Neutrinos in the early universe: Speculations on sterile neutrino dark matter and the telltale light element nucleosynthesis clues its creation may leave behind

This talk will give a brief overview of the evolution of the neutrino component in the early universe, identifying what we know and don’t know about neutrino physics and neutrino effects. In fact, we will see that the uncertainties in neutrino properties are manifold and invite speculation about what could be. One such possibility is that sterile neutrinos comprise some of the dark matter. X-ray observations of clusters of galaxies may have found this, or ruled it out! (Ah . . . astronomy!) In any case, the net lepton numbers (an asymmetry of neutrinos over antineutrinos of about one part in 10^4) required to manufacture a dark matter relic density for these sterile neutrinos at the QCD epoch (age of the universe ~ 10 microseconds) may induce flavor transformation among the ordinary, “active” neutrinos at the epoch (age of the universe ~ 1 second) when these particles decouple and the light nuclei are formed in Big Bang Nucleosynthesis (BBN). If so, there may be subtle alterations in the calculated yields of deuterium, helium, and 7Be/7Li in BBN. It is not at all all clear that we could measure these alterations and, truth be told, we cannot really do any of the required flavor physics calculations right now! But we have ideas and new insights and new computational tools, and we are making progress.