  We show that time dependent gravitational potential can be directly detected
from the cosmic microwave background (CMB) anisotropies. The signature can be
measured by cross-correlating the CMB with the projected density field
reconstructed from the weak lensing distortions of the CMB itself. The
cross-correlation gives a signal whenever there is a time dependent
gravitational potential. This method traces dark matter directly and has a well
defined redshift distribution of the window projecting over the density
perturbations, thereby avoiding the problems plaguing other proposed
cross-correlations. We show that both MAP and Planck will be able to probe this
effect for observationally relevant curvature and cosmological constant models,
which will provide additional constraints on the cosmological parameters.
