Effects of Non-Standard Neutrinos on Gravitational Waves

It is well known that the primordial stochastic background of gravitational waves (GWs) can be damped due to the free-streaming of neutrinos during the propagation of GWs through cosmic structures. However, for some frequencies of the GWs an opposite process can take place due to free-streaming neutrinos, with an enhancement of the amplitude of GWs. This thesis aims first of all at reviewing such a phenomenon (that has direct implications for the GWs that can be measured through Cosmic Microwave Background anisotropies). It will then focus on scenarios of non-standard neutrinos (typically considered both in particle physics and cosmology as a possible solution to some experimental data anomalies). In particular, the aim is to understand whether the enhancement or further damping takes place and, if so, if it can place at the frequencies of Pulsar Timing Arrays (PTA) collaborations, given the importance of their first measurement of a stochastic gravitational wave background in 2023. This analysis can allow us to understand whether such measurements can put constraints on non-standard neutrino physics. We incorporate realistic thermodynamic evolution, including Standard Model particle de-relativization and phase transitions. Using numerical solutions of the tensor perturbation equation, we compute the primordial gravitational waves (PGWs) transfer function. The effects are evaluated across frequencies relevant to PTA experiments.