Why flavour physics demands a cold early universe

Extensions of the Standard Model that explain the hierarchy of particle masses through flavour-dependent gauge groups inevitably produce light magnetic monopoles in the early universe. Combining constraints from cosmology, astrophysics, and direct searches, the authors show these monopoles require reheating temperatures around 10³–10⁴ TeV—roughly a thousand times hotter than the TeV scale but far cooler than the Grand Unified Theory scale. This creates a direct link: evidence for flavour physics would imply a cold early universe, while gravitational wave observations could pin down the scale at which these gauge structures emerge.