Cosmic Neutrinos and Other Light Relics
  Cosmological measurements of the radiation density in the early universe can
be used as a sensitive probe of physics beyond the standard model. Observations
of primordial light element abundances have long been used to place non-trivial
constraints on models of new physics and to inform our understanding of the
thermal history to the first few minutes of our present phase of expansion.
Precision measurements of the angular power spectrum of the cosmic microwave
background temperature and polarization will drastically improve our
measurement of the cosmic radiation density over the next decade. These
improved measurements will either uncover new physics or place much more
stringent constraints on physics beyond the standard model, while pushing our
understanding of the early universe to much earlier times.
