CORDIS - EU research results

Testing galaxy formation with gravitational-wave and X-ray observations of massive black holes

Final Report Summary - GALFORMBHS (Testing galaxy formation with gravitational-wave and X-ray observations of massive black holes)

The goal of GALFORMBHS is to study the evolution of massive black holes in competing scenarios for the formation of cosmic structures and for their seeds at high redshift; to work out the predictions of these scenarios for gravitational-wave detectors and X-ray missions, in order to assess with what accuracy they may be observationally distinguished; and to explore the potential of gravitational-wave detectors to test the existence of Dark Matter and Dark Energy. This project is particularly timely as it will help maximize the impact of European missions such as LISA (a space-based gravitational-wave detector), which was selected in 2013 by the European Space Agency as the flagships of its Cosmic Vision Program. It became even more relevant in 2015-2016, with the first direct detections of gravitational waves by Advanced LIGO, the continuously improving constraints from pulsar timing arrays on the background of gravitational waves from massive black holes, and the spectacular success of the LISA Pathfinder mission. As a result of these developments, the project also became of urgent relevance for the European Space Agency, as it proved crucial to allow them to quantify the scientific output of competing LISA mission designs, a necessary and urgent step to proceed with mission design selection and eventually launch.

Main results:

1) We introduced one of the most detailed studies of the coevolution of massive black holes and their galactic hosts, with particular attention to their mass and spin evolution, in several competing models for the black-hole accretion properties - which were related to the morphological properties of the galactic host - as well as for the black-hole seeds at high redshift. We have compared this model to existing X-ray measurements, and worked out predictions for future gravitational-wave experiments.

2) We have delivered the most sophisticated model to date for determining the accuracy with which LISA will be able to measure the luminosity distance - redshift relation thanks to the coincident detection of gravitational-wave and electromagnetic signals from mergers of massive black holes.

3) We have worked out the most precise semi-analytical predictions to date for the final spin from the merger of two black holes.

4) We have shown that existing observations of nuclear star clusters and massive black holes provide already fossil evidence that binaries of massive black holes merge, which has crucial implications for LISA and its science case.

5) We have performed the most thorough study to date of the interaction of massive black-hole binaries with the gas and stars that surround them, aiming to understand how the presence of realistic astrophysical environments will affect missions such as LISA.

6) We have obtained promising results showing that gravitational-wave observations can help test whether gravity is described by Einstein's General Relativity or by a modified theory (thus potentially shedding light on the nature of Dark Matter and Dark Energy). In particular, we have shown that these tests can be carried out already by existing gravitational-wave detectors, though the accuracy of these tests will improve significantly with LISA.