Cosmological test with the QSO Hubble diagram

Abstract

A Hubble diagram (HD) has recently been constructed in the redshift range [Formula: see text] using a nonlinear relation between the ultraviolet (UV) and X-ray luminosities of quasi stellar objects (QSOs). The Type Ia Supernovae (SN) HD has already provided a high-precision test of cosmological models, but the fact that the QSO distribution extends well beyond the supernova range ([Formula: see text]), in principle provides us with an important complementary diagnostic whose significantly greater leverage in [Formula: see text] can impose tighter constraints on the distance versus redshift relationship. In this paper, we therefore perform an independent test of nine different cosmological models, among which six are expanding, while three are static. Many of these are disfavored by other kinds of observations (including the aforementioned Type Ia SNe). We wish to examine whether the QSO HD confirms or rejects these earlier conclusions. We find that four of these models (Einstein–de Sitter, the Milne universe, the static universe with simple tired light and the static universe with plasma tired light) are excluded at the [Formula: see text] C.L. The quasi-steady state model is excluded at [Formula: see text] C.L. The remaining four models ([Formula: see text]CDM/[Formula: see text]CDM, the [Formula: see text] universe, the Friedmann open universe and a static universe with a linear Hubble law) all pass the test. However, only [Formula: see text]CDM/[Formula: see text]CDM and [Formula: see text] also pass the Alcock–Paczyński (AP) test. The optimized parameters in [Formula: see text]CDM/[Formula: see text]CDM are [Formula: see text] and [Formula: see text] (the dark energy equation-of-state). Combined with the AP test, these values become [Formula: see text] and [Formula: see text]. But whereas this optimization of parameters in [Formula: see text]CDM/[Formula: see text]CDM creates some tension with their concordance values, the [Formula: see text] universe has the advantage of fitting the QSO and AP data without any free parameters.