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The cosmic X-ray background (CXB), which peaks at an energy of ~30 keV, isproduced primarily by emission from accreting supermassive black holes (SMBHs).The CXB therefore serves as a constraint on the integrated SMBH growth in theUniverse and the accretion physics and obscuration in active galactic nuclei(AGNs). This paper gives an overview of recent progress in understanding thehigh-energy (>~10 keV) X-ray emission from AGNs and the synthesis of the CXB,with an emphasis on results from NASA's NuSTAR hard X-ray mission. We thendiscuss remaining challenges and open questions regarding the nature of AGNobscuration and AGN physics. Finally, we highlight the exciting opportunitiesfor a next-generation, high-resolution hard X-ray mission to achieve thelong-standing goal of resolving and characterizing the vast majority of theaccreting SMBHs that produce the CXB.

Resolving the cosmic X-ray background with a next-generation high-energy X-ray observatory