Title

Intermediate-mass black holes: past, present, future

Abstract

Despite their key importance from stellar to cosmological scales, intermediate-mass black holes (IMBHs) are one of the unsolved puzzles of modern astronomy with no conclusive evidence for their existence. While the classical approach to detect them based on the use of optical and infrared data is limited to nearby systems, gravitational wave (GW) missions have the potential to shed light on IMBHs up to the distant Universe. IMBH sources are most likely to be produced in dense stellar environments, where IMBHs can form GW-emitting binaries through dynamical interactions with other compact objects. The intermediate mass-ratio inspiral of a stellar compact remnant into an IMBH is a potential target for multi-band detection, since LISA measurements will alert astronomers of an incoming merger detectable within the next few years by LIGO/Virgo/Kagra, Einstein Telescope, and Cosmic Explorer. I will discuss the formation and evolution of IMBHs, which characterize the typical GW signal expected for current and upcoming missions, offering for the first time the opportunity to demonstrate the existence of IMBHs beyond any reasonable doubt. The next decade may bring hundreds of events, promising a spectacular range of new science from stellar evolution to cosmology. The future of the darkest black holes appears bright.