Researchers at the University of Southampton and the Indian Institute of Technology Kanpur have proposed that a hypothetical, long-lived heavy neutrino may have helped determine why matter survived the early universe. Their calculations suggest that the particle could briefly have dominated the universe’s energy density after the Big Bang, altering the conditions that allowed matter to outnumber antimatter and potentially leaving traces in primordial gravitational waves.
One of the central unanswered questions in physics is why the universe contains matter rather than nothing. Matter and antimatter are thought to have been created in roughly equal quantities in the early universe. When they meet, they annihilate and release energy. If the two had existed in exactly equal amounts, almost all matter would have disappeared, leaving nothing from which stars, planets and life could form.
The researchers’ explanation focuses on a little-understood relative of the neutrino. Neutrinos are extremely light particles that interact only weakly with ordinary matter.
A forgotten particle in the early universe
The team studied a hypothetical type of particle known as an “isolated” heavy neutrino. Unlike ordinary neutrinos, this heavier relative has generally been considered too weakly connected to other particles to have a significant effect on the universe.
However, its unusually long lifetime may have made it important. The researchers’ calculations suggest that, as the universe cooled after the Big Bang, the particle could have survived long enough to temporarily dominate the universe’s energy density.
This brief period of dominance may 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 findings are reported in the scientific journal Physical Review Letters, published by the American Physical Society.
How heavy neutrinos could explain why matter survived
Previous attempts to explain the dominance of matter over antimatter have generally excluded isolated heavy neutrinos. Because these particles interact so weakly with other particles, scientists had assumed that they would have little influence 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 over antimatter.
This imbalance, known as the matter–antimatter asymmetry, is essential to understanding our existence. For reasons physicists are still investigating, 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 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.”
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 way, a particle long considered unimportant could have helped determine why the universe contains matter at all.
Primordial gravitational waves could provide evidence
The theory could also have implications for understanding how the universe expanded during its earliest stages.
Study co-author Dr Sudipta Shaw of the Indian Institute of Technology Kanpur said the findings suggest that a neglected particle may have dramatically altered the expansion history of the young universe.
He added:
“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 offer a way to test whether the proposed particles influenced the early evolution of the universe.
For now, the explanation remains theoretical. It nevertheless gives physicists a new possible route to investigating one of cosmology’s deepest mysteries: how matter survived when the early universe might otherwise have been almost completely emptied by annihilation with antimatter.
Source: www.sciencedaily.com