Institute for Advanced Study/Princeton University Early Universe/Cosmology Lunch Discussion

Abstract 1: Gravitational waves (GWs) are novel signals that probe the behavior of gravity in both the strong and weak-field regimes. As these waves propagate towards us over cosmological distance, they are sensitive to the presence of inhomogeneities in the Universe. 
Similarly to light, GWs can be lensed by intervening galaxies or clusters of galaxies, and lead to multiple magnified and delayed images from the same source. One important difference with light though is that the detection of GWs is sensitive to the phase of the wave, which can change from emission to detection due to strong lensing. In this talk I will discuss what this change of phase is, how it can affect future observations, and how it can be used as a unique effect to help identify GW lensed events. 

Abstract 2: Inflationary perturbations are approximately Gaussian and deviations from Gaussianity are usually calculated using in-in perturbation theory. This method, however, fails for unlikely events on the tail of the probability distribution: in this regime non-Gaussianities are important and perturbation theory breaks down. I show that this regime is amenable to a semiclassical treatment. In this limit the wavefunction of the Universe can be calculated in saddle-point, corresponding to a resummation of all the tree-level Witten diagrams. The saddle can be found by solving numerically the classical (Euclidean) non-linear equations of motion, with prescribed boundary conditions. I apply these ideas to a model with a quartic inflaton self-interaction. Numerical and analytical methods show that the tail of the probability distribution of the curvature perturbation has a clear non-perturbative dependence on the coupling. These results are relevant for the calculation of the abundance of primordial black holes.