Tracing Dark Energy History with Gamma-Ray Bursts

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 wCDM model, where w is thought to evolve with redshift, adopting two priors over the Hubble constant in tension at 4.4σ, i.e., H₀ = (67.4 ± 0.5) km s^-1 Mpc^-1 and H₀ = (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 1σ 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 2σ and 4σ 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.