Supernova Constraints on Modified Theories of Gravity

Most attempts to unify gravitation with the standard model of particle physics involve new fields and/or additional (usually compact) dimensions. The dynamics of these compact extra dimensions can, however, act back on the dynamics of macroscopic space and time. We investigate a particular class of models with $n$ compact dimensions plus a scalar field with negative kinetic energy (``phantom''), and show that they are strongly disfavored by recent data on the magnitudes of Type Ia supernovae.