  We derive an expression for the luminosity distance as a function of redshift
for a flat Robertson-Walker spacetime perturbed by arbitrary scalar
perturbations possibly produced by a modified gravity theory with two different
scalar perturbation potentials. Measurements of the luminosity distance as
function of redshift provide a constraint on a combination of the scalar
potentials and so they can complement weak lensing and other measurements in
trying to distinguish among the various alternative theories of gravity.
