Big Bang Nucleosynthesis constraints on sterile neutrino and lepton
  asymmetry of the Universe
  We consider the cosmological effects of sterile neutrinos with the masses of
$150- 450$ MeV. The decay of sterile neutrinos changes the thermal history of
the Universe and affects the energy density of radiation at the recombination
and the big bang nucleosynthesis (BBN) results. We derive severe constraints on
the parameters of sterile neutrinos from the primordial abundances of helium-4
and deuterium. We also find that in a particular model the constraints can be
considerably relaxed by assuming a large lepton asymmetry in the active
neutrinos. In this case, the consistent parameters result in $N_{\mathrm{eff}}
\simeq 3.2- 3.4$ and can alleviate the Hubble tension.
