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Observations of the CMB and large-scale structure 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, c₄₅₅^2, using the most recent CMB and large-scale structure data. Additionally, we report improved constraints to the CDRB viscosity parameter, cₕ₈ₒ^2. For a noninteracting species, these parameters both equal 1/3. Using current data we find that a standard CDRB, composed entirely of three noninteracting neutrino species, is ruled out at the 99% confidence level (C. L. ) with c₄₅₅^2=0. 30-₀. ₀₂₆^+0. 027 and cₕ₈ₒ^2=0. 44-₀. ₂₁^+0. 27 (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.
Smith et al. (Fri,) studied this question.