Inflationary cosmology and the scale-invariant spectrum

The claim of inflationary cosmology to explain certain observable facts, which the Friedmann-Roberston-Walker models of 'Big-Bang' cosmology were forced to assume, has already been the subject of significant philosophical analysis. However, the principal empirical claim of inflationary cosmology, that it can predict the scale-invariant power spectrum of density perturbations, as detected in measurements of the cosmic microwave background radiation, has hitherto been taken at face value by philosophers. The purpose of this paper is to expound the theory of density perturbations used by inflationary cosmology, to assess whether inflation really does predict a scale-invariant spectrum, and to identify the assumptions necessary for such a derivation. The first section of the paper explains what a scale-invariant power-spectrum is, and the requirements placed on a cosmological theory of such density perturbations. The second section explains and analyses the concept of the Hubble horizon, and its behaviour within an inflationary space-time. The third section expounds the inflationary derivation of scale-invariance, and scrutinises the assumptions within that derivation. The fourth section analyses the explanatory role of 'horizon-crossing' within the inflationary scenario. (C) 2017 Elsevier Ltd. All rights reserved.