Abstract: Early Universe phenomena are an open window to study particle physics at energy scales well above those that can be probed by ground-based experiments. In certain situations, these phenomena are expected to develop non-perturbative and non-linear dynamics, which precludes the use of analytical techniques to obtain accurate predictions. A notorious example are cosmic strings. These are one-dimensional topological defects that are predicted to form after phase transitions in some extensions of the Standard Model, ranging from axion models to grand-unified theories. After forming, cosmic strings are expected to decay via the emission of particles and gravitational waves (GWs), leading to the formation of a GW background that may be detected by ongoing or future GW experiments. Traditionally, predictions of the produced GWB are based on expectations from the Nambu-Goto limit, in which the internal structure of the strings and their decay into particles is neglected. However, a correct interpretation of a positive GW signal requires of accurate predictions that go beyond the Nambu-Goto approximation.

In this talk, I will briefly explain how lattice techniques can be used used to study non-linear early-universe phenomena, with special interest on the evolution and decay of string loops of global and local type. I will discuss recent studies in which we have computed the simultaneous emission of particles and GWs from cosmic string loops using lattice simulations, finding that, for both global and local strings, the emission of GWs is very suppressed compared to particle production.