Early Universe phenomena are an open window to the study of particle physics at energy scales that cannot be reached in ground-based experiments. These phenomena, however, are characterized by the presence of large non-linearities, limiting the use of analytical techniques to obtain precise predictions. One example is cosmic strings. These are one-dimensional topological defects that generate after phase transitions in some extensions of the Standard Model, such as in some axion models. Typically, they are studied in the Nambu-Goto limit, that neglects their core width.

In this talk, I will briefly explain how lattice techniques are used to study non-linear early-universe phenomena, with special interest on the evolution and decay of global string loops. I will present some recent results that indicate that the gravitational wave emission from global strings is very suppressed compared to their decay into particle rediation.