Abstract
Abstract Observations of gamma-ray bursts up to z ∼ 9 are best suited to study the possible evolution of the universe equation of state at intermediate redshifts. We apply the Combo relation to a sample of 174 gamma-ray bursts to investigate possible evidence of evolving dark energy parameter w ( z ). We first build a gamma-ray burst Hubble’s diagram and then we estimate the set (Ω m , Ω Λ ) in the framework of flat and non-flat ΛCDM paradigm. We then get bounds over the w CDM model, where w is thought to evolve with redshift, adopting two priors over the Hubble constant in tension at 4.4 σ , i.e., H 0 = (67.4 ± 0.5) km s −1 Mpc −1 and H 0 = (74.03 ± 1.42) km s −1 Mpc −1 . We show our new sample provides tighter constraints on Ω m since at z ≤ 1.2 we see that w ( z ) agrees within 1 σ with the standard value w = −1. The situation is the opposite at larger z , where gamma-ray bursts better fix w ( z ) that seems to deviate from w = −1 at 2 σ and 4 σ level, depending on the redshift bins. In particular, we investigate the w ( z ) evolution through a piecewise formulation over seven redshift intervals. From our fitting procedure we show that at z ≥ 1.2 the case w < −1 cannot be fully excluded, indicating that dark energy’s influence is not negligible at larger z . We confirm the Combo relation as a powerful tool to investigate cosmological evolution of dark energy. Future space missions will significantly enrich the gamma-ray burst database even at smaller redshifts, improving de facto the results discussed in this paper.