So Close, yet So Far

[P]hotons are massless, whereas neutrinos have a tiny amount of mass, which makes them travel more slowly than light. In a question and answer session at a 2009 summer school, Mika Vesterinen, a graduate student in particle physics at the University of Manchester in England, asked Scott Dodelson, of the Fermi National Accelerator Lab in Illinois, how the mass of neutrinos would affect the distance the relic neutrinos had traveled since they last interacted. So Dodelson and Vesterinen set out to calculate the answer.

What they found surprised them: even though the relic neutrinos have been traveling for far longer than the CMB, their slower speed means they’ve covered much less distance. The cosmic neutrino background (CNB) originates from a distance of about a billion to ten billion light years away, much less than the 40-billion light-years for the CMB.