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We study the dark energy equation of state as a function of redshift in a non-parametric way, without imposing any a priori w (z) (ratio of pressure over energy density) functional form. As a check of the method, we test our scheme through the use of synthetic data sets produced from different input cosmological models which have the same relative errors and redshift distribution as the real data. Using the luminosity-time Lₗ-T₀ correlation for GRB X-ray afterglows (the Dainotti et al. correlation), we are able to utilize GRB sample from the Swift satellite as probes of the expansion history of the Universe out to z 10. Within the assumption of a flat FLRW universe and combining SNeIa data with BAO constraints, the resulting maximum likelihood solutions are close to a constant w=-1. If one imposes the restriction of a constant w, we obtain w=-0. 99 0. 06 (consistent with a cosmological constant) with the present day Hubble constant as H₀=70. 0 0. 6 km \, s^-1 Mpc^-1 and density parameter as ₀=0. 723 0. 025, while non-parametric w (z) solutions give us a probability map which is centred at H₀=70. 04 1 km \, s^-1 Mpc^-1 and ₀=0. 724 0. 03. Our chosen GRB data sample with full correlation matrix allows us to estimate the amount, as well as quality (errors) of data, needed to constrain w (z) in the redshift range extending an order of magnitude in beyond the farthest SNeIa measured.
Postnikov et al. (Mon,) studied this question.
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