Neutrinos don't interact strongly with regular matter. Countless neutrinos stream through us all the time with no effect. But neutrinos weren't always so weakly interacting. In the earliest cosmic moments neutrinos still interacted strongly with protons, neutrons, and electrons. All of these particles were in thermal equilibrium for a time until the Universe could expand to a critical point. At about a second after the Big Bang neutrons decoupled from other material particles. They began to act similar to the neutrinos we see today.
These decoupled neutrinos still fill the Universe. They form a cosmic neutrino background. A particle echo of the Big Bang, similar to the cosmic microwave background. In the present epoch these neutrinos play no strong role in the evolution of the Universe, but some of them may be the reason we live in a matter-dominated universe.
One of the basic symmetries of the Universe is that every elementary particle has a twin. Electrons have the positron. Quarks have anti-quarks. Protons have anti-protons. Under the right circumstances you can create particles, but they always come in pairs. So one of the big questions in cosmology is why we live in a matter-dominated cosmos. If the Big Bang created an equal amount of matter and antimatter, where did all the antimatter go? The answer could be heavy decoupled neutrinos.
Although neutrinos don't have an electric charge, there are both matter and antimatter neutrinos. Rather than charge, they are distinguished by their helicity or handedness. Matter neutrinos are always left-handed, while anti-matter neutrinos are right-handed. In the modern Universe only left-handed neutrinos interact with matter. The right-handed ones have some teeny-tiny gravitational effect but are otherwise "sterile" neutrinos.
Neutrinos also come in three different varieties or flavors. For matter, the three flavors are associated with the electron, muon, and tauon, which is why neutrinos have three possible masses. Anti-neutrinos correspond to the anti-ons and also have three mass values. This latest study focuses on the anti-neutrinos with the greatest mass. What they call the decoupled heavy neutrino.
Decoupled heavy neutrinos could have dominated the Universe for a time. Credit: Roshan & Show
In their model, the authors note that heavy neutrinos can undergo decay. They argue that since the half-life of these heavy neutrinos is relatively long, they would have decayed well after things like electrons, protons, positrons, and anti-protons formed. When heavy anti-matter decayed, their by-products kept the Universe a bit warm and annihilated some of the antimatter particles to create neutral photons. As a result, the balance between matter and antimatter was shifted in favor of matter. The remaining antimatter of the cosmos was destroyed via regular particle interactions, leaving matter as the stuff we see today.
If this model is correct, the neutrino-dominated period would have a measurable effect on primordial gravitational waves. When gravitational wave observatories improve to the point where we can detect primordal waves, we could also test this model. There would also be a measurable effect on the cosmic neutrino background. So perhaps in the future we will discover that one of the strongest consequences of the Big Bang was caused by one of the Universes weakest interactions.
Reference: Roshan, Rishav, and Sudipta Show. "Delayed Charged Lepton Yukawa Equilibration in Minimal Seesaw." Physical Review Letters 137.11 (2026): 111802.