The discovery of gravitational waves from merging black holes and / or neutron stars has opened a new window in exploring the universe. They constitute a unique tool for the study of black holes, allowing astrophysical production models to be tested, and their description in terms of General Relativity, paving the way for a new quantum theory of gravity. Signals from neutron star mergers, accompanied by measurements from optical telescopes, provide an accurate new way to determine the Hubble constant for the expansion of the universe. Likewise, the study of the propagation of gravitational waves in space-time puts the equation of state of dark energy and the current cosmological model to the test, in addition to opening new ways to understand the nature of dark matter. The penetrating power of gravitational waves will allow us to explore the first moments of the universe, when it was opaque to light, and to test inflationary models, cosmological phase transitions related to the unification of forces, or string theory predictions. The field of gravitational waves, which has seen the first experimental results with LIGO-Virgo, at the rate of one or two events per week, will witness a dramatic increase in the rate of observations in the next two decades, culminating in the future telescope Einstein, where new events are expected every minute, exploring virtually the entire universe and bringing the field into precision physics.