1. Problem "Estimate the flux of neutrinos passing through your body per second if the present energy density of neutrinos from the Big Bang is 0.2 MeV/m3. Assume that you are a standard size covering 0.01 m2". 2. Relevant equations nv = Uv(T) / <Ev> 3. The attempt at a solution I've assumed that the neutrinos have a temperature of 1.95 K. Now I'm not sure whether to presume that the neutrinos are relativistic (hence, zero mass and velocity of c) or non-relativistic (i.e. mv < 1 eV), since the question does not specify. Although I believe the Tv = 1.95 K value comes from assuming neutrinos are massless (I think). I've attempted both and have different answers (although I doubt whether they are correct). Regardless, I've not had much success converting the neutrino density to a flux density. I assume that the neutrinos are travelling in all directions with the same velocity.