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Abstract:
This thesis deals with some aspects concerning the fascinating nature of the dark
matter of the Universe. After an overview of the solid body of observational evidence
in favour of the existence of dark matter, both from astrophysical and cosmological
sources, we concentrate our attention on two of the most prominent dark matter
particle candidates: the generic Weakly Interacting Massive Particle (WIMP) and
the sterile neutrino. We study the most common production mechanism of WIMPs
in the early Universe (freeze-out from thermal equilibrium), and derive carefully the
analytical solution to the Boltzmann equation, mentioning every approximation and
assumption along the way.
We then motivate the sterile neutrino as a dark matter candidate by elaborating
on the observation of neutrino oscillations and the problem of generating neutrino
masses in the Standard Model. The most prominent production mechanisms of sterile
neutrino dark matter and their experimental constrains are discussed.
Finally, we show that within the minimal seesaw type I model, the introduction of a
varying neutrino Yukawa coupling opens the possibility of producing sterile neutrinos
by freeze-out from thermal equilibrium in the early Universe. Until now, this has been
ruled out due to the stringent bounds on the active-sterile neutrino mixing angle.
Furthermore, we show that there is a thin region in the parameter space in which
the resulting relic abundance can account for 100% or a fraction of the observed
dark matter density. This possibility is open at all masses investigated, i.e. 1 GeV ~
105 GeV, but potentially even further.
