Modifying Gravity at Low Redshift

We consider the growth of cosmological perturbations in modified gravity models where a scalar field mediates a non-universal Yukawa force between different matter species. The growth of the density contrast is altered for scales below the Compton wave-length of the scalar field. As the universe expands, the Compton wave-length varies in time in such a way that scales which where outside the range of the scalar field force may feel it at a lower redshift. In this case, both the exponent $\gamma$ measuring the growth of Cold Dark Matter perturbations and the shift function representing the ratio of the two Newtonian potentials $\psi$ and $\phi$ may differ from their values in General Relativity at low redshift.