Geodesic-light-cone coordinates and the Bianchi I spacetime

TL;DR

This paper introduces geodesic-light-cone coordinates, explores their properties, and applies them to Bianchi I cosmology to reveal effects of anisotropy on gravitational lensing and cosmic observations.

Contribution

It provides a detailed introduction to GLC coordinates and demonstrates their application to anisotropic cosmology, highlighting violations of the Weinberg conjecture.

Findings

Gravitational lensing affects proper area in anisotropic universes.

GLC coordinates reveal anisotropy impacts on the Hubble diagram.

Violations of the Weinberg conjecture in Bianchi I models.

Abstract

The geodesic-light-cone (GLC) coordinates are a useful tool to analyse light propagation and observations in cosmological models. In this article, we propose a detailed, pedagogical, and rigorous introduction to this coordinate system, explore its gauge degrees of freedom, and emphasize its interest when geometric optics is at stake. We then apply the GLC formalism to the homogeneous and anisotropic Bianchi I cosmology. More than a simple illustration, this application (i) allows us to show that the Weinberg conjecture according to which gravitational lensing does not affect the proper area of constant-redshift surfaces is significantly violated in a globally anisotropic universe; and (ii) offers a glimpse into new ways to constrain cosmic isotropy from the Hubble diagram.