How massless neutrinos affect the cosmic microwave background damping tail
How massless neutrinos affect the cosmic microwave background damping tail
We explore the physical origin and robustness of constraints on the energy density in relativistic species prior to and during recombination, often expressed as constraints on an effective number of neutrino species, ${N}_{\mathrm{eff}}$. If the primordial helium abundance, ${Y}_{\mathrm{P}}$, follows the prediction of the big bang nucleosynthesis (BBN) theory, the …