BBN as a Probe of Particle Physics
Neutrinos:
§may have mass but they are
known to be relatively light.
§During BBN, neutrinos moved
at almost the speed of light,
forming a background neutrino
`radiation’ in addition to the
cosmic photon background: today, nn ~
100 per cm3
§Current Temperature of the n background is about 1 degree,
slightly colder than the CMB.
§The cosmic neutrino
background has not been detected and there
are so far no good ideas for doing so,
due to the weakness of their
interactions.