Why do Cosmological Perturbations Look Classical to Us?

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