Testing non-standard cosmologies with gravitational waves

In theories beyond standard model, the presence of additional degrees of freedom, such as scalars, is ubiquitous. These fields may give rise to a non-standard period of cosmological evolution between the end of inflation and the onset of primordial BBN. While this epoch is poorly constrained by current cosmological experiments, gravitational waves constitute a powerful new tool to test this epoch, and thus these theories.

In this talk, I will discuss the modified cosmological evolution due to the presence of a class of scalar-tensor theories, motivated by string theory. The modified evolution can give rise to changes in the thermal dark matter abundance,  the net number of e-folds of inflation, PBH production. Moreover, it also provides a natural set-up for early dark energy models. I will then discuss how such an epoch can be tested by future GW experiments.