LUSSO, ELISABETAELISABETALUSSOVALIANTE, RosaRosaVALIANTEVITO, FabioFabioVITO2025-05-082025-05-082023978-981-16-4544-0http://hdl.handle.net/20.500.12386/37104In the last decades, luminous accreting super massive black holes have been discovered within the first Gyr after the Big Bang, but their origin is still an unsolved mystery. We discuss our state-of-the-art theoretical knowledge of their formation physics and early growth and describe the results of dedicated observational campaigns in the X-ray band. We also provide an overview of how these systems can be used to derive cosmological parameters. Finally, we point out some open issues, in light of future electromagnetic and gravitational-wave astronomical facilities.STAMPAenBook part10.1007/978-981-16-4544-0_116-1https://link.springer.com/referenceworkentry/10.1007/978-981-16-4544-0_116-12023hxga.book..122LFIS/05 - ASTRONOMIA E ASTROFISICA