Generation and evolution of cosmological second-order gravitational waves and density perturbations

At first-order, scalar, vector and tensor perturbations do not mix. This can happen from second-order on: scalar modes can be sourced by lower-order tensor modes and vice versa. In this work, second-order cosmological perturbations are calculated in an Einstein-de Sitter Universe of perfect irrotational fluid, in two gauges. The focus is mainly on second-order scalar perturbations generated by linear tensor modes (gravitational waves), whose contribution as a source has usually been neglected in the literature. In particular, the evolution in time of density perturbations generated by tensors is studied, to verify that the appearance of a new growing mode is possible.