A new study published in Physical Review Letters suggests that a rare particle existing at the birth of the universe could explain why matter dominates the cosmos. The research points to the heaviest decoupled right-handed neutrino as the key to this mystery.
The modern universe is built on matter, yet it also contains small amounts of antimatter. This antimatter consists of particles with the same mass as normal matter but opposite electrical charges. For example, protons are positively charged while antiprotons are negatively charged. Electrons carry a negative charge, whereas positrons carry a positive charge.
When matter and antimatter meet, they destroy each other instantly. This mutual annihilation creates a major problem for cosmology. The Big Bang should have produced equal amounts of both. Instead, matter became the dominant substance. Scientists have yet to explain this asymmetry.
Rishav Roshan, a research fellow at the University of Southampton, and Sudipta Show, a postdoctoral researcher at the Indian Institute of Technology Kanpur, have identified the overlooked particle as a solution. They predict their hypothesis can be tested in future experiments.
“No one talks about this decoupled right-handed neutrino,” Roshan told 404 Media. “We realized that this decoupled right-handed neutrino could alter the expansion of the universe and affect the matter-antimatter asymmetry of the universe.”
“This thing was present in the literature for a very long time, and for some reason people did not pay that much attention to this particular point,” he added.
Scientists have previously identified a process called leptogenesis as a leading explanation for how matter dominated the universe. Roshan and Show now demonstrate that the heaviest decoupled right-handed neutrino is essential to this hypothesis.
The particle and its behaviour
This particle is the heaviest of three hypothetical decoupled right-handed neutrinos that existed briefly after the Big Bang. These particles are central to the minimal seesaw model, which explains the origin of modern neutrinos. Modern neutrinos are extremely lightweight and abundant today.
“The heaviest decoupled right-handed neutrino is a particle that exists in the minimal seesaw model but interacts very weakly with regular matter, making it nearly invisible to current experiments,” Roshan said. He noted that researchers often focus on lighter neutrinos and view heavy states as irrelevant due to their minimal influence on observable physics.
Roshan and Show included these particles in models of the primordial universe. The models suggest the particle could have slowly decayed, creating an imbalance in leptons. Leptons include different flavors such as electrons, muons, and neutrinos. This initial imbalance was then amplified into the larger matter-antimatter imbalance.
“In the early universe, when this heavy neutrino was produced, the temperature of the universe was extremely high, much greater than the mass energy of this neutrino, and it behaved as radiation,” Roshan explained. “As the universe began to expand, its temperature slowly decreased.”
“Once the temperature fell below the mass energy of the heavy neutrino, the particle became non-relativistic and started to behave like matter due to its extremely weak interactions, influencing the dynamics of the universe’s expansion and affecting the creation of matter versus antimatter,” he continued.
How to find it
The team noted that new observatories could potentially detect this process hidden in gravitational waves. These are ripples in spacetime. Current detectors like the Laser Interferometer Gravitational-Wave Observatory (LIGO) pick up waves from cataclysmic events, such as mergers between black holes.
However, a new generation of advanced detectors is due to come online in the 2030s and 2040s. These instruments are designed to tune into primordial waves from the split-seconds after the Big Bang. These waves may contain spectral signs of the heaviest decoupled right-handed neutrino.
“Future detectors like SKA, LISA, DECIGO, and ET could potentially observe these signatures,” Roshan wrote. “These instruments are being developed to detect gravitational waves, with some expected to come online within the next few years. If successful, they could pick up signals related to the heavy neutrino’s effects on the early universe.”
“Solving the matter-antimatter asymmetry question could transform our understanding of cosmic evolution and the fundamental laws of physics,” he added. “It may challenge or refine the Standard Model and influence theories of the universe’s formation and structure. A new understanding of this asymmetry could also lead to insights about dark matter and the early moments of the universe, potentially reshaping our view of cosmic history.”
What it means
For scientists, this discovery shifts focus from lighter neutrinos to a heavier, previously ignored particle. It changes the approach to testing the minimal seesaw model. Instead of relying solely on current experiments that struggle to see heavy particles, researchers can now look for gravitational wave signatures in upcoming decades. This provides a clear path to verify if this particle drove the creation of matter.
