A neglected heavy neutrino could have reshaped the early universe
Physicists in Southampton and Kanpur propose that a long-lived heavy neutrino could briefly have dominated the early universe’s energy. Their theoretical work, published in Physical Review Letters with an author preprint on arXiv, links altered expansion to the conditions behind the matter–antimatter imbalance. The scenario depends on assumed particle properties; it reports no detection of the hypothetical neutrino.
Science··Midday
A long-lived hypothetical particle
A heavy neutrino usually treated as largely irrelevant could have changed the early universe's expansion, according to physicists in Southampton and Kanpur. Their theoretical proposal examines how a slowly decaying particle might affect the conditions in which matter gained an advantage over antimatter. The work offers a possible cosmic history, without reporting the detection of that particle.[1], [2]
Rishav Roshan of the University of Southampton and Sudipta Show of the Indian Institute of Technology Kanpur developed the work, published in Physical Review Letters. The authors' full preprint is also available on arXiv. Heavy right-handed neutrinos appear in models addressing the very small masses of known neutrinos, particles that interact only rarely with ordinary matter.[1]
How slow decay changes the scenario
The researchers reconsider a heavy neutrino whose exceptionally weak interactions can give it a long lifetime. As the universe cools, the particle can become a major contributor to its total energy density. In the calculated scenario, matter temporarily dominates before the heavy particle decays and radiation again becomes the leading component in the expanding universe.[1]
Changing that expansion history alters when interactions involving charged leptons and the Higgs field reach equilibrium. Leptons are a family that includes electrons. The timing affects when differences among lepton types matter to the generation of a matter–antimatter imbalance. The authors track cosmic expansion, particle decay and energy densities together through a set of coupled equations.[1]
Possible traces of an early expansion phase
The outcome depends on the masses, interaction strengths and initial conditions assumed in the model. An apparently decoupled particle can have a cosmological role because weak interactions allow it to survive for a long time. The proposal describes the consequences if those assumptions apply; it leaves the existence and properties of the heavy neutrino to be tested.[1]
The authors suggest that a temporary change in expansion could leave features in primordial gravitational waves, disturbances in space-time originating in the early universe. They describe those possible signatures as a way to examine the scenario. The study presents no gravitational-wave detection. Neutrino measurements provide another possible route for investigating the particle assumptions behind the calculations.[1]
