# Testing the Cosmological Principle in the radio sky

## TL;DR

This paper tests the Cosmological Principle's assumption of isotropy using NVSS radio galaxy counts, finding the data consistent with isotropy after analysis and simulations.

## Contribution

It introduces a local variance estimator method to test isotropy in radio sky data, confirming the Cosmological Principle with NVSS observations.

## Key findings

- NVSS data is consistent with statistical isotropy.
- The local variance estimator effectively tests isotropy.
- Simulations support the observational results.

## Abstract

The Cosmological Principle states that the Universe is statistically isotropic and homogeneous on large scales. In particular, this implies statistical isotropy in the galaxy distribution, after removal of a dipole anisotropy due to the observer's motion. We test this hypothesis with number count maps from the NVSS radio catalogue. We use a local variance estimator based on patches of different angular radii across the sky and compare the source count variance between and within these patches. In order to assess the statistical significance of our results, we simulate radio maps with the NVSS specifications and mask. We conclude that the NVSS data is consistent with statistical isotropy.