  We show that, as a result of non-linear self-interactions, it is feasible, at
least in light of the bounds coming from terrestrial tests of gravity,
measurements of the Casimir force and those constraints imposed by the physics
of compact objects, big-bang nucleosynthesis and measurements of the cosmic
microwave background, for there to exist, in our Universe, one or more scalar
fields that couple to matter much more strongly than gravity does. These scalar
fields behave like chameleons: changing their properties to fit their
surroundings. As a result these scalar fields can be not only very strongly
coupled to matter, but also remain relatively light over solar system scales.
These fields could also be detected by a number of future experiments provided
they are properly designed to do so. These results open up an altogether new
window, which might lead to a completely different view of the role played by
light scalar fields in particle physics and cosmology.
