What they do in the shadows: accessing high-redshift black hole mergers with next-generation detectors

Even at design sensitivity, advanced ground-based gravitational-wave detectors will only be able to detect binary black holes up to redshifts of 2. While this enables studying local populations of binary black holes, existing detectors are not likely to detect black holes that might have formed from the first generation of stars, or even primordial black holes, as those are expected to merge at redshift of several. By contrast next-generation detectors such as Cosmic Explorer and Einstein Telescope will access stellar-mass black holes to redshifts of 50 and higher. In this talk I will review the prospects for detecting and characterizing both populations and individual binary black holes at very high redshifts with next-generation detectors.