The particle, a hypothetical "right-handed" neutrino, briefly could have held more energy than everything else in the infant universe combined, which would have sped up cosmic expansion and delayed key particle interactions, the study proposes. The research was published Sept. 8 in the journal Physical Review Letters.
One popular fix involves "right-handed" neutrinos. Particles spin a bit like tiny tops, and physicists call them left- or right-handed depending on how they spin relative to their direction of travel. A right-handed particle rotates like an ordinary screw being driven in, turning clockwise as it moves away from you. Every neutrino detected so far is left-handed and turns counterclockwise as it flies away, like a screw with a reverse thread.
"Right-handed neutrinos may mix with left-handed neutrinos, helping to explain the mass of the latter" and why they switch flavors, Roshan said.
The IceCube facility, photographed here beneath the Southern Lights, is Earth's premier neutrino detection facility. The proposed 'right-handed' neutrino has not yet been detected. (Image credit: IceCube/NSF)The particles could solve other puzzles, too. They "aim to address several issues, including the origin of neutrino masses, the matter-antimatter asymmetry of the universe, and the nature of dark matter," Roshan said.
In this picture, decaying heavy neutrinos left behind a slight excess of leptons — the particle family that includes electrons — that later became today's surplus of matter, he explained.
How the heaviest neutrino took over
In the new study, Roshan and Sudipta Show, a physicist at the Indian Institute of Technology Kanpur, examined what this neglected particle — which, in the team's main example, was about 100 trillion times as heavy as a proton — would have done after the Big Bang.
The study reveals that the neglected right-handed neutrino "significantly impacts the early universe's expansion dynamics, driving a period of early matter domination," Roshan said. This change "affects various Standard Model interactions" that could help to explain the matter-antimatter asymmetry, Roshan added.
These interactions would have let the early universe distinguish lepton flavors, which would have affected how the matter surplus formed. Thanks to the delay, the team found, a lighter right-handed neutrino about 100 billion times as heavy as a proton could have produced the observed amount of matter before the flavors could be told apart — even after the heavy neutrino's decay diluted part of it.
Hard to test, but not out of reach
Related storiesThe heavy neutrino is far too massive to create in any particle accelerator, so tests must be indirect.
LISA, a space-based European Space Agency mission, is scheduled to launch around 2035, while SKAO, a giant radio telescope project being built in Australia and South Africa, could catch such waves by timing distant pulsars. Tests could come "as these facilities come online and begin data collection," Roshan said. A faint hum from the hot primordial plasma would carry the era's imprint too, but at frequencies beyond what existing detectors could pick up.
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