A TEST OF COSMOLOGICAL MODELS USING HIGH- z MEASUREMENTS OF H ( z )

Abstract

The recently constructed Hubble diagram using a combined sample of SNLS and SDSS-II Type Ia SNe, and an application of the Alcock-Paczy\'nski (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 ($0\lesssim z\lesssim 2$) 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 $R_{\rm h}=ct$ Universe (an FRW cosmology with zero active mass) and $w$CDM/$\Lambda$CDM, 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 $R_{\rm h}=ct$ over $w$CDM/$\Lambda$CDM with a BIC likelihood of $\sim 95\%$ versus only $\sim 5\%$, 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.