In Einstein’s theory of General Relativity, gravity is a side-effect of the curvature of spacetime. In the words of John Wheeler: “matter tells spacetime how to curve, and spacetime tells matter how to move”. An important prediction of Einstein’s theory are gravitational waves: tiny ripples in spacetime caused by violent events in the Universe, for example the collisions of two neutron stars or black holes. Since 2015, large laser interferometers such as LIGO in the US and Virgo in Italy have been detecting gravitational waves on a regular basis; a similar instrument in Japan called KAGRA is also getting ready to make observations, and in a few years’ time a gravitational wave detector will be built in India as well.
For the first time we have access to spacetime curvatures that are millions of times stronger than in the Solar System
The ability to directly detect gravitational waves has opened up a new observational field at the intersection of fundamental physics, astrophysics, and cosmology. For the first time we have access to spacetime curvatures that are millions of times stronger than in the Solar System, caused by macroscopic objects like black holes that are smashing into each other at a significant fraction of the speed of light. This has already enabled more penetrating tests of General Relativity than any that were done before, and is allowing us to probe some of the most fundamental properties of black holes by direct observation. Gravitational waves also allow us to “look” inside neutron stars, with implications for our understanding of the strong nuclear force that keeps atomic nuclei together. The latter can be tied to heavy ion collision experiments at the LHC, which look at the strong interaction in a complementary way; combining the two efforts has the potential to greatly increase our insight into the strong force and its wealth of implications. At much larger length scales, gravitational wave sources can be used as cosmic distance markers to map out the large-scale evolution of the Universe. Further into the future we also hope to find primordial gravitational waves that originated a split-second after the Big Bang.
The future of this field promises to be very exciting. We have not yet begun to access the full, rich discovery space of possible GW signals
Our researchers are at the forefront of this emerging field, developing methods to search for new sources of gravitational waves, as well as techniques to extract astrophysical information from signals once they are detected. We are members of the Virgo Collaboration, which together with LIGO and KAGRA jointly analyzes all the data coming out of the global detector network. Additionally, we are involved in the effort towards future detectors, such as the space-based LISA, which will consist of three spacecraft in orbit around the Sun at mutual distances of about a million kilometers, exchanging laser beams to form an interferometer. Finally, we are active members of a Europe-wide collaboration to prepare for Einstein Telescope (ET), a large underground gravitational wave observatory which will detect signals from throughout the visible Universe. Though it has not yet been decided where ET will be built, the two possible locations are the island of Sardinia, and the border region of the Netherlands, Belgium and Germany.