Modifying Gravity at Low Redshift

Abstract

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 were outside the range of the scalar field force may feel it at a lower redshift. In this case, both the exponent ${\hat{I}}^3$ measuring the growth of Cold Dark Matter perturbations and the slip function representing the ratio of the two Newtonian potentials $\ddot{I}ˆ$ and $\ddot{I}•$ may differ from their values in General Relativity at low redshift.

27 pages, 5 figures; revised version, to appear in JCAP.