# Apparent cosmic acceleration from type Ia supernovae

## TL;DR

This paper compares the timescape cosmology model to the standard Lambda-CDM model using supernova data, finding similar fits and suggesting the importance of inhomogeneous models in understanding cosmic acceleration.

## Contribution

It demonstrates that the timescape model, an inhomogeneous cosmology without dark energy, fits supernova data as well as the standard model, highlighting the role of spatial curvature evolution.

## Key findings

- Timescape model fits supernova data as well as Lambda-CDM.
- Cosmic acceleration in the timescape model is marginal and non-zero.
- Systematic effects influence supernova light curve parameters independently of cosmology.

## Abstract

Parameters that quantify the acceleration of cosmic expansion are conventionally determined within the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) model, which fixes spatial curvature to be homogeneous. Generic averages of Einstein's equations in inhomogeneous cosmology lead to models with non-rigidly evolving average spatial curvature, and different parametrizations of apparent cosmic acceleration. The timescape cosmology is a viable example of such a model without dark energy. Using the largest available supernova data set, the JLA catalogue, we find that the timescape model fits the luminosity distance-redshift data with a likelihood that is statistically indistinguishable from the standard spatially flat $\Lambda$ cold dark matter cosmology by Bayesian comparison. In the timescape case cosmic acceleration is non-zero but has a marginal amplitude, with best-fitting apparent deceleration parameter, $q_0=-0.043^{+0.004}_{-0.000}$. Systematic issues regarding standardization of supernova light curves are analysed. Cuts of data at the statistical homogeneity scale affect light curve parameter fits independent of cosmology. A cosmological model dependence of empirical changes to the mean colour parameter is also found. Irrespective of which model ultimately fits better, we argue that as a competitive model with a non-FLRW expansion history, the timescape model may prove a useful diagnostic tool for disentangling selection effects and astrophysical systematics from the underlying expansion history.