Cosmology of an Axion-Like Majoron

Abstract

We propose a singlet majoron model that defines an inverse seesaw mechanism in the ν sector. The majoron φ has a mass m φ ≈ 0.5 eV and a coupling to the τ lepton similar to the one to neutrinos. In the early universe it is initially in thermal equilibrium, then it decouples at T ≈ 500 GeV and contributes with just ∆N eff = 0.026 during BBN. At T = 26 keV (final stages of BBN) a primordial magnetic field induces resonant γ ↔ φ oscillations that transfer 6% of the photon energy into majorons, implying ∆N eff = 0.55 and a 4.7% increase in the baryon to photon ratio. At T ≈ m φ the majoron enters in thermal contact with the heaviest neutrino and it finally decays into ν ν pairs near recombination, setting ∆N eff = 0.85. This boost in the expansion rate at later times solves the Hubble tension, while the neutrino-majoron interactions suppress the ν free streaming and make the model consistent with large scale structure observations. Its lifetime and the fact that it decays into neutrinos instead of photons lets this axion-like majoron avoid the strong bounds that affect other axion-like particles of similar mass and coupling to photons.