Structure formation with strongly interacting neutrinos - implications for the cosmological neutrino mass bound
Steen Hannestad
TL;DR
The paper tests a Beacom-like scenario where neutrinos strongly couple to a light scalar, causing annihilation and forming a tightly coupled neutrino–scalar fluid. While this fluid leaves the matter power spectrum almost unchanged, it dramatically alters the CMB by removing anisotropic stress, leading to tension with observations. Likelihood analyses using WMAP and SDSS data show that such models poorly fit the data, implying that the relativistic energy density at recombination must be in the form of free-streaming, weakly interacting particles, thereby supporting the standard ΛCDM picture and the conventional cosmological neutrino mass bounds.
Abstract
We investigate a model where neutrinos are strongly coupled to a new, light scalar field. In this model neutrinos annihilate as soon as they become non-relativistic in the early universe, and a non-zero neutrino mass has a marginal effect on the matter power spectrum. However, the angular power spectrum of the cosmic microwave background (CMB) is changed significantly because the strongly interacting fluid of neutrinos and scalars does not experience any anisotropic stress. Such models are strongly disfavoured by current observations. Interestingly, this leads to the conclusion that the relativistic energy density around the epoch of recombination must be in the form of very weakly interacting particles. This conclusion is independent of the specific interaction model.