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 mϕ ≈ 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 ΔN 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 ΔN eff = 0.55 and a 4.7% increase in the baryon to photon ratio. At T ≈ mϕ the majoron enters in thermal contact with the heaviest neutrino and it finally decays into νν̅ pairs near recombination, setting ΔN eff = 0.85. The boost in the expansion rate at later times may relax the Hubble tension (we obtain H 0 = (71.4 ± 0.5) km/s/Mpc), while the processes νν̅ ⟷ ϕ suppress the free streaming of these particles 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.