Constraints on neutrino and dark radiation interactions using
  cosmological observations
  Observations of the cosmic microwave background (CMB) and large-scale
structure (LSS) provide a unique opportunity to explore the fundamental
properties of the constituents that compose the cosmic dark radiation
background (CDRB), of which the three standard neutrinos are thought to be the
dominant component. We report on the first constraint to the CDRB rest-frame
sound speed, ceff^2, using the most recent CMB and LSS data. Additionally, we
report improved constraints to the CDRB viscosity parameter, cvis^2. For a
non-interacting species, these parameters both equal 1/3. Using current data we
find that a standard CDRB, composed entirely of three non-interacting neutrino
species, is ruled out at the 99% confidence level (C.L.) with ceff^2 = 0.30
+0.027 -0.026 and cvis^2 = 0.44 +0.27 -0.21 (95% C.L.). We also discuss how
constraints to these parameters from current and future observations (such as
the Planck satellite) allow us to explore the fundamental properties of any
anomalous radiative energy density beyond the standard three neutrinos.
