  It has been suggested that the recent acceleration of the expansion of the
Universe is due a modified gravitational action consisting of the
Einstein-Hilbert term plus a term proportional to the reciprocal of the Ricci
scalar. Although the original version of this theory has been shown to be in
conflict with solar system observations, a modified Palatini version of the
theory, in which the metric and connection are treated as independent variables
in the variational principle, has been suggested as a viable model of the
cosmic acceleration. We show that this theory is equivalent to a type of
scalar-tensor theory in which the scalar field kinetic energy term in absent
from the action, and in which the scalar field is therefore not an independent
dynamical degree of freedom. Integrating out the scalar field gives rise to
additional interactions among the matter fields of the standard model of
particle physics at a energy scale of order 10^(-3) eV (the geometric mean of
the Hubble scale and the Planck scale), and so the theory is excluded by, for
example, electron-electron scattering experiments.
