  Light scalar fields are expected to arise in theories of high energy physics
(such as string theory), and find phenomenological motivations in dark energy,
dark matter, or neutrino physics. However, the coupling of light scalar fields
to ordinary (or dark) matter is strongly constrained from laboratory, solar
system, and astrophysical tests of fifth force. One way to evade these
constraints in dense environments is through the chameleon mechanism, where the
field's mass steeply increases with ambient density. Consequently, the
chameleonic force is only sourced by a thin shell near the surface of dense
objects, which significantly reduces its magnitude.
  In this paper, we argue that thin-shell conditions are equivalent to
"conducting" boundary conditions in electrostatics. As an application, we use
the analogue of the method of images to calculate the back-reaction (or
self-force) of an object around a spherical gravitational source. Using this
method, we can explicitly compute the violation of equivalence principle in the
outskirts of galactic haloes (assuming an NFW dark matter profile):
Intermediate mass satellites can be slower than their larger/smaller
counterparts by as much as 10% close to a thin shell.

Cosmology and Nongalactic Astrophysics;General Relativity and Quantum Cosmology