A neglected heavy neutrino may have helped matter survive the early universe
Physicists Rishav Roshan and Sudipta Show propose that a hypothetical heavy neutrino, N3, briefly dominated the early universe and altered its expansion. Their calculations suggest this could have delayed reactions that erode the matter–antimatter imbalance, helping preserve the small excess that later formed the matter we see today; the proposal remains theoretical.
Bottom line — The model predicts an almost massless lightest neutrino, but current experiments cannot test a mass that small.
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The study, published in Physical Review Letters, builds on leptogenesis, in which heavy-neutrino decays can produce an imbalance between matter and antimatter.
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In one calculation, N3 was about 100 trillion times heavier than a proton and interacted so weakly that it could survive unusually long.
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As the universe cooled, N3 would have shifted from behaving like radiation to behaving like matter, then released energy when it decayed.
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The researchers calculated that relevant lepton interactions began when the universe was seven times cooler than usual.
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The authors say the lightest known neutrino would weigh between one trillionth and one hundred-trillionth of an electronvolt in this scenario.
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KATRIN has set an upper limit of 0.45 electronvolts on a combined measure of neutrino mass; it cannot directly confirm the much smaller predicted mass.
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Primordial gravitational waves could preserve evidence of an early matter-dominated era, but no such background has been detected.
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The proposal depends on N3 having been produced in the early universe, a condition the authors have not established.