Exploring the cosmic microwave background dipole direction using gamma-ray bursts

¹ Università di Camerino, Divisione di Fisica, Via Madonna delle Carceri 9, 62032 Camerino, Italy
² Department of Nanoscale Science and Engineering, University at Albany-SUNY, Albany, New York 12222, USA
³ INAF, Osservatorio Astronomico di Brera, 20121 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
⁷ Département de Physique Théorique and Center for Astroparticle Physics, Université de Genève, 24 quai Ernest Ansermet, 1211 Genève 4, Switzerland

^⋆ Corresponding authors: orlando.luongo@unicam.it; marco.muccino@unicam.it; francesco.sorrenti@unige.ch

Received: 10 August 2025
Accepted: 17 September 2025

Abstract

Context. Cosmic dipole measurements from diverse cosmological probes consistently reveal enhanced dipole amplitudes – and at times even mild directional discrepancies – relative to the cosmic microwave background dipole.

Aims. Using gamma-ray burst (GRB) data, we searched for dipole variations in the Hubble constant H₀, as such anisotropies may also shed light on the Hubble tension.

Methods. We employed the most recent and reliable GRB catalogs from the E_p − E_iso and the L₀ − E_p − T correlations. Despite their large uncertainties, GRBs are particularly suited for this analysis due to three factors: their redshift coverage up to z ∼ 9; their isotropic sky distribution, which minimizes directional bias; and their strong correlations, whose normalizations act as proxies for H₀. To this aim, a whole sky scan – partitioning GRB data into hemispheres – enabled us to define dipole directions by fitting relevant GRB correlation parameters and cosmological parameters. The statistical significance across the full H₀ dipole maps, one per correlation, was then evaluated through the normalization differences between hemispheres and compared against the cosmic microwave background dipole direction. The method was then validated by simulating directional anisotropies via Markov chain Monte Carlo analyses for the two correlations.

Results. Comparison with previous literature confirms the robustness of the method, while no significant dipole evidence is consistently detected with the expected isotropy of GRBs.

Conclusions. To confirm this null result, additional studies are needed to understand how much the degree of anisotropy would influence the net dipole and to infer the fundamental properties behind its possible presence.