Spherical Collapse in the Symmetron Model

The Symmetron model is a scalar field model for dark energy in which the scalar field is coupled to matter and is invariant under a reflection symmetry → −. The coupling of the symmetron to matter is such that the symmetry is exact in high density environments and spontaneously broken when the density drops below a critical value. We study the nonlinear evolution of density perturbations using the spherical collapse model. We calculate the extrapolated linear density contrast δc which is a fundamental quantity to obtain the halo-mass function for this model.