Large Scale Structure as a Probe of Gravitational Slip

Many modified gravity schemes predict a non-zero difference (``gravitational slip'') between the Newtonian and longitudinal perturbed metric potentials. Such a slip would affect the growth of large scale structure without altering the expansion history of the universe. We quantify the slip with a new parameter $\varpi$, show the effect of non-zero $\varpi$ on the growth of cosmic overdensities, and constrain its value using CMB and weak lensing data.