# Coordinate independent approach to the calculation of the effects of local structure on the luminosity distance

## TL;DR

This paper develops a coordinate-independent method to relate local inhomogeneities directly to luminosity distance measurements, improving accuracy over previous linear and curvature-based formulas, especially in the non-linear regime.

## Contribution

It introduces a coordinate-independent low-redshift expansion of luminosity distance in terms of density contrast, enhancing the accuracy of inhomogeneity effects on cosmological observations.

## Key findings

- The new formulas agree well with numerical calculations.
- They outperform linear perturbation theory in the non-linear regime.
- They are more accurate than curvature-based formulas.

## Abstract

Local structure can have important effects on luminosity distance observations, which could for example affect the local estimation of the Hubble constant based on low red-shift type Ia supernovae. Using a spherically symmetric exact solution of the Eistein's equations and a more accurate expansion of the solution of the geodesic equations, we improve the low red-shift expansion of the monopole of the luminosity distance in terms of the curvature function. Based on this we derive the coordinate independent low red-shift expansion of the monopole of the luminosity distance in terms of the monopole of the density contrast. The advantage of this approach is that it relates the luminosity distance directly to density observations, without any dependency on the radial coordinate choice. We compute the effects of different inhomogeneities on the luminosity distance, and find that the formulae in terms of the density contrast are in good agreement with numerical calculations, in the non linear regime are more accurate than the results obtained using linear perturbation theory, and are also more accurate than the formulae in terms of the curvature function.