Constraints on neutrino and dark radiation interactions using cosmological observations

TL;DR

This paper uses recent cosmological data to constrain the properties of dark radiation, specifically the neutrino-like components, revealing deviations from the standard non-interacting neutrino model and exploring potential new physics.

Contribution

First to constrain the dark radiation rest-frame sound speed and viscosity parameters using current CMB and LSS data, providing insights into neutrino properties and beyond-standard-model physics.

Findings

Standard neutrino model ruled out at 99% confidence level

Measured ceff^2 = 0.30 with small uncertainties

Measured cvis^2 = 0.44 with larger uncertainties

Abstract

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.