Microwave background bispectrum. II. A probe of the low redshift universe - art. no. 103002

Gravitational fluctuations along the line of sight from the surface of last scatter to the observer distort the microwave background in several relate d ways: The fluctuations deflect the photon path (gravitational lensing), t he decay of the gravitational potential produces additional fluctuations (I SW effect) and scattering off of hot gas in clusters produce additional flu ctuations (Sunyaev-Zel'dovich effect). Even if the initial fluctuations gen erated at the surface of last scatter were Gaussian, the combination of the se 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 th e fraction of ionized gas and to probe the time evolution of the gravitatio nal potential. [S0556-2821(99)01210-2].