Estimate what neutrino mass (in eV/c2) would provide the critical density to close the universe. Assume the

Question:

Estimate what neutrino mass (in eV/c2) would provide the critical density to close the universe. Assume the neutrino density is, like photons, about 109 times that of nucleons, and that nucleons make up only
(a) 2% of the mass needed,
(b) 5% of the mass needed.