B Massive · Binary massive black hole astrophysics

Massive black hole binaries (MBHBs) are the most extreme, fascinating yet elusive astrophysical objects in the Universe. 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 MBHBs. Remarkably, besides arguably being among the brightest variable objects shining in the Cosmos, MBHBs are also the loudest gravitational wave (GW) 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 (PTAs) respectively. B MASSIVE leverages on a unique comprehensive approach combining theoretical astrophysics, radio and gravitational-wave astronomy and time-domain surveys, with state of the art data analysis techniques to: i) observationally prove the existence of MBHBs, ii) understand and constrain their astrophysics and dynamics, iii) enable and bring closer in time the direct detection of GWs with PTA. As European PTA (EPTA) executive committee member and former IInternational PTA (IPTA) chair, I am a driving force in the development of pulsar timing science world-wide, and the project will build on the profound knowledge, broad vision and wide collaboration network that established me as a world leader in the field of MBHB and GW astrophysics. B MASSIVE is extremely timely; a pulsar timing data set of unprecedented quality is being assembled by EPTA/IPTA, and Time-Domain astronomy surveys are at their dawn. In the long term, B MASSIVE will be a fundamental milestone establishing European leadership in the cutting-edge field of MBHB astrophysics in the era of LSST, SKA and LISA.