Light-cone averaging in cosmology: formalism and applications.
Marozzi, Giovanni (College de France, Paris)

I will show a general gauge invariant formalism for defining cosmological averages that are relevant for observations based on light-like signals. Such averages involve either null hypersurfaces corresponding to a family of past light-cones or compact surfaces given by their intersection with timelike hypersurfaces. Generalized Buchert-Ehlers commutation rules for derivatives of these light-cone averages will be also shown. To conclude, after introducing some adapted geodesic light-cone coordinates, I will give explicit expressions for averaging the redshift to luminosity-distance relation and the so-called redshift drift in a generic inhomogeneous Universe.