The universe's dark matter may not have needed a calm, cold start to help build the cosmos, according to a new study from researchers at the University of Minnesota Twin Cities and Université Paris-Saclay. This groundbreaking research challenges decades-old assumptions about dark matter's origins, suggesting that it could have formed while moving near the speed of light and then cooled over time, allowing it to behave like the cold dark matter essential for the formation of galaxies and larger cosmic structures. This new perspective opens up exciting possibilities for understanding the early universe and the role of dark matter in its evolution.
The study, published in Physical Review Letters, focuses on a period just after inflation, the rapid expansion of the early universe. Instead of assuming the universe instantly heated up after inflation, the researchers explored the reheating phase, where the inflaton field slowly decayed and transferred energy into particles and radiation. This timing is crucial, as it determines how quickly dark matter particles can cool and become cold.
The key mechanism, called ultrarelativistic freeze-out (UFO), describes dark matter particles stopping their interactions with ordinary matter while still moving extremely fast. As the universe expands, particle momenta decrease, and by the time cosmic structures begin to form, the once-fast dark matter can behave like cold dark matter. This process is influenced by the timing of freeze-out during reheating.
The study's findings challenge the idea that neutrinos, which decoupled while moving close to the speed of light, were the prime example of hot dark matter. Keith Olive explains that the neutrino's behavior was ruled out over 40 years ago because it would have wiped out galactic-scale structures. However, the new research suggests that a similar candidate, if produced just as the hot big bang universe was being created, could have cooled to the point where it would act as cold dark matter.
The study's broader implications point back to a poorly understood chapter of cosmic history. Many dark matter scenarios erase most memories of inflation and reheating, but this research does not. If the relic abundance of dark matter was set during reheating, observations or experiments could provide insights into the conditions of the universe before the hot big bang. This opens up new avenues for understanding the early universe and the transition from inflation to the formation of galaxies.
The practical implications of this research are significant. It widens the search map for dark matter by reviving models that were previously dismissed as too hot to match the observed universe. Theorists now have a larger set of viable candidates to test, which could impact the design and interpretation of experiments in colliders, scattering experiments, and cosmological observations. This work also provides a new connection between dark matter physics and the least understood stages of cosmic history, offering a deeper understanding of the universe's evolution.
