Forgotten particle may hold key to universe’s existence

Researchers from the University of Southampton and the Indian Institute of Technology Kanpur believe they’ve identified a crucial element in this puzzle: a ‘decoupled’ heavy neutrino, a lesser-known relative of the neutrino. These particles are incredibly lightweight and rarely interact with ordinary matter.

The team’s focus centered on this type of particle, which has typically been considered too weakly connected to other particles to significantly impact the universe. However, their calculations reveal that its unusually long lifespan could have played a far more significant role than previously imagined.

According to the research, this particle may have temporarily dominated the universe’s energy density as it cooled following the Big Bang. This brief period of dominance could have altered the early universe’s evolution, influencing the conditions that allowed matter to outnumber antimatter.

“This decoupled heavy neutrino has long been dismissed as an irrelevant detail in particle physics,” explained Dr. Rishav Roshan of the University of Southampton, a study author. “However, it decays incredibly slowly. As the cosmos cooled after the Big Bang, the particle lived long enough that it temporarily took over the energy density of the universe and triggered a matter-dominated period.”

The findings were published in Physical Review Letters, a publication of the American Physical Society.

Previous attempts to explain the matter-antimatter asymmetry often overlooked the decoupled heavy neutrino, assuming its weak interactions would be inconsequential. This new research challenges that assumption, suggesting even weakly interacting particles can indirectly influence conditions that gave matter its advantage.

This imbalance, known as matter and antimatter asymmetry, is fundamental to our existence. The early universe possessed a slight surplus of matter, allowing some to persist after most particles and their antiparticles annihilated each other.

“The fact that we are here at all proves that an asymmetry between matter and antimatter occurred,” added Dr. Roshan. “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 Standard Model Higgs boson. This temperature acts like a switch that shapes how the matter-antimatter imbalance was built up.”

The Higgs boson, a fundamental particle associated with mass, is central to this theory. The heavy neutrino, according to the researchers, altered the temperature conditions governing particle interactions involving the Higgs boson.

By influencing when these interactions occurred, the particle may have affected how the matter-antimatter imbalance developed. This previously insignificant element may have played a role in determining why the universe contains matter.

The theory also has implications for understanding the universe’s early expansion. Dr. Sudipta Show of the Indian Institute of Technology Kanpur, a study co-author, stated that the findings suggest the forgotten particle could have dramatically altered the cosmos’ expansion history.

“It offers us a new explanation for how matter won,” he said. “The dramatic shift in the early expansion of the cosmos leaves distinct but detectable imprints of changes, for example on primordial gravitational waves. These gravitational wave signatures open up new avenues for us to unravel the evolution of the early universe after the Big Bang.”

Gravitational waves, ripples in space-time, may hold clues to the universe’s earliest moments. The temporary dominance of the heavy neutrino could have left recognizable signatures in these ancient signals, potentially providing a way to investigate its influence on the universe’s earliest evolution. The explanation remains theoretical, but offers a new possibility for solving a profound cosmological mystery: how matter survived when annihilation with antimatter could have left the universe empty.