  The symmetron is a scalar field associated with the dark sector whose
coupling to matter depends on the ambient matter density. The symmetron is
decoupled and screened in regions of high density, thereby satisfying local
constraints from tests of gravity, but couples with gravitational strength in
regions of low density, such as the cosmos. In this paper we derive the
cosmological expansion history in the presence of a symmetron field, tracking
the evolution through the inflationary, radiation- and matter-dominated epochs,
using a combination of analytical approximations and numerical integration. For
a broad range of initial conditions at the onset of inflation, the scalar field
reaches its symmetry-breaking vacuum by the present epoch, as assumed in the
local analysis of spherically-symmetric solutions and tests of gravity. For the
simplest form of the potential, the energy scale is too small for the symmetron
to act as dark energy, hence we must add a cosmological constant to drive
late-time cosmic acceleration. We briefly discuss a class of generalized,
non-renormalizable potentials that can have a greater impact on the late-time
cosmology, though cosmic acceleration requires a delicate tuning of parameters
in this case.
