Improved constraints on the dark energy equation of state using Gaussian processes

We perform a comprehensive study of the dark energy equation of state (EoS) utilizing the model-independent Gaussian processes (GP). Using a combination of the Union 2. 1 data set, the 30 newly added H (z) cosmic chronometer data points and Planck's shift parameter, we modify the usual GaPP code and provide a tighter constraint on the dark energy EoS than the previous literature about GP reconstructions. Subsequently, we take the ``controlling variable method '' to investigate directly the effects of the variable matter density parameter ₌₀, variable cosmic curvature ₊₀, and variable Hubble constant H₀ on the dark energy EoS. We find that too small or large ₌₀, ₊₀, and H₀ are all disfavored by our GP reconstructions based on current cosmological observations. Subsequently, we find that variables ₌₀ and ₊₀ affect the reconstructions of the dark energy EoS but hardly affect the reconstructions of the normalized comoving distance D (z) and its derivatives D^' (z) and D^'' (z). However, variable H₀ affects the reconstructions of the dark energy EoS by affecting obviously those of D (z), D^' (z), and D^'' (z). Furthermore, we find that the results of our reconstructions support substantially the recent local measurement of H₀ reported by Riess et al.