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Quasars at high redshift provide direct information on the mass growth ofsupermassive black holes and, in turn, yield important clues about how theUniverse evolved since the first (Pop III) stars started forming. Yet evenbasic questions regarding the seeds of these objects and their growth mechanismremain unanswered. The anticipated launch of eROSITA and ATHENA is expected tofacilitate observations of high-redshift quasars needed to resolve theseissues. In this paper, we compare accretion-based supermassive black holegrowth in the concordance LCDM model with that in the alternativeFriedmann-Robertson Walker cosmology known as the R_h=ct universe. Previouswork has shown that the timeline predicted by the latter can account for theorigin and growth of the > 10^9 M_sol highest redshift quasars better than thatof the standard model. Here, we significantly advance this comparison bydetermining the soft X-ray flux that would be observed for Eddington-limitedaccretion growth as a function of redshift in both cosmologies. Our resultsindicate that a clear difference emerges between the two in terms of the numberof detectable quasars at redshift z > 6, raising the expectation that the nextdecade will provide the observational data needed to discriminate between thesetwo models based on the number of detected high-redshift quasar progenitors.For example, while the upcoming ATHENA mission is expected to detect ~0.16(i.e., essentially zero) quasars at z ~ 7 in R_h=ct, it should detect ~160 inLCDM---a quantitatively compelling difference.

Unseen Progenitors of Luminous High-z Quasars in the R_h=ct Universe