Abstract
The Hubble diagram constructed using HII galaxies (HIIGx) and Giant extragalactic HII regions (GEHR) as standard candles already extends beyond the current reach of Type Ia SNe. A sample of 156 HIIGx and GEHR sources has been used previously to compare the predictions of ΛCDM and Rh = ct, the results of which suggested that the HIIGx and GEHR sources strongly favour the latter over the former. But this analysis was based on the application of parametric fits to the data and the use of information criteria, which disfavour the less parsimonious models. In this paper, we advance the use of HII sources as standard candles by utilizing Gaussian processes (GP) to reconstruct the distance modulus representing these data without the need to pre-assume any particular model, none of which may in the end actually be the correct cosmology. In addition, this approach tightly constrains the 1σ confidence region of the reconstructed function, thus providing a better tool with which to differentiate between competing cosmologies. With this approach, we show that the Planck concordance model is in tension with the HII data at more than 2.5σ, while Rh = ct agrees with the GP reconstruction very well, particularly at redshifts ≳ 10−3.