  We tested a fifth force using cold atom experiments. The accelerated
expansion of the universe implies the possibility of the presence of a scalar
field throughout the universe driving the acceleration. This field would result
in a detectable force between normal-matter objects. Theory of the chameleon
field states that the force should be strong in a thin shell near the surface
of a source object but greatly suppressed inside and outside of the source
object. We used two atom clouds: one as the source and the other as the test
mass; so the test mass can pass through the thin-shell region of the source
mass. We detected the chameleon force and obtained the couple constant of about
4.5E11 between matter and the field. The chameleon force is considerably larger
than Newtonian gravity at short distance; the interaction range is short enough
to satisfy all experimental bounds on deviations from general relativity.
