Physicists from the University of Southampton and the Indian Institute of Technology Kanpur, studying the mystery of why we exist, have proposed that an often-overlooked hypothetical particle known as a decoupled heavy neutrino could help explain why matter survived in the early universe.
If correct, the proposed mechanism could help explain how matter came to dominate over antimatter in the early phases after the Big Bang, ultimately making possible the material universe we see today.
Mystery of Why We Exist: “One of the Greatest Conundrums of Modern Physics”
Current models of the early universe suggest that the Big Bang should have produced nearly equal amounts of matter and antimatter. If these particles had existed in perfectly equal quantities, their mutual annihilation would have left a universe dominated by radiation, rather than the stars, galaxies, and planets we observe today.
The fact that our universe contains far more matter than antimatter indicates that some process created an imbalance between the two. Explaining how that happened remains one of the greatest unanswered questions in modern physics.
“The fact that we are here at all proves that an asymmetry between matter and antimatter occurred,” explained Southampton’s Dr. Rishav Roshan, author of the study detailing the new theory of why we exist.
Now, Dr. Roshan and colleagues believe they may have found a clue to a possible solution in an often-overlooked ancient particle called a decoupled heavy neutrino.
Decoupled Heavy Neutrinos “Triggered a Matter-Dominated Period”
According to the university’s announcement, previous theoretical studies have often neglected this proposed particle, a heavy relative of ordinary neutrinos, because its extremely weak interactions were thought to make its influence negligible.
“This decoupled heavy neutrino has long been dismissed as an irrelevant background detail in particle physics,” Dr. Roshan explained.
In their new theory, the Southampton and Kanpur researchers found a mechanism where this particle, which decays very slowly, could alter the particle–antiparticle asymmetry essential for all current matter in the universe to exist.
According to Dr. Roshan, after the Big Bang, when the cosmos began to cool, the slowly decaying ancient particle lived long enough “that it temporarily took over the energy density of the universe and triggered a matter-dominated period.”
“The decoupled neutrino shifted the temperature in the early universe at which fundamental particles – like electrons and other leptons – interacted with their anti-particles with the help of the Standard Model Higgs boson,” the research explained, adding that this increased temperature “acts like a switch that shapes how the matter-antimatter imbalance was built up.”
In their theory, this energy imbalance affected the early universe’s background energy levels long enough for some matter particles to remain after all antimatter was annihilated.
New Explanation for How Matter Won
When discussing the theory’s significance, paper co-author Dr. Sudipta Show from Kanpur said it could offer a firm solution to the mystery of why we exist.
“It offers us a new explanation for how matter won,” Dr. Show said.
The researcher also said the concept could change the fundamental understanding of “how the early universe expanded so quickly after the Big Bang.”
“The dramatic shift in the early expansion of the cosmos leaves distinct but detectable imprints of changes, for example on primordial gravitational waves,” Dr. Show explained.
The team said such gravitational wave signatures will also “open up new avenues” for scientists to unravel the evolution of the early universe immediately after the Big Bang.
The team’s paper was published in Physical Review Letters.
Christopher Plain has spent the last six years as Associate News Editor and Head Science Reporter at The Debrief. Follow and connect with him on X, learn about his novels at plainfiction.com, or email him at christopher@thedebrief.org.