Cosmological Parameters of a Smoothed Inhomogeneous Spacetime

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.