Expansion ofthe Universe [duplicate]

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If the universe is expanding how come Andromeda is heading for us? In isolated spatial areas does gravity overcome universal expansion? What is the gravitational force between us and Andromeda?

marked as duplicate by Jan Doggen, Rory Alsop, Steve Linton, FJC, GlorfindelSep 27 '18 at 16:46

The masses of the Milky Way and Andromeda (depending on exactly whay you consider "part of" each) are estimated as about 850 Billion solar masses and 1.7 Trillion solar masses respectively. Their mean distance is about 2.54 million light years, so it's easy to calculate the force using $$F = GMM'/R^2$$ We get a force of $$6.6\times 10^{29} N$$.