fluctuations, but which keeps a well-defined FL averaged behavior. Such conditions are satisfied by the Swiss-cheese model [@EinStr45]: one starts with a spatially homogeneous and isotropic FL geometry, and then cuts out spherical vacuoles in which individual masses are embedded. Thus, the masses are contained in vacua within a spatially homogeneous fluid-filled cosmos (see bottom panel of Fig. \[fig:FL\_VS\_SC\]). By construction, this exact solution is free from any backreaction: its cosmic dynamics is identical to the one of the underlying FL spacetime.

From the kinematical point of view, Swiss-cheese models allow us to go further than perturbation theory, because not only the density of matter exhibits finite fluctuations, but also the metric itself. Hence, light propagation is expected to be very different in a Swiss-cheese universe compared to its underlying FL model. Moreover, the inhomogeneities of a Swiss cheese are introduced in a way that addresses the so-called “Ricci-Weyl problem.” Indeed, the standard FL geometry is characterized by a vanishing Weyl tensor and a nonzero Ricci tensor, while in reality light mostly travels in vacuum, where conversely the Ricci tensor vanishes—apart from the contribution of $\Lambda$, which does not focus light—and the Weyl tensor is nonzero (see Fig. \[fig:FL\_VS\_reality\]). A 