Could Heavy Right-Handed Neutrinos Explain Why Matter Survived the Big Bang?
An overlooked particle may have altered the early universe in ways that helped create the imbalance between matter and antimatter, according to new theoretical research.
Rishabh Roshan and Sudipta Shaw have demonstrated that isolated right-handed neutrinos can trigger an early epoch of matter dominance and profoundly change the history of the expansion of the universe. Image credit: M. Weiss / Harvard-Smithsonian Center for Astrophysics.
A possible explanation for matter’s dominance
Why there is matter in the universe in the first place remains one of physics’ most enduring mysteries.
The Big Bang produced approximately equal amounts of matter and antimatter, allowing the two to annihilate each other. Yet enough matter survived to form galaxies, stars, planets and, eventually, life.
Dr Rishabh Roshan of the University of Southampton and Dr Sudipta Shaw of the Indian Institute of Technology Kanpur propose that a little-considered particle called the heavy dextral neutrino could help explain how matter became dominant.
“This detached heavy neutrino has long been ignored as an irrelevant background detail in particle physics,” Roshan explained.
“But it decays incredibly slowly. As the universe cooled after the Big Bang, the particles lived long enough to temporarily take over the energy density of the universe and trigger an era dominated by matter.”
How heavy neutrinos could affect leptogenesis
When the particles eventually decay, the resulting changes in the expansion history of the universe may alter the temperature at which certain interactions involving charged leptons reach equilibrium.
These interactions are important for a process known as leptogenesis. In leptogenesis, differences in the behavior of particles and antiparticles help generate the excess matter needed to explain the universe observed today.
The physicists’ calculations show that changing conditions can affect how this asymmetry evolves. They also indicate that relatively light heavy neutrinos could produce an imbalance consistent with observations.
“The fact that we are here proves that an asymmetry has arisen between matter and antimatter,” Roshan said.
“The detached neutrinos changed the temperature of the early universe, where fundamental particles such as electrons and other leptons interacted with antiparticles with the help of the Standard Model Higgs boson.”
“This temperature acts like a switch that determines how the matter-antimatter imbalance builds up.”
Could the theory be tested?
The proposal is theoretical, and the study does not prove that the hypothetical particle existed in the early universe.
However, the researchers suggest that the scenario could be tested through neutrino experiments and observations of gravitational waves.
Identifying such signals could help scientists determine whether overlooked particles played a role in forming the universe or creating the conditions that allowed matter to survive.
“This discovery changes our understanding of how the early universe expanded rapidly after the Big Bang,” Shaw said.
“It gives us a new explanation of how matter triumphs.”
“Drastic changes in the initial expansion of the universe leave distinct but detectable traces of changes in, for example, primordial gravitational waves.”
“These gravitational wave signatures open up new avenues for understanding the evolution of the early universe after the Big Bang.”
Study publication
The team’s paper was published in Physical Review Letters in September 2026.
_____
Rishav Roshan and Sudipta Shaw. 2026. “Delayed charged lepton Yukawa equilibrium in a minimal seesaw.” Physical Review Letters 137, 111802. doi: 10.1103/nfkx-q6z7
Source: www.sci.news