Two merging black holes
Artist's impression of two black holes that are spiralling towards each other and will eventually coalesce. Black hole mergers are extremely energetic events that release gravitational waves – fluctuations in the fabric of spacetime.
Since 2015, the ground-based LIGO and Virgo observatories have been detecting high-frequency gravitational waves from pairs of colliding compact objects such as black holes and neutron stars. ESA's future mission, LISA, will detect gravitational waves from orbit, looking for the low-frequency fluctuations that are released when two supermassive black holes merge and can only be detected from space.