This thesis collects in a unitary presentation the results of a series of papers
[25, 54, 55, 56] in which we studied the effects of matter on oscillations of cosmic
neutrinos from various sources.
After a first introductory chapter, in chapter 2 we consider the conditions for
significant matter effect; in particular we show that a minimum width of medium
is required, independently of the neutrino oscillation parameters and energy, and of the matter density profile. In chapter 3 this condition is applied to high-energy
neutrinos propagating across the universe: -vve find that substantial effect is produced
by the relic neutrino background provided that it has a large CF-asymmetry and
the neutrino source is at cosmological distance. The effect and its observability
are discussed in detail. The chapters 4-6 are devoted to supernova neutrinos and
their regeneration in the matter of the Earth. A detailed description of the Earth
matter effect is presented in chapter 4, and in chapter 5 we give an application to
the neutrino signal from SN1987 A. vVe find that the Earth effect could reconcile the
difference in the energy spectra of the Kamiokande and IMB data. In chapter 6
we consider future supernova neutrino detections: we show that the Earth matter
effect is observable and has important implications on the reconstruction of the
neutrino mixing scheme and mass spectrum. The discussion is completed by three
appendices.
