# Testing Isotropic Universe Using the Gamma-Ray Burst Data of Fermi / GBM

## TL;DR

This study investigates the isotropy of the universe by analyzing properties of Gamma-Ray Bursts from Fermi/GBM data, finding a potential anisotropic feature near specific galactic coordinates, though it may be due to statistical fluctuations or observational biases.

## Contribution

It introduces a novel method to test universe isotropy through GRB property analysis, beyond traditional sky distribution studies, applied to a large Fermi/GBM dataset.

## Key findings

- Detected a potential anisotropic feature near galactic coordinates (30°, 15°).
- Probability of this feature occurring in an isotropic universe is less than 1%.
- Results are consistent with isotropy in some tests, indicating possible statistical fluctuation.

## Abstract

The sky distribution of Gamma-Ray Bursts (GRBs) has been intensively studied by various groups for more than two decades. Most of these studies test the isotropy of GRBs based on their sky number density distribution. In this work we propose an approach to test the isotropy of the Universe through inspecting the isotropy of the properties of GRBs such as their duration, fluences and peak fluxes at various energy bands and different time scales. We apply this method on the {\em Fermi} / Gamma-ray Burst Monitor (GBM) data sample containing 1591 GRBs. The most noticeable feature we found is near the Galactic coordinates $l\approx 30^\circ$, $b\approx 15^\circ$ and radius $r\approx 20^\circ-40^\circ$. The inferred probability for the occurrence of such an anisotropic signal (in a random isotropic sample) is derived to be less than a percent in some of the tests while the other tests give results consistent with isotropy. These are based on the comparison of the results from the real data with the randomly shuffled data samples. Considering large number of statistics we used in this work (which some of them are correlated to each other) we can anticipate that the detected feature could be result of statistical fluctuations. Moreover, we noticed a considerably low number of GRBs in this particular patch which might be due to some instrumentation or observational effects that can consequently affect our statistics through some systematics. Further investigation is highly desirable in order clarify about this result, e.g. utilizing a larger future {\em Fermi} / GBM data sample as well as data samples of other GRB missions and also looking for possible systematics.