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

Orlando Luongo, Marco Muccino, Francesco Sorrenti

TL;DR

The paper investigates whether a sky-dependent variation in the Hubble constant, i.e., an $H_0$ dipole, exists by applying hemisphere-based MCMC fits to GRB-derived proxies for $H_0$ using the Amati ($E_p$-$E_{iso}$) and Combo ($L_0$-$E_p$-$T$) correlations. Their method scans the sky in grid directions, splits the GRB samples into hemispheres, and measures directional changes in the correlation intercept as a proxy for $H_0$, validating the approach with mock catalogs containing injected dipoles. The results show no significant dipole detected in either GRB data set, with maximal hemispheric signals well below discovery thresholds and directions inconsistent with the CMB dipole, thereby supporting isotropy. The study highlights limitations due to small GRB catalogs and large uncertainties and calls for larger, more comprehensive data sets and cross-checks with other probes to robustly test potential intrinsic anisotropies in the expansion rate.

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.

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

TL;DR

The paper investigates whether a sky-dependent variation in the Hubble constant, i.e., an dipole, exists by applying hemisphere-based MCMC fits to GRB-derived proxies for using the Amati (-) and Combo (--) correlations. Their method scans the sky in grid directions, splits the GRB samples into hemispheres, and measures directional changes in the correlation intercept as a proxy for , validating the approach with mock catalogs containing injected dipoles. The results show no significant dipole detected in either GRB data set, with maximal hemispheric signals well below discovery thresholds and directions inconsistent with the CMB dipole, thereby supporting isotropy. The study highlights limitations due to small GRB catalogs and large uncertainties and calls for larger, more comprehensive data sets and cross-checks with other probes to robustly test potential intrinsic anisotropies in the expansion rate.

Abstract

We search for dipole variations in the Hubble constant 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 and the correlations. Despite their large uncertainties, GRBs are particularly suited for this analysis due to their redshift coverage up to , their isotropic sky distribution that minimizes directional bias, and their strong correlations whose normalizations act as proxies for . 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 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.

Paper Structure

This paper contains 8 sections, 9 equations, 2 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Dipole determinations across cosmic data sets
  3. Search for a $H_0$ dipole in GRB data
  4. The $E_{p}-E_{iso}$ correlation data set
  5. The $L_0$--$E_{\rm p}$--$T$ correlation data set
  6. Dipole determination methodology
  7. Mock catalogs with artificial $H_0$ dipoles
  8. Physical discussions and conclusions

Figures (2)

  • Figure 1: Mollweide RA--DEC projection maps of the significance $\sigma$ (see right side color-coded bars) computed from Eq. \ref{['sigma']} for the A118 (top) and the C182 (bottom) catalogs. The maximum $\Delta a$ direction $(\alpha_{\rm m},\delta_{\rm m})=(144^\circ,-54^\circ)$ for the A118 catalog (top panel, black cross), the maximum direction $(\alpha_{\rm m},\delta_{\rm m})=(270^\circ,-6^\circ)$ for the C182 catalog (bottom panel, black cross), and the CMB dipole direction $(\alpha_\star,\delta_\star) = (168^{\circ}, -7^{\circ})$ (black dots) are also shown.
  • Figure 2: Mollweide RA--DEC projection maps of the significance $\sigma$ (see right side color-coded bars) computed from Eq. \ref{['sigma']} for the mock A118 (top) and the C182 (bottom) catalogs. The artificial dipole direction $(\alpha_0,\delta_0) = (240^{\circ}, 30^{\circ})$ (black crosses) and the CMB dipole direction $(\alpha_\star,\delta_\star) = (168^{\circ}, -7^{\circ})$ (black dot) are also shown.