The Action B Massive is framed in the context of theoretical and observational astrophysics, the branch of science that has the goal of understanding the evolution of the Universe and of the astrophysical objects (from galaxies to planets) within it. Among those, massive black hole binaries are perhaps the most extreme, fascinating yet elusive ones. In fact, as the mutual gravitational pull causes galaxies to merge with each other frequently along cosmic history, the massive black holes hosted at their center are expected to sink into the nucleus of the merger remnant and form a binary. Still, the evidence of the existence of these binaries is sparse and their role in galaxy evolution is poorly understood. Establishing observationally their existence will be a milestone for contemporary astronomy, providing a fundamental missing piece in the puzzle of galaxy formation, piercing through the (hydro)dynamical physical processes shaping dense galactic nuclei from parsec scales down to the event horizon, and probing gravity in extreme conditions.
We can both see and listen to massive black hole binaries. Remarkably, besides arguably being among the brightest objects shining in the Cosmos, they are also the loudest gravitational wave sources in the Universe. As such, we shall take advantage of both the type of messengers – photons and gravitons – they are sending to us, which can now be probed by all-sky time-domain surveys and radio pulsar timing arrays respectively. The main goal of B Massive is to combine theoretical astrophysics, radio an gravitational-wave astronomy and time-domain surveys, with state of the art data analysis techniques to: i) observationally prove the existence of massive black hole binaries, ii) understand and constrain their astrophysics and dynamics, iii) enable and bring closer in time the direct detection of gravitational waves with pulsar timing arrays.
By seeking its goals, the Action made a decisive contribution to one of the most important scientific breakthrough of the decade: the first detection of a low frequency gravitational wave signal with pulsar timing arrays. This was achieved within the European PTA (EPTA) collaboration, in which the B Massive project is framed. This marks the opening of a new observational window on the universe, widening the gravitational wave spectrum that has been already probed in the kiloHz range by the ground based interferometers LIGO and Virgo. Although the origin of this signal cannot yet be firmly established observationally, its properties match our theoretical expectation. In fact, this is exactly the type of signal we would expect to see if a cosmic population of massive black hole binaries were to emit low frequency gravitational waves. The interpretation framework developed within the Action allowed us to infer from the signal the main characteristics of this cosmic populations, confirming that massive black hole binaries do indeed exist, and efficiently emit gravitational waves.
The impact of this discovery has profound implications both for the scientific community and for the society at large. In fact, although fairly abstract theoretical objects, the impact of massive black hole binaries on our society cannot be underestimated. Black holes are among the most iconic physical objects, teasing the imagination and inspiring people of all ages and socio-economic backgrounds. The discovery of this signal made headlines all around the world contributing to the engagement of the public with science.Moreover massive black hole binaries are the main driver of future space borne observatories like the Laser Interferometer Space Antenna (LISA), an ESA-led endeavor to observe gravitational waves in space which will have a huge economic and technological impact for the European Union. The detection of this signal give us more confidence that the sources that LISA is targeting are out there awaiting discovery, to unveil the deepest secrets of the universe.
