The impact of cosmological neutrinos on large-scale structure observables

In the last couple of decades, several cosmological experiments probing the Cosmic Microwave Background (CMB) and the large-scale structure of the Universe have confirmed the wonderful agreement between data and the standard cosmological model, the ΛCDM paradigm. According to the latter, our Universe is filled, besides ordinary baryonic matter, with a cosmological constant which makes the expansion accelerateand cold dark matter (CDM) as the main driver for structure formation. A small amount of energy is carried by cosmological neutrinos. While the Standard Model of particle physics predicts them to be massless, the detection of flavor oscillations highlighted how they do indeed have a mass. Unfortunately, these experiments are not able to constrain the mass scale. On the other hand cosmology has the power to do so, thanks to the considerable impact that neutrinos have on the cosmological observables. Neutrinos decouple from the photon-baryon plasma in the very early Universe, when they are still in the relativistic regime. While on large scales they essentially behave like CDM, the high thermal velocities they possess prevent them from clustering, at linear level, on scales smaller than the free-streaming length. This induces a back-reaction on the growth of CDM density perturbations, which becomes scaledependent, and affects the matter power spectrum and all the observables that depend upon it. Future surveys like Euclid, the Large Synoptic Survey Telescope (LSST), the Dark Energy Survey Instrument (DESI) and the Square Kilometer Array (SKA) will likely measure the sum of the three neutrino masses (Mν) for the first time. In order to have a correct estimate of Mν, considerable efforts must be made on the theoretical side to assess which observables are the most suitable for the detection, to accurately quantify the impact of neutrino mass on such observables and to carefully study of the systematics, nuisances and biases that can affect such measurements. The research I have been carrying out during my Ph.D. was developed with this goal in mind. This thesis presents the main and most relevant results of papers published on refereed journals, sorted according to the degree of non-linearity involved in the problem. The first analysis presented extends previous works on the linear point (LP) of the two-point correlation function (2PCF) to the case of massive neutrino cosmologies. So far, the LP has been shown to be an excellent standard ruler for cosmology. By using state-of-art N-body simulations, we show that also in cosmologies with massive neutrinos the LP retains its nature of standard ruler for the CDM and halo real-space 2PCF. To do so, we use a model-independent parametric fit in the range of scales of the Baryon Acoustic Oscillations (BAOs). We also propose a procedure to constrain neutrino masses by comparing the measured LP from data to the LP of a mock galaxy catalog with massless neutrinos and the same remaining cosmological parameters. We find that the sum of the neutrino masses could in principle be detected provided that several redshift bins are used, the survey volume is sufficiently large and the shot noise of the galaxy sample is sufficiently low. In the second work we investigate the possibility that the degeneracies between the effects of neutrino mass and those of baryons on the large-scale matter distribution (e.g. AGN feedback, galactic winds) could bias the measurement of Mν in future surveys probing galaxy clustering and cosmic shear. To this end, we generate synthetic data sets and fit them using the Markov Chain Monte Carlo (MCMC) technique. Baryon feedback is modelled with fitting functions that describe the suppression to the matter power spectrum through free parameters with well-established physical meaning, while neutrinos are modelled through the HALOFIT operator calibrated on N-body simulations. The covariance matrix entering in the likelihood function contains cosmic variance and shot/shape noise as sources of statistical uncertainties, while theoretical inaccuracies are accounted for through a mode-coupling function with a given correlation length. For the weak lensing analysis we also take into account the systematic carried by the intrinsic alignment effect. Overall, for both clustering and shear, we are always able to recover the right input neutrino mass well within 1-σ. In the sheAar survey, we also report some interesting degeneracy between Mν and the parameter controlling the amplitude of the intrinsic alignment effect. Finally, the third work concerns the clustering of relic neutrinos in the Milky Way. Since neutrinos are massive, they feel the gravitational attraction of the Galaxy and should therefore be more abundant at the Earth position than the average cosmological value. This could enhance the event rate of future experiments aiming at a direct detection of the cosmic neutrino background. This work improves past analyses by performing full 3-D calculations and including in the budget close-by structures like the Virgo cluster and the Andromeda galaxy. The neutrino clustering is computed by back-tracking particles in the Milky Way gravitational field using the N-one body technique. Our results overall confirm previous findings, but highlight how the contribution of the Virgo cluster is relevant. The local neutrino density (and in turn the detection rate) is found to be enhanced by 0.53% for a neutrino mass of 10 meV, 12% for 50 meV, 50% for 100 meV or 500% for 300 meV.