Researchers at the University of Southampton and the Indian Institute of Technology Kanpur have proposed that a little-studied heavy neutrino may have helped determine why matter survived the Big Bang. Their calculations suggest that the particle could have lived long enough to temporarily dominate the early universe’s energy density, altering its expansion and the conditions that allowed matter to become more abundant than antimatter.
One of the fundamental questions in physics is why the universe contains matter rather than nothing. Scientists believe that matter and antimatter were produced in roughly equal amounts in the early universe. When they meet, the two annihilate and release energy. If their quantities had been exactly equal, almost all matter would have disappeared, leaving nothing from which stars, planets and life could form.
The new explanation involves a little-understood relative of the neutrino. Neutrinos are extremely lightweight particles that interact only weakly with ordinary matter.
A forgotten particle in the early universe
The researchers focused on a hypothetical particle known as an “isolated” heavy neutrino. Unlike regular neutrinos, this heavier relative has generally been considered too weakly connected to other particles to significantly affect the universe.
However, the team found that its unusually long lifetime could have made it important. Their calculations suggest that, as the universe cooled after the Big Bang, the particle may have survived long enough to temporarily dominate the universe’s energy density.
This brief period of dominance could have changed the evolution of the early universe and influenced the conditions under which matter eventually came to outnumber antimatter.
Study author Dr. Rishabh Roshan of the University of Southampton said:
“This isolated heavy neutrino has long been ignored as an irrelevant background detail in particle physics.
“But it decays incredibly slowly. As the universe cooled after the Big Bang, the particles temporarily took over the energy density of the universe and survived long enough to trigger an era dominated by matter.”
The study’s results are reported in Physical Review Letters, the scientific journal of the American Physical Society.
How heavy neutrinos could explain matter’s survival
Previous explanations for the dominance of matter over antimatter have generally left out isolated heavy neutrinos. Because these particles interact so weakly with other particles, scientists had assumed they would have little effect on the processes responsible for the imbalance.
The new study challenges that assumption. The researchers found that even particles with extremely weak interactions could indirectly alter the conditions under which matter gained an advantage.
This imbalance, known as the matter-antimatter asymmetry, is central to understanding our existence. For reasons physicists are still working to explain, the early universe contained slightly more matter than antimatter. That small difference allowed some matter to remain after most particles and antiparticles had annihilated one another.
“The fact that we are here proves that an asymmetry has arisen between matter and antimatter,” Roshan added.
He continued:
“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.”
The Higgs boson is a fundamental particle involved in the mechanism that gives mass to many elementary particles. In the scenario proposed by the researchers, heavy neutrinos changed the temperature conditions governing important particle interactions involving the Higgs boson.
Those changes may have influenced how the imbalance between matter and antimatter developed. In this picture, a particle once considered unimportant could help explain why the universe contains matter at all.
Ancient gravitational waves could provide evidence
The theory could also have implications for the early expansion of the universe.
Study co-author Dr. Sudipta Shaw of the Indian Institute of Technology Kanpur said the findings suggest that the forgotten particle may have dramatically altered the expansion history of the young universe.
He said:
“This study provides us with 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.”
Gravitational waves are ripples in the fabric of space-time. Scientists believe that processes in the very early universe may have produced primordial gravitational waves carrying information about conditions at that time.
According to the new theory, the temporary dominance of heavy neutrinos may have left a recognizable imprint on these ancient signals. Detecting such a pattern could eventually help researchers determine whether the proposed particles influenced the universe’s early evolution.
For now, the explanation remains theoretical. It nevertheless offers a possible new approach to one of cosmology’s deepest mysteries: how matter survived when the universe could otherwise have been almost completely emptied by annihilation with antimatter.
Source: www.sciencedaily.com
