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 produces non-Gaussian features in the microwave sky. We discuss the microwave bispectrum as a tool for measuring and 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.

• Received 17 November 1998

DOI:https://doi.org/10.1103/PhysRevD.59.103002