  In the standard cosmological framework, the Hubble diagram is interpreted by
assuming that the light emitted by standard candles propagates in a spatially
homogeneous and isotropic spacetime. However, the light from "point
sources"--such as supernovae--probes the Universe on scales where the
homogeneity principle is no longer valid. Inhomogeneities are expected to
induce a bias and a dispersion of the Hubble diagram. This is investigated by
considering a Swiss-cheese cosmological model, which (1) is an exact solution
of the Einstein field equations, (2) is strongly inhomogeneous on small scales,
but (3) has the same expansion history as a strictly homogeneous and isotropic
universe. By simulating Hubble diagrams in such models, we quantify the
influence of inhomogeneities on the measurement of the cosmological parameters.
Though significant in general, the effects reduce drastically for a universe
dominated by the cosmological constant.
