Characterizing the equivalence between dark energy and radiation using gamma-ray bursts

¹ Università di Camerino, Divisione di Fisica, Via Madonna delle carceri 9, 62032 Camerino, Italy
² SUNY Polytechnic Institute, 13502 Utica, New York, USA
³ INAF – Osservatorio Astronomico di Brera, Milano, Italy
⁴ INFN, Sezione di Perugia, Perugia 06123, Italy
⁵ Al-Farabi Kazakh National University, Al-Farabi av. 71, 050040 Almaty, Kazakhstan
⁶ ICRANet, Piazza della Repubblica 10, Pescara 65122, Italy

^⋆ Corresponding authors: orlando.luongo@unicam.it; marco.muccino@unicam.it

Received: 17 January 2025
Accepted: 22 April 2025

Abstract

Context. In contrast to the equivalence time between either matter and radiation or dark energy and matter, the equivalence between dark energy and radiation occurs between two subdominant fluids because it takes place in the matter-dominated epoch. However, the equivalence of dark energy to radiation may correspond to a cosmographic bound since it strongly depends on how dark energy evolves.

Aims. A possible model-independent bound on this time would indicate how dark energy evolves in time. In this respect, gamma-ray bursts (GRBs) may be used as tracers to obtain cosmic constraints on this equivalence.

Methods. Consequently, based on observed GRB data from the E_p–E_iso correlation, we proceeded by simulating additional GRB data points and investigating two distinct equivalence epochs: 1) the epoch of dark energy–radiation, and 2) the epoch of dark energy–radiation with matter.

Results. We extracted constraints on the corresponding two redshifts by adopting Monte Carlo Markov chain simulations by means of two methods: The first method calibrated the GRB and the cosmological fit steps independently, and the second method performed these steps simultaneously with a hierarchical Bayesian regression. To keep the analysis model independent, we considered a generic dark energy model, with the unique constraint to reproduce the ΛCDM at z = 0.

Conlcusions. Our findings were compared to theoretical predictions, and the comparison indicated that the ΛCDM model is statistically favored to predict such an equivalence time, but a slow evolution with time cannot be fully excluded. Finally, we critically reexamined the Hubble constant tension in view of our outcomes.