Two études in Gravity for particle physicists

In this Thesis, we will work at the interface between particle physics and gravity. At first we will discuss a broad class of possible extensions of General Relativity, namely Metric-Affine Gravity. In general, this kind of theo ries predict the existence of a large number of new degrees of freedom, which in general could be pathological. In fact, for general values of the coupling in the Lagrangian, these new states could be tachyons or ghosts. Our aim will be the construction of prototypical metric-affine theories which are healthy and “simple”, in the sense that they contain only a new propagating particle on top of the graviton. The second issue that we will investigate is the cosmological constant problem. We will see that computing the back-reaction of quantum f ields on the spacetime in a self-consistent way combined with the use of a dimensionless cutoff results in a radically different prediction with respect to the usual one: the spacetime tends to become flat in the limit in which the cutoff goes to infinity.