Evidence for a positive cosmological constant from flows of galaxies and distant supernovae

Recent observations(1,2) of high-redshift supernovae seem to suggest that t he global geometry of the Universe may be affected by a 'cosmological const ant: which acts to accelerate the expansion rate with time. But these data by themselves still permit an open universe of low mass density and no cosm ological constant. Here we derive an independent constraint on the lower bo und to the mass density, based on deviations of galaxy velocities from a sm ooth universal expansion(3-7). This constraint rules out a low-density open universe with a vanishing cosmological constant, and together the two favo ur a nearly flat universe in which the contributions from mass density and the cosmological constant are comparable. This type of universe, however, s eems to require a degree of fine tuning of the initial conditions that is i n apparent conflict with 'common wisdom'.