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

TL;DR

This study investigates potential dipole anisotropies in the Hubble constant using gamma-ray burst data, finding no significant evidence for directional variation, thus supporting the universe's isotropy.

Contribution

It introduces a novel method to analyze Hubble constant anisotropies with GRB correlations across the entire sky, validated through simulations and comparison with previous studies.

Findings

No significant dipole anisotropy detected in H_0 using GRB data.

Method validated with simulated anisotropies, confirming robustness.

Results support the isotropy of the universe at large scales.

Abstract

We search for dipole variations in the Hubble constant $H_0$ using gamma-ray burst (GRB) data, as such anisotropies may shed light on the Hubble tension. We employ the most recent and reliable GRB catalogs from the $E_{p}-E_{iso}$ and the $L_0-E_{p}-T$ correlations. Despite their large uncertainties, GRBs are particularly suited for this analysis due to their redshift coverage up to $z\sim9$, their isotropic sky distribution that minimizes directional bias, and their strong correlations whose normalizations act as proxies for $H_0$. To this aim, a whole sky scan - partitioning GRB data into hemispheres - enabled to define dipole directions by fitting the relevant GRB correlation and cosmological parameters. The statistical significance across the full $H_0$ dipole maps, one per correlation, is then evaluated through the normalization differences between hemispheres and compared against the CMB dipole direction. The method is then validated by simulating directional anisotropies via Markov Chain Monte Carlo analyses for both correlations. Comparison with previous literature confirms the robustness of the method, while no significant dipole evidence is detected, consistently with the expected isotropy of GRBs. This null result is discussed in light of future analyses involving larger datasets.