If the universe is expanding, why are we on a collision course with the Andromeda galaxy?

Gemma Lavender heads straight into this one to find us the answer.


The expansion of the Universe, as measurements carried out by astronomer Edwin Hubble in the 1920s show, mean that galaxies are rushing away from us at a rate, recently measured by today’s cosmologists, to be 74 kilometres per second per megaparsec (where one megaparsec equals around 3.26 million light years).

While it is easy to envision all galaxies moving away from each other, the evidence of smash-ups between these gigantic structures litter the Universe. This means that galaxies are both moving away and crashing into one another – this happens much more often than you think.

So often, in fact, that our galactic neighbour, Andromeda is moving towards the Milky Way Galaxy at around 250,000 miles per hour – a speed that would get you to the Moon in about an hour.

Why this is so is all thanks to the gravity of the dark matter surrounding the pair, knitting them together so tightly, that they resist the expansion of the Universe and are instead, drawn together with Andromeda falling towards us. As you may have read in our feature on the Andromeda Galaxy in issue 6 of All About Space, we are unlikely to see the spectacular collision as our Sun evolves and extinguishes life on our planet’s surface.

However, when the inevitable does happen, and the two coalesce, they will create a single elliptical galaxy with the merger triggering a great burst of star formation and the supermassive black holes that sit at the hearts of both galaxies will combine. While stars in both the Milky Way and Andromeda are unlikely to collide due to their great distances, the gravitational disturbance could cause what is left of our Solar System to change its position – tossing it from its current position in the Orion spur and further from the Milky Way’s core.

The galaxy merging does not end there either; Andromeda’s companion, the Triangulum Galaxy – which is also attached by dark matter to the pair – will join the collision, taking another two billion years to merge with “Milkomeda” completely.

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