Precision detection of the cosmic neutrino background

In the standard big bang cosmology the canonical value fur the ratio of rel ic neutrinos to cosmic microwave background (CMB) photons is 9/11. Within t he framework of the standard model of particle physics there are small corr ections, in sum about 1%, due to slight heating of neutrinos by electron-po sitron annihilations and finite-temperature QED effects. We show that this leads to changes in the predicted CMB anisotropies that will bias determina tion of the other cosmological parameters if not correctly taken into accou nt. These changes might be detected by future satellite experiments.