Pseudo-Nambu-Goldstone Boson Motion from Thermal Thresholds

Abstract

Coherent motion of light scalar fields in the early Universe is typically attributed to an initial displacement, a primordial velocity, or a prescribed temperature-dependent potential. We identify a distinct microscopic mechanism in which differential thermal evolution of multiple non-aligned symmetry-breaking sources rotates the effective vacuum direction of a pseudo-Nambu--Goldstone boson (pNGB), driving motion from an initially stationary minimum. The mechanism is realized in the Majoron sector of a Type-I seesaw mechanism, where heavy neutrinos with separated thermal thresholds supply the required differential thermal evolution. For a representative benchmark, the thermal source direction rotates by $|Δθ_T|\simeq0.76$~rad and drives the Majoron to a peak velocity $|\dotθ|/H\simeq0.62$, while the field tracks the moving minimum to sub-milliradian accuracy. This establishes a microscopic finite-temperature origin for pNGB motion, with implications for spontaneous leptogenesis and, more generally, for any system with multiple non-aligned symmetry-breaking sources that evolve differently with temperature.