Symmetron Fields: Screening Long-Range Forces Through Local Symmetry
  Restoration
  We present a screening mechanism that allows a scalar field to mediate a long
range (~Mpc) force of gravitational strength in the cosmos while satisfying
local tests of gravity. The mechanism hinges on local symmetry restoration in
the presence of matter. In regions of sufficiently high matter density, the
field is drawn towards \phi = 0 where its coupling to matter vanishes and the
\phi-> -\phi symmetry is restored. In regions of low density, however, the
symmetry is spontaneously broken, and the field couples to matter with
gravitational strength. We predict deviations from general relativity in the
solar system that are within reach of next-generation experiments, as well as
astrophysically observable violations of the equivalence principle. The model
can be distinguished experimentally from Brans-Dicke gravity, chameleon
theories and brane-world modifications of gravity.
