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The U(1)$_{B-L}$ symmetry could be restored during inflation, since theBICEP2 results suggest a GUT-scale inflation with the Hubble parameter, $H_{\rminf} \simeq 10^{14}$ GeV, close to the U(1)$_{B-L}$ breaking scale. We considera scenario in which the ${B-L}$ Higgs field dominates the Universe afterinflation, and mainly decays into the U(1)$_{B-L}$ gauge bosons, whosesubsequent decays reheat the Universe. Interestingly, if one of theright-handed neutrinos is extremely light and behaves as dark radiation or hotdark matter, its abundance is determined by the $B-L$ charge assignment and therelativistic degree of freedom in plasma. We find that $\Delta N_{\rm eff}$takes discrete values between $0.188$ and $0.220$ in the standard model plusthree right-handed neutrinos, depending on whether the decay into heavierright-handed neutrinos are kinematically accessible or not. In the fivenessU(1)$_{\bf 5}$ case, we find that $\Delta N_{\rm eff}$ takes discrete valuesbetween $0.313$ and $0.423$. The tension between BICEP2 and {\it Planck} can bepartially relaxed by dark radiation.

U(1)$_{B-L}$ Symmetry Restoration and Effective Neutrino Species