If the universe is expanding, how are the Milky Way and Andromeda getting closer together?

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The Universe is certainly expanding in the aftermath of the Big Bang 13.82 billion years ago, but you yourself are not expanding. Neither is the Earth, nor our Solar System, nor our Milky Way, nor the local group of galaxies of which the Milky Way and Andromeda are the two most prominent members.

This is because only on the largest scales is cosmic expansion strong enough to overcome the gravity that’s binding together astronomical objects. On the smaller scales – masses equivalent to stars and galaxies and even galaxy clusters – gravity is strong enough to hold things together and defy the expansion of the Universe.

The Milky Way and Andromeda are not only bound to each other but approaching each other. In about five billion years’ time, it is estimated that they will collide and form a new galaxy, already dubbed ‘Milkomeda’.

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Asked by: Hadden Diamond Holgate, aged 7, via email

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