Photon to axion conversion during BBN

Abstract

We discuss the cosmology of an axion-like particle associated to the generation of neutrino masses: an axion-like majoron or ALM. In particular, we consider the possibility that at the end of BBN a fraction of the CMB photons converts into majorons that then decay into neutrinos during recombination. We show that this could define an interesting variation of the Λ CDM model where (i) the increase in 𝑁 eff relaxes the so called 𝐻 0 tension; (ii) the larger baryon to photon ratio after BBN provides a better agreement of the deuterium abundance with CMB observables; and (iii) the sudden drop in the CMB temperature interrupts the synthesis of lithium and beryllium alleviating the so called lithium problem.