Detecting the cosmological neutrino background

Thursday, 18 December, 2014

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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 particles 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 gas. We show that free streaming particles are highly preferred over a relativistic perfect gas with $\Delta\chi^2\simeq 250$. We also study the possibility to replace the neutrinos by a viscous gas and find that a relativistic viscous gas with the standard values $c_{\rm eff}^2=c_{\rm vis}^2=1/3$ cannot provide a good fit to present CMB data. It has $\Delta \chi^2=52$ with respect to free streaming neutrinos. Even if we allow for arbitrary $c_{\rm eff}^2$ and $c_{\rm vis}^2$, the best fit still has $\Delta\chi^2=22$, which shows that also this possibility is highly disfavoured.


Ruth Durrer
Elena Sellentin