Italy/USA and France

Alessandra Buonanno and Thibault Damour

2021 Balzan Prize for Gravitation: Physical and Astrophysical Aspects

For their leadership in the prediction of the gravitational-wave signals produced when compact objects like neutron stars and black holes spiral together and eventually merge. Their work was instrumental in the detection of gravitational waves, providing an extremely accurate confirmation of general relativity as the theory of gravitation, and allowing the LIGO and Virgo detectors to promote a type of astronomy which uses gravitational waves as new, powerful messengers of the universe.

Gravitational radiation provides a new observational window on the universe, and professors Alessandra Buonanno and Thibault Damour successfully developed new theoretical methods to fully exploit its scientific potential.  

Starting in the late 90s, their research focused on the problem of predicting the gravitational waves from inspiralling and merging black holes. Accurate predictions are essential to extract the tiny gravitational-wave signal from overwhelming noise, and to infer the physical characteristics of the system of merging objects. A very large number of theoretical waveforms (corresponding to different masses, spins and orbital parameters of the system) must be computed and compared to the data, so that accurate and rapid calculations are essential.  

Alessandra Buonanno and Thibault Damour conceived a sophisticated, novel formalism (the effective-one-body, EOB) to analytically solve the two-body problem in general relativity, enabling the fast calculation of the waveforms. In almost 20 years of hard work with their students, postdoctoral scholars, and collaborators, the EOB method was extended and perfected to include higher harmonics, spin precession, and tidal effects: all these extensions are crucial to predict waveform models for asymmetric binaries and binaries comprising neutron stars. Within a large collaboration between researchers working in numerical and analytical relativity and in data analysis, the combination of the EOB analytical method with numerical-relativity simulations was implemented into the LIGO/Virgo data-analysis pipelines and was instrumental for the first detection – and especially the interpretation – of gravitational waves from a binary-black-hole merger and for the analyses of the gravitational-wave signals observed since then. The Buonanno-Damour EOB method also played a leading role in using these data to perform the first tests of general relativity in the ultra-strong gravity regime, beautifully confirming the validity of general relativity as an incredibly accurate theory of gravitation. 

In addition, the synergetic combination of the LIGO and Virgo interferometer data made it possible to pin down the position in the sky of the sources of gravitational waves. This allowed identification of the source of a gravitational wave with associated γ-rays, X-rays, visible and infrared light, and it was possible to ascribe it to the fusion of two neutron stars, understanding its physical implications. This can be considered a pivotal point in the so-called multi-messenger astronomy.  

The theoretical progresses initiated by Alessandra Buonanno and Thibault Damour will also be fundamental in the interpretation of future precision gravitational-wave astronomy data from the space-borne LISA interferometer, and future facilities on the ground, such as the Einstein Telescope and Cosmic Explorer, where the accurate knowledge of the waveforms will be the key to fully exploit the observed data, shaping the future of astronomy.