On the accelerated expansion of the cosmos

TL;DR

This paper reviews evidence for the Universe's accelerated expansion, primarily using type Ia supernovae, and discusses the robustness of this evidence against various assumptions and potential inhomogeneities.

Contribution

It provides a model-independent analysis of supernova data supporting cosmic acceleration and highlights the importance of data calibration and local structure effects.

Findings

Cosmic acceleration detected at 7 sigma with the Union set and SALT fitter.

Inconsistent results across different light-curve fitters for the SDSS SN data.

Local supernovae may affect the Hubble diagram due to inhomogeneities.

Abstract

We present a short (and necessarily incomplete) review of the evidence for the accelerated expansion of the Universe. The most direct probe of acceleration relies on the detailed study of supernovae (SN) of type Ia. Assuming that these are standardizable candles and that they fairly sample a homogeneous and isotropic Universe, the evidence for acceleration can be tested in a model- and calibration-independent way. Various light-curve fitting procedures have been proposed and tested. While several fitters give consistent results for the so-called Constitution set, they lead to inconsistent results for the recently released SDSS SN. Adopting the SALT fitter and relying on the Union set, cosmic acceleration is detected by a purely kinematic test at 7 sigma when spatial flatness is assumed and at 4 sigma without assumption on the spatial geometry. A weak point of the described method is the local set of SN (at z < 0.2), as these SN are essential to anchor the Hubble diagram. These SN are drawn from a volume much smaller than the Hubble volume and could be affected by local structure. Without the assumption of homogeneity, there is no evidence for acceleration, as the effects of acceleration are degenerate with the effects of inhomogeneities. Unless we sit in the centre of the Universe, such inhomogeneities can be constrained by SN observations by means of tests of the isotropy of the Hubble flow.