Abstract
The matter content of the Universe is strongly inhomogeneous on small scales. Motivated by this fact, we consider a model of the Universe that has regularly spaced discrete masses, rather than a continuous fluid. The optical properties of such space-times can differ considerably from the continuous fluid case, even if the `average' dynamics are the same. We show that these differences have consequences for cosmological parameter estimation, and that fitting to recent supernovae observations gives a correction to the inferred value of ΩΛ of ∼ 10%.