Testing gravity with non-Gaussianity
  We show that modified gravity presents distinctive nonlinear features on the
Cosmic Microwave Background (CMB) anisotropies comparing with General
Relativity (GR). We calculate the contribution to the CMB non-Gaussianity from
nonlinear Sachs-Wolfe effect in $f(R)$ gravity and show that, contrary to GR's
contribution which is typically $\lesssim \mathcal{O}(1)$, the contribution in
$f(R)$ gravity is sensitive to the nonlinear structure of $f(R)$ and can be
large in principle. Optimistically, this gives an alternative origin for the
possibly observed large CMB non-Gaussianities besides the primordial ones. On
the other hand, such nonlinear features can be employed to provide a new
cosmological test of $f(R)$ or other modified theories of gravitation, which is
unique and independent of previously known tests.
