Gravitational Astronomy

This chapter is about opening the gravitational window to observe the Universe. Although the weakest of all known forces, gravity plays a dominant role in forming stars and galaxies, shaping the large-scale structure, and driving the expansion of the Universe. Gravity has so far played a passive role in our understanding. We only witness its influence indirectly by observing its effect on star light (Doppler effect, cosmological redshift, gravitational lensing, etc.). However, we are at a momentous period that could soon transform our picture of the Universe by opening the gravitational window for observational astronomy. Gravitational waves have already been critical for understanding how neutron star binaries evolve [1][2]. However, we have not directly observed the waves themselves. This will change before the end of this decade when several different methods of observing gravitational waves will reach sensitivity levels at which we should finally begin to unravel some of the deepest questions in astronomy, cosmology, and fundamental physics. The chapter by van den Broeck will deal with the two latter topics. In this chapter, we will discuss what gravitational waves are (Sect. 26.2), how they interact with matter (Sect. 26.3), on-going and future projects aimed at detecting cosmic gravitational waves (Sect. 26.4), expected and speculative astronomical sources, and a list of open problems on which gravitational astronomy could shed some light (Sect. 26.5).