Measuring cosmic dipole with the GRB luminosity-time relation

TL;DR

This study tests the cosmological principle at high redshift by using gamma-ray bursts (GRBs) standardized with the Dainotti L-T relation as probes of large-scale anisotropy. It employs two complementary dipole analyses—the Dipole Fitting Method and the novel Anisotropic Residual Analysis—to detect a dipolar modulation in the GRB distance modulus from 176 Swift long GRBs. The results yield a consistent dipole amplitude of $A_d \approx 0.6$ with direction $\mathrm{RA} \approx 134^{\circ}$, $\mathrm{DEC} \approx -36^{\circ}$, and robust isotropy tests via $10^4$ simulations reinforce that the signal is not due to chance or sampling biases; the dipole model also improves the fit relative to isotropic $\Lambda$CDM and removes residual correlations. While aligned roughly with the CMB dipole, the inferred boost direction is antipodal to the dipole direction, highlighting potential systematics from sky exposure effects and underscoring GRBs as a powerful, high-redshift complement to traditional CP tests; future work should address exposure maps and joint analyses with other high- and low-redshift tracers.

Abstract

We present a new analysis of cosmic dipole anisotropy using gamma-ray bursts (GRBs) as high-redshift standardizable candles. GRBs are ideal probes for testing the cosmological principle thanks to their high luminosity, wide redshift range, and nearly isotropic sky coverage. For the first time, we employ the luminosity-time (L-T) relation, known in the literature as the bidimensional X-ray Dainotti relation, corrected for redshift evolution, to standardize a sample of 176 long GRBs detected by \textit{Swift}. We test for dipolar modulations in the GRB Hubble diagram using both the Dipole Fit Method and a new approach introduced here, the Anisotropic Residual Analysis Method. Both methods yield consistent results: a dipole amplitude of $A_d \simeq 0.6 \pm 0.2$ pointing towards (RA, DEC) $\approx (134^\circ \pm 30^{\circ}, -36^\circ \pm 21^{\circ})$ (equatorial coordinates). As shown in the Appendix, this corresponds to a boost velocity of the observer with respect to the GRB rest-frame in the antipodal direction from the dipole direction. Extensive isotropy tests and 20,000 Monte Carlo simulations confirm that the detected signal cannot be explained by chance alignments or by the angular distribution of the GRB sample. We also show how, by incorporating a dipole term, residual correlations are eliminated, showing that the dipole model provides a better fit than standard isotropic $Λ$CDM.

Figures (7)

• Figure 1: Sky distribution in galactic coordinates (left) and redshift distribution (right) for individual data samples.
• Figure 2: (a) The L-T diagram showing the sample correlation, (b) best-fit standardizing parameters and (c) Hubble diagram applying the best-fit values and redshift evolution. In case of duplicates among samples, only the Platinum value is displayed.
• Figure 3: Bayesian analysis results and comparison with other dipole works.
• Figure 4: Anisotropic residual results
• Figure 5: Number counts per Quadrant (left) and Mollweide projection (right) for the individual GRB subsamples.