Dark matter might have had a turbulent youth. According to an international team, this invisible substance may have formed at extremely high temperatures, moving almost at the speed of light shortly after the Big Bang. This proposal directly challenges several decades of cosmological theories that favored cold and slow-moving dark matter.
Invisible yet omnipresent, dark matter does not emit light. However, its gravitational influence is necessary to explain the formation of galaxies. Comprising a major part of cosmic mass, it guides the assembly of stars and planets. Its absence would make the vast structures we observe today very different, if not inconceivable.
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Published in
Physical Review Letters, this work indicates that dark matter particles could have decoupled from ordinary matter while still being very energetic. This phenomenon would have occurred during the critical post-inflation reheating phase. The Universe, then expanding and gradually cooling, would have allowed these initially very hot particles to lose energy.
This new hypothesis differs from models involving neutrinos, long considered unsuitable as hot candidates. It shows that dark matter could have cooled down early enough not to hinder galaxy formation. The researchers specify that the dynamics of cosmic expansion would have given these ultrarelativistic particles time to slow down before the emergence of the first galactic structures.
This proposal thus broadens the scenarios concerning the nature and behavior of dark matter, while offering new ways to consider its interactions with visible matter. Scientists can now examine alternative models incorporating this high-temperature formation.
To test these ideas, the team plans to search for clues through particle collisions or via observations of the cosmos. These approaches could uncover signatures of dark matter in ancient cosmic events.
Cosmic inflation and the subsequent reheating
Cosmic inflation refers to an extremely brief period occurring just after the Big Bang, during which the Universe underwent exponential expansion. This phase smoothed out and homogenized the cosmos, laying the foundation for future structures.
Post-inflation reheating is the era that immediately follows, where the Universe fills with particles and energy. It is during this phase that dark matter, according to the new study, could have formed at extreme temperatures. The conditions were conducive to violent interactions and the generation of exotic particles.