the true nature of the cosmos’s unseen majority.
