Cosmological constraints on the seesaw scale

We will analyse the simplest extension of the Standard Model that can account for neutrino masses: the Type-I seesaw with 2 and 3 right-handed neutrinos. The model introduces a New Physics scale, $M$, which is usually assumed to be much larger than the electroweak scale. However, it is experimentally unconstrained and the light neutrino masses and mixing can be generated for any value of M above $O(eV)$. Paying particular attention to the contribution of the sterile neutrinos to $N_{eff}$as a function of $M$, we will show that a large part of the $M$ parameter space (8 orders of magnitude) can be excluded due to cosmological measurements. The phenomenological implications for neutrinoless double beta decay will also be discussed.