Determining the mass of cosmological neutrinos using Lyman-alpha forests

In the work presented in this thesis, I use the power spectrum of the transmitted flux in the Lyman-alpha (Ly-α) forest of distant quasars to constrain the mass of cosmological neutrinos in the context of four separate projects. Neutrinos leave a signature imprint on large scale structures in the Universe through their free-streaming, which manifests as a deficit of matter density fluctuations on typical length scales that are inversely proportional to their rest mass. This typical free-streaming scale, of order a few Mpc, can be probed by Ly-α forests which are imprints of the neutral atomic Hydrogen density along the background quasar's line-of-sight. I use the Ly-α flux power spectrum from mainly two large scale structure surveys: the 13, 821 low-resolution quasar spectra from the ninth data release of SDSS (BOSS) in 12 redshift bins from (Ζ)= 2.2 to 4.4; and the 100 high-resolution quasar spectra from the XQ-100 survey (of the VLT's XShooter spectrograph) in 3 redshift bins, (Ζ)= 3.20, 3.56 and 3.93. This enables us to probe scales from k ≥ 0.001 s/km to k ≤ 0.02 and k ≤ 0.07 s/km respectively. Modeling the flux power spectrum requires solving the non-linear regime of structure formation and the intergalactic gas in the cosmological hydrodynamics simulations that are used to that effect. I controlled for several of many systematic uncertainties related to the simulations. First, I ran simulations with different initial conditions to quantify the sampling variance. I then tested the accuracy of a splicing technique that we use to construct the flux power spectrum from lower size and lower resolution simulations. This required producing a complete run of a (100 h^-1Mpc)³ comoving cube containing 2 * 2048³ dark matter particles and baryons. Moreover, I quantified the impact of the neutrino mass ordering on the flux power spectrum. This enabled our working group to enhance the previously established constraints on the sum of neutrino masses from Σ m_v ≤ 0.15 eV to the most stringent constraint to date Σ m_v ≤ 0.12 eV with 95% confidence. I then worked on implementing right-handed neutrinos in non-cold dark matter cosmological frameworks. A substantial amount of work has gone into applying plausible initial conditions that would accurately model the free-streaming effect of these types of particles. I put the most stringiest constraints (at the time of publication) on the mass of non-resonantly produced sterile neutrinos as pure warm dark matter candidates, m_v ≥ 25 keV at 95% confidence. I extended this investigation into a mixed warm plus cold dark matter cosmology. Finally, I implement right-handed neutrinos produced in presence of a lepton asymmetry which boosts their production and lowers their free-streaming scale. I started a collaboration with a team of theoretical physicists involved in searching for astrophysical evidence for the existence of such resonantly-produced right-handed neutrinos in dark matter rich systems. Our new-born collaboration has enabled the first ever constraints on their mass using the Ly-α forest power spectrum. (author)