  Gravitational fluctuations along the line-of-sight from the surface of last
scatter to the observer distort the microwave background in several related
ways: The fluctuations deflect the photon path (gravitational lensing), the
decay of the gravitational potential produces additional fluctuations (ISW
effect) and scattering off of hot gas in clusters produce additional
fluctuations (Sunyaev-Zel'dovich effect). Even if the initial fluctuations
generated at the surface of last scatter were Gaussian, the combination of
these effects produce non-Gaussian features in the microwave sky. We discuss
the microwave bispectrum as a tool for measuring a studying this signal. For
MAP, we estimate that these measurements will enable us to determine the
fraction of ionized gas and to probe the time evolution of the gravitational
potential.
