  Quasars at high redshift provide direct information on the mass growth of
supermassive black holes and, in turn, yield important clues about how the
Universe evolved since the first (Pop III) stars started forming. Yet even
basic questions regarding the seeds of these objects and their growth mechanism
remain unanswered. The anticipated launch of eROSITA and ATHENA is expected to
facilitate observations of high-redshift quasars needed to resolve these
issues. In this paper, we compare accretion-based supermassive black hole
growth in the concordance LCDM model with that in the alternative
Friedmann-Robertson Walker cosmology known as the R_h=ct universe. Previous
work has shown that the timeline predicted by the latter can account for the
origin and growth of the > 10^9 M_sol highest redshift quasars better than that
of the standard model. Here, we significantly advance this comparison by
determining the soft X-ray flux that would be observed for Eddington-limited
accretion growth as a function of redshift in both cosmologies. Our results
indicate that a clear difference emerges between the two in terms of the number
of detectable quasars at redshift z > 6, raising the expectation that the next
decade will provide the observational data needed to discriminate between these
two 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 in
LCDM---a quantitatively compelling difference.
