Probing the Dark Universe with Gravitational Waves

Gravitational wave (GW) astronomy has come to revolutionize our understanding of astrophysics, cosmology and fundamental physics. GWs from binary black-hole (BH) mergers allow us to learn about the population of BHs, their origin and their role through the history of the universe. If these BHs are produced abundant enough at early times, they could comprise a large fraction of the Dark Matter (DM). As a case of study, I will present how a copious production of primordial BHs can arise in Critical Higgs Inflation and discuss its GWs signatures.
Moreover, GWs detected with an associated counterpart can probe the expansion of the universe and Dark Energy (DE). The recent measurement of the GW speed following GW170817 is an excellent example of the immense potential available to GWs tests of gravity. I will present the conditions for anomalous GW speed and classify the theories for DE that remain viable after GW170817. Finally, I will discuss how other propagation effects could also shed light on the quest for the nature of DE.