Space with distance of 1megaparsec is expanding with rate of 70km/s, and if space with a distance of 4296 megaparsec is expanding at rate of 300,720km/s, then why Andromeda is coming to us? Even there has a dark energy that accelerate the space expansion? Why?


marked as duplicate by Mike G, Chappo Says Reinstate Monica, PM 2Ring, Glorfindel, peterh says reinstate Monica Jun 15 at 13:02

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    $\begingroup$ For the same reason why your head is not expanding away from your feet - they are bound together, and that bond overcomes the expansion of space. $\endgroup$ – Florin Andrei Jun 15 at 5:05

The Andromeda galaxy is about 2.5 Mly away, less than 1 MPc, so if the Hubble flow were relevant, it would have Andromeda moving at about 55 km/s away from us. Andromeda is actually approaching at about 300 km/s, or almost 6X that calculated due to the expansion of space. However, Andromeda and the Milky Way are graivitationally bound along with the rest of the galaxies in our local cluster. The individual (so called, peculiar) velocities of these local galaxies are the result of orbital motion within groups and around the local cluster barycenter.

Expansion dominates on larger spatial scales, such as distances between clusters and superclusters of galaxies.


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