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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, Glorfindel Sep 27 '18 at 16:46

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What is the gravitational force between us and Andromeda?

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 $$6.6\times 10^{29} N$$.

  • $\begingroup$ And another way to frame that number is to ask what is the timescale for that force to significantly alter the location of Andromeda, as that timescale can be compared with the age of the universe. The timescale is the square root of the quantity of distance over acceleration, which comes out to ten billion years. Only if that were much larger than the age of the universe could we neglect the impact of that attraction on the overall dynamics. $\endgroup$ – Ken G Sep 27 '18 at 11:42

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