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Published online by Cambridge University Press: 19 July 2016
Three ways to estimate Ω from the motions of nearby galaxies are reviewed: (i) comparisons can be made between observed local streaming motions and observed density fluctuations, (ii) a timing argument can give a value of f(Ω) = to Ho, and (iii) the motions in groups provide solid evidence regarding the amount of matter that is distributed like galaxies on a scale of 0.5 Mpc. In each case, the evidence suggests the Universe is open, but only by a factor of 3–5 and, given the uncertainties, a closed Universe definitely is not precluded.