Researchers propose that knotted structures in the early universe could account for why matter dominates over antimatter. By combining two symmetries in particle physics, these cosmic knots formed after the Big Bang, briefly dominated cosmic energy, and decayed to produce a slight matter surplus. The model also addresses neutrino masses, dark matter, and the strong CP problem.
In the early moments after the Big Bang, the universe underwent phase transitions that broke symmetries and potentially created topological defects like cosmic strings. A new study suggests these strings intertwined to form stable 'cosmic knots' through the interplay of gauged Baryon Number Minus Lepton Number (B-L) symmetry and Peccei-Quinn (PQ) symmetry.
The B-L symmetry generates magnetic flux tubes, while the PQ symmetry produces superfluid vortices. Their coupling creates knot solitons that resist decay, allowing them to dominate the universe's energy density over radiation as spacetime expanded. This knot-dominated era ended when quantum tunneling unraveled the structures, releasing heavy right-handed neutrinos.
These neutrinos, with masses around 10^12 giga-electronvolts, decayed preferentially into matter, generating the observed asymmetry—one extra matter particle per billion pairs. The process reheated the universe to about 100 GeV, aligning with the threshold for electroweak processes that convert neutrino imbalance to baryon excess.
"This study addresses one of the most fundamental mysteries in physics: why our Universe is made of matter and not antimatter," said Muneto Nitta, corresponding author from Hiroshima University's WPI-SKCM2. "This question is important because it touches directly on why stars, galaxies, and we ourselves exist at all."
The model extends the Standard Model, incorporating axions for dark matter via PQ symmetry and explaining neutrino masses through B-L. It predicts a shifted gravitational-wave spectrum detectable by future observatories like LISA, Cosmic Explorer, and DECIGO.
Co-author Yu Hamada noted, "The right-handed neutrinos are special because their decay can naturally generate the imbalance between matter and antimatter... they are the parents of all matter in the universe today."
Minoru Eto added, "Cosmic strings are a kind of topological soliton... our result isn't tied to the model's specifics."
Published in Physical Review Letters (2025; 135(9)), the work revives Lord Kelvin's knot idea in a modern context, offering a testable path to understanding cosmic origins.