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

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 Ep − Eiso and the L0 − Ep − 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.