Cosmology and Neutrinos
Julien Lesgourgues (right) continued the discussion of cosmology, focusing on neutrinos. He started by pointing out that one can divide observables derived from first principles, and ones derived from modelling of complex phenomena. To be more explicit, examples of the first kind is CMB data, which uses GR and QED, and integration of linearized Einstein and Boltzmann equations. An example of the second kind is Supernovae, Cepheids, small-scale structures, which depend on non-linear simulations, phenomenological fits and scaling laws. Large scale structure measurements have a foot on each side in that picture.
He discussed what neutrino effects we are testing in cosmology. One is the sum of active neutrino masses, another is the abundance of light sterile neutrinos mass and abundance; another is the non-standard interaction of active neutrinos. Some of these observables are constrained through some complex modeling. For instance in putting bounds on the sum of neutrino masses, one only needs CMB and few modeling assumptions. If one wants to put bounds on the keV-order mass of sterile neutrinos, you specifically rely on data from galaxy clusters, Lyman-alpha, data on which you have to put great care as it is dependent on complex modeling issues.
The bound on neutrino masses depend on the number of new ingredients in cosmological model beyond the set of six minimal parameters. As time goes on, we have better and better data, and the bound keeps going down, but also become more model independent. The bound is nowadays at 118 meV by combining CMB, LSS, galaxies, Lyman-alpha data, BAOs. With that you can discuss how much the normal hierarchy is preferred. We are getting close to the point of ruling out the inverted hierarchy. Arxiv:1308.4164 discusses the future bounds we will be able to put on the sum of masses.
The Euclid satellite should be launched in the next two to three years. Some new CMB experiments, both space-based and ground-based projects, are also planned. If we add the sensitivity projected for CORE, we see that the bound should go from 40 to 10 meV, and even more model independent. In conclusion, it is not a dream to achieve a 5-sigma detection of neutrino masses, possible even if the mass is in the 60 meV range. The forecast we make are including non-minimal cosmological assumptions. The cosmology bounds are more sensitive than many beta and double-beta decay experiments.