Why do Cosmological Perturbations Look Classical to Us?

Abstract

According to the inflationary scenario of cosmology, all structure in the\nUniverse can be traced back to primordial fluctuations during an accelerated\n(inflationary) phase of the very early Universe. A conceptual problem arises\ndue to the fact that the primordial fluctuations are quantum, while the\nstandard scenario of structure formation deals with classical fluctuations. In\nthis essay we present a concise summary of the physics describing the\nquantum-to-classical transition. We first discuss the observational\nindistinguishability between classical and quantum correlation functions in the\nclosed system approach (pragmatic view). We then present the open system\napproach with environment-induced decoherence. We finally discuss the question\nof the fluctuations' entropy for which, in principle, the concrete mechanism\nleading to decoherence possesses observational relevance.\n