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The cosmic X-ray background (XRB) spectrum and active galaxy number countsencode essential information about the spectral evolution of active galacticnuclei (AGNs) and have been successfully modeled by XRB synthesis models formany years. Recent measurements of the $8$--$24$ keV AGN number counts byNuSTAR and Swift-BAT are unable to be simultaneously described by existing XRBsynthesis models, indicating a fundamental breakdown in our understanding ofAGN evolution. Here we show that the $8$--$24$ keV AGN number counts can besuccessfully modeled with an XRB synthesis model in which the reflectionstrength ($R$) in the spectral model increases with redshift. We show that anincrease in reflection strength with redshift is a natural outcome of (1)connecting $R$ to the incidence of high column density gas around AGNs, and (2)the growth in the AGN obscured fraction to higher redshifts. In addition to theredshift evolution of $R$, we also find tentative evidence that an increasingCompton-thick fraction with $z$ is necessary to describe the $8$--$24$ keV AGNnumber counts determined by NuSTAR. These results show that, in contrast to thetraditional orientation-based AGN unification model, the nature and coveringfactor of the absorbing gas and dust around AGNs evolve over time and must beconnected to physical processes in the AGN environment. Future XRB synthesismodels must allow for the redshift evolution of AGN spectral parameters. Thisapproach may reveal fundamental connections between AGNs and their hostgalaxies.

The evolving X-ray spectrum of active galactic nuclei: evidence for an increasing reflection fraction with redshift