Impact of Massive Neutrinos and Dark Radiation on the High-redshift
  Cosmic Web. I. Lyman-Alpha Forest Observables
  With upcoming high-quality data from surveys such as the Extended Baryon
Oscillation Spectroscopic Survey (eBOSS) or the Dark Energy Spectroscopic
Instrument (DESI), improving the theoretical modeling and gaining a deeper
understanding of the effects of neutrinos and dark radiation on structure
formation at small scales are necessary, to obtain robust constraints free from
systematic biases. Using a novel suite of hydrodynamical simulations that
incorporate dark matter, baryons, massive neutrinos, and dark radiation, we
present a detailed study of their impact on Lyman-Alpha forest observables. In
particular, we accurately measure the tomographic evolution of the shape and
amplitude of the small-scale matter and flux power spectra and search for
unique signatures along with preferred scales where a neutrino mass detection
may be feasible. We then investigate the thermal state of the intergalactic
medium (IGM) through the temperature-density relation. Our findings suggest
that at k~5h/Mpc the suppression on the matter power spectrum induced by
M_nu=0.1 eV neutrinos can reach ~4% at z~3 when compared to a massless neutrino
cosmology, and ~10% if a massless sterile neutrino is included; surprisingly,
we also find good agreement (~2%) with some analytic predictions. For the 1D
flux power spectrum, the highest response to free-streaming effects is achieved
at k~0.005 s/km when M_nu=0.1 eV; this k-limit falls in the Lyman-Alpha forest
regime, making the small-scale 1D flux power spectrum an excellent probe for
detecting neutrino and dark radiation imprints. Our results indicate that the
IGM at z~3 provides the best sensitivity to active and sterile neutrinos.
