Messengers from the Early Universe: Cosmic Neutrinos and Other Light
  Relics
  The hot dense environment of the early universe is known to have produced
large numbers of baryons, photons, and neutrinos. These extreme conditions may
have also produced other long-lived species, including new light particles
(such as axions or sterile neutrinos) or gravitational waves. The gravitational
effects of any such light relics can be observed through their unique imprint
in the cosmic microwave background (CMB), the large-scale structure, and the
primordial light element abundances, and are important in determining the
initial conditions of the universe. We argue that future cosmological
observations, in particular improved maps of the CMB on small angular scales,
can be orders of magnitude more sensitive for probing the thermal history of
the early universe than current experiments. These observations offer a unique
and broad discovery space for new physics in the dark sector and beyond, even
when its effects would not be visible in terrestrial experiments or in
astrophysical environments. A detection of an excess light relic abundance
would be a clear indication of new physics and would provide the first direct
information about the universe between the times of reheating and neutrino
decoupling one second later.
