Detecting the cosmological neutrino background in the CMB

TL;DR

This paper investigates whether the cosmic microwave background (CMB) can distinguish between free streaming neutrinos and relativistic fluids, finding that free streaming neutrinos are strongly preferred by the data.

Contribution

The study demonstrates that CMB data favor free streaming neutrinos over relativistic fluids, providing evidence for the neutrinos' free streaming nature at decoupling.

Findings

Free streaming particles are preferred over a relativistic perfect fluid with Δχ²≈21.

A relativistic viscous fluid with standard or best-fit parameters cannot fit the CMB data well, with Δχ²≈20.

CMB data can distinguish between free streaming neutrinos and relativistic fluids.

Abstract

Three relativistic particles in addition to the photon are detected in the cosmic microwave background (CMB). In the standard model of cosmology, these are interpreted as the three neutrino species. However, at the time of CMB-decoupling, neutrinos are not only relativistic but they are also freestreaming. Here, we investigate, whether the CMB is sensitive to this defining feature of neutrinos, or whether the CMB-data allow to replace neutrinos with a relativistic fluid. We show that free streaming particles are preferred over a relativistic perfect fluid with $\Delta\chi^2\simeq 21$. We also study the possibility to replace the neutrinos by a viscous fluid and find that a relativistic viscous fluid with either the standard values $c_{\rm eff}^2=c_{\rm vis}^2=1/3$ or best fit values for $c_{\rm eff}^2$ and $c_{\rm vis}^2$ has $\Delta\chi^2=20$ and thus cannot provide a good fit to present CMB data either.