The Cosmic Quartet - Cosmological Parameters of a Smoothed Inhomogeneous Spacetime

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We discuss the relation between `bare' cosmological parameters as the true spatial average characteristics that determine the cosmological model, and the parameters interpreted by observers with a `Friedmannian bias', i.e., within a homogeneous space geometry. We may say that the latter are `dressed' by the smoothed-out geometrical inhomogeneities of the surveyed spatial region. We identify two effects that quantify the difference between `bare' and `dressed' parameters: `curvature backreaction' and `volume effect'. An estimate of the latter is given in terms of a simple geometrical example.