Constraints on modified gravity from the Atacama Cosmology Telescope and the South Pole Telescope

The Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT) have recently provided new and precise measurements of the cosmic microwave background anisotropy damping tail. This region of the cosmic microwave background angular spectra, thanks to the angular distortions produced by gravitational lensing, can probe the growth of matter perturbations and provide a new test for general relativity. Here we make use of the ACT and SPT power spectrum measurements (combined with the recent WMAP9 data) to constrain $f(R)$ gravity theories. Adopting a parametrized approach, we obtain an upper limit on the length scale of the theory of B₀<0.86 at 95% C.L. from ACT, while we get a much stronger limit from SPT with B₀<0.14 at 95% C.L.