In 2015, the two LIGO interferometers in the US obtained the first direct detection of gravitational waves:  GW150914, interpreted as a binary black hole merger (Abbott et al. 2016). After the first event, the two LIGO interferometers and Virgo, in Italy, which started taking data in 2017, observed several other binary black holes and one binary neutron star (e.g. Abbott et al. 2019). These are exciting times for gravitational wave astronomy!

  The observations of LIGO – Virgo pose new fundamental questions about black holes. Two black holes can merge within a Hubble time by gravitational wave emission only if their initial orbital separation is tremendously small (a few to a few ten solar radii); what are the astrophysical processes that can bring two black holes so close to each other?

  The DEMOBLACK project (full title: Demography of black hole binaries in the era of gravitational-wave astronomy) aims to shed light on the formation channels of binary black holes, by means of innovative numerical models. We focus on two possible scenarios: the formation of binary black holes from the evolution of massive binary stars and the dynamical pairing of black holes in dense star clusters. Our new population-synthesis codes, MOBSE (Mapelli et al. 2017; Giacobbo et al. 2018) and SEVN (Spera et al. 2015, 2017, 2019; Mapelli et al. 2019), are the key tool for our study. SEVN and MOBSE can be run as stand-alone codes to investigate the evolution of massive binary stars and can be interfaced with direct N-body codes to unravel the complex dynamics of black holes in star clusters (Mapelli 2016; Di Carlo et al. 2019).

  Binary black holes form and merge all the time through cosmic history, from the epoch of the first stars until the present day. In about ten years from now, the third generation ground-based gravitational wave detectors (Einstein Telescope and Cosmic Explorer) will observe black hole mergers up to redshift 10 or even more. To be ready for third-generation detectors, we make predictions about the cosmic evolution of binary black holes, by taking advantage of semi-analytic models (Giacobbo & Mapelli 2019; Bouffanais et al. 2019) and cosmological simulations (Mapelli & Giacobbo 2018; Mapelli et al. 2019; Artale et al. 2019), interfaced with our population-synthesis and star-cluster simulations.

  From our numerical models, we obtain detailed information on the mass, the redshift, the merger rate density and other fundamental properties of binary black holes: these results will provide us with a key to interpret current and future gravitational-wave data.