Cosmology of Chameleons with Power-Law Couplings
  In chameleon field theories a scalar field can couple to matter with
gravitational strength and still evade local gravity constraints due to a
combination of self-interactions and the couplings to matter. Originally, these
theories were proposed with a constant coupling to matter, however, the
chameleon mechanism also extends to the case where the coupling becomes
field-dependent. We study the cosmology of chameleon models with power-law
couplings and power-law potentials. It is found that these generalized
chameleons, when viable, have a background expansion very close to LCDM, but
can in some special cases enhance the growth of the linear perturbations at low
redshifts. For the models we consider it is found that this region of the
parameter space is ruled out by local gravity constraints. Imposing a coupling
to dark matter only, the local constraints are avoided, and it is possible to
have observable signatures on the linear matter perturbations.
