The forgotten particle explained to astrophysicists why the Universe exists

Matter defeated antimatter thanks to a heavy neutrino that physicists had considered an inconsequential detail for decades. Due to its slow decay, the particle survived in the cooling cosmos and temporarily seized dominance over the Universe. This hypothesis was proposed by a team from the University of Southampton in England and their colleagues. The results of the study have been published in the journal Physical Review Letters.

The problem it addresses is as old as cosmology itself. According to textbooks, the Big Bang should have produced matter and antimatter in strictly equal amounts, and upon meeting, they should have completely converted into radiation. This means the Universe was supposed to become a sea of pure light with no planets and no people. Since we exist, the balance was broken somewhere. By a widely accepted estimate, the imbalance was microscopic: roughly one extra particle of matter for every billion annihilated pairs, and it was precisely this remnant that assembled all the galaxies.

The solution was found in a particle with the reputation of an extra: the "decoupled" heavy neutrino, interacting so weakly with the rest of the world that most previous theories simply overlooked it. "This particle was long dismissed as an inconsequential detail in particle physics," explains Rishav Roshan from Southampton. "But because it decays very slowly, it survived the cooling of the cosmos, temporarily took over the energy density of the Universe, and triggered an era of matter domination."

The heavy neutrino shifted the temperature at which leptons, such as electrons, interacted with their antiparticles through the Higgs boson. It is precisely this temperature that determines how strongly the balance tips in favor of matter. This produced a tiny shift in the threshold in the early Universe, as a result of which we observe galaxies, stars, and life.

Space itself will allow the hypothesis to be tested: the predicted changes would leave distinct but detectable imprints on primordial gravitational waves, the echo of the first moments of the world. The work, according to co-author Sudipta Shaw, changes the picture of the rapid early expansion of the Universe and offers "a new explanation for how matter won." If the imprint is found, a particle from the background noise will become the central figure of cosmic genesis, and the question of why we exist at all will receive the most direct answer possible.