  An alternative to dark energy as an explanation for the present phase of
accelerated expansion of the Universe is that the Friedmann equation is
modified, e.g. by extra dimensional gravity, on large scales. We explore a
natural parametrization of a general modified Friedmann equation, and find that
the present supernova type Ia and cosmic microwave background data prefer a
correction of the form 1/H over a cosmological constant. We also explore the
constraints that can be expected in the future, and find that there are good
prospects for distinguishing this model from the standard cosmological constant
to very high significance if one combines supernova data with a precise
measurement of the matter density.
