Intermediate to massive black holes with masses in the interval between 10 000 and 10 000 000 solar masses will be detected by LISA, to explore for the first time the low-mass end of the massive Black hole population, at cosmic times as early as z ~ 10.
The Gravitational Universe will make it possible to survey the vast majority of all coalescing massive black hole binaries throughout the whole universe. This will expose an unseen population of objects which will potentially carry precious information about the black hole population as a whole. It will provide both the widest and deepest survey of the sky ever, since gravitational wave detectors are non-directional in nature, and operate as non-pointed and weakly directional full-sky monitors. The range of black hole redshifts and masses that will be explored is complementary to the space explored by electromagnetic observations.
LISA will detect all binary black hole mergers even when the black holes are not active. With this unbiased and complete survey, it will be possible to investigate the link between the growing seed population with the rich population of active supermassive black holes evolving during cosmic dawn and high noon, probing the light end of the mass function at the largest redshifts.
Black hole coalescence events will illuminate the physical processes of black hole formation and feeding. While the mass distribution carries information about the seeds, the spin distribution charts the properties of the accretion flows, whether they are chaotic or coherent. Gravitational wave observations alone will be able to distinguish between the different massive Black hole formation and evolution scenarios.