Gravitational waves (GWs) are ripples in the spacetime, generated by extremely energetic astrophysical sources. It took almost 100 years since their prediction by A. Einstein in 1916 to detect them and, since the first detection in 2015, more and more astrophysical sources have been discovered through gravitational-waves. After three observing runs, the LIGO and Virgo collaborations have observed almost 100 compact binaries coalescences, including binary black holes and binary neutron star mergers. GWs observations can be employed to study many of the open questions in current physics such as : Is general relativity valid on cosmological scales? Is there a massive graviton? How fast the Universe is expanding? What is the formation channel of binary black holes?
In this seminar I will show how GW sources can address these questions and also potentially introduce systematic biases in this kind of studies. After reminding how GWs are generated from compact binaries and detected by ground-based detectors, I will highlight how astrophysical information on the source are imprinted in their GW emission. Then, I will show how GWs from compact objects provide a ''standard siren'', a new way to measure cosmic distances, which can be used to measure cosmological parameters, such as the Hubble constant. I will then discuss how GWs can be used to test General relativity, in particular to test the nature of Dark Energy and the presence of a possible massive gravition. I will conclude by showing how populations of GWs sources can be studied to infer the formation channels of compact binaries systems.