  The evidence for the accelerated expansion of the universe and the
time-dependence of the fine-structure constant suggests the existence of at
least one scalar field with a mass of order H_0. If such a field exists, then
it is generally assumed that its coupling to matter must be tuned to
unnaturally small values in order to satisfy the tests of the Equivalence
Principle (EP). In this paper, we present an alternative explanation which
allows scalar fields to evolve cosmologically while having couplings to matter
of order unity. In our scenario, the mass of the fields depends on the local
matter density: the interaction range is typically of order 1 mm on Earth
(where the density is high) and of order 10-10^4 AU in the solar system (where
the density is low). All current bounds from tests of General Relativity are
satisfied. Nevertheless, we predict that near-future experiments that will test
gravity in space will measure an effective Newton's constant different by order
unity from that on Earth, as well as EP violations stronger than currently
allowed by laboratory experiments. Such outcomes would constitute a smoking gun
for our scenario.
