Abstract: The Early Universe is characterized by very high energies, many orders of magnitude above those achievable by current ground-based experiments, and so its study can allow to infer new physics beyond the Standard Model of particles. One of the primary avenues for exploring the early universe is the detection of a cosmological background of gravitational waves, which carry information of high energy phenomena occurring during the first seconds of the universe. However, analyzing such a potential detection would require deep understanding of the possible phenomena generating them. Fortunately, the large energies of the early universe make it possible to study it using classical-field-theory simulations. Unluckily, those high energies also mean the early universe is out of equilibrium, and so non-linear phenomena occurr, precluding analytical predictions.
In this talk I will explain how processes occurring in the early universe can be studied using classical-field-theory lattice simulations. I will show different examples of how these techniques can be used to study early-universe phenomena, such as particle-production at the end of inflation, and to predict the associated production of gravitational waves. I will later focus on cosmic strings, one-dimensional topological defects predicted to form in the early universe by many dark-matter models. I will present results related to their decay into particles and gravitational waves, and explain how they compare to approximate analytical predictions.