The recently constructed Hubble diagram using a combined sample of SNLS and
SDSS-II Type Ia SNe, and an application of the Alcock-Paczynski (AP) test using
model-independent Baryon Acoustic Oscillation data, have suggested that the
principal constraint underlying the cosmic expansion is the total
equation-of-state of the cosmic fluid, rather than that of its dark energy.
These studies have focused on the critical redshift range (0 < z < 2) within
which the transition from decelerated to accelerated expansion is thought to
have occurred, and they suggest that the cosmic fluid has zero active mass,
consistent with a constant expansion rate. The evident impact of this
conclusion on cosmological theory calls for an independent confirmation. In
this paper, we carry out this crucial one-on-one comparison between the R_h=ct
Universe (an FRW cosmology with zero active mass) and wCDM/LCDM, using the
latest high-z measurements of H(z). Whereas the Type Ia SNe yield the
integrated luminosity distance, while the AP diagnostic tests the geometry of
the Universe, the Hubble parameter directly samples the expansion rate itself.
We find that the model-independent cosmic chronometer data prefer R_h}=ct over
wCDM/LCDM with a BIC likelihood of ~95% versus only ~5%, in strong support of
the earlier SNeIa and AP results. This contrasts with a recent analysis of H(z)
data based solely on BAO measurements which, however, strongly depend on the
assumed cosmology. We discuss why the latter approach is inappropriate for
model comparisons, and emphasize again the need for truly model-independent
observations to be used in cosmological tests.Comment: 22 pages, 1 figure, 1 table. Accepted for publication in the
Astronomical Journa
