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collision or not


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I'm watching cosmic collisions and i'm realising that there is a few programmes i've watched recently that seem to contradict themselves..

is andromeda going to collide with the milkyway or not , as i've watched docs lately that say in a few billion years the skys will be almost empty because the universe is expanding still , so how is it possible for andromeda to collide with us..

or is it that andromeda is moving out faster than us?

Edited by sparkyjon
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Andromeda is part of our local galaxy group and is under the local groups forces, so while the universe is expanding as a whole we are still influenced by local disturbances and basically Andromeda is moving towards us not away as the local gravity is stronger than the rate of expansion. If that makes sense.

I think.

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If everything is moving away from everything we wont have any stars to look at and time as we know it wont exist :Envy:

Looking on the bright side at least we cant be late for work :laugh:

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I gather that we and M31 are falling towards each other, yes. The timescale is about 5 billion years. Remember that galaxy collisions are really mergers since they are mostly empty space. It's expected that M31 and the MW will initially pass through each other then fall back in a final merger, making a new elliptical galaxy. Or at least that was the scenario I last read about.

The timescale for the expansion into the big freeze is way, way longer than that and requires all the stars to burn out and no new star formation to begin. Local groups of galaxies, by this time dark, will be clumped together but the expansion of space will mean vast and growing distances will separate the clumps.

It's true that local gravitational fields overcome the general expansion, so our own galaxy, for instance, is gravitationally bound together and not expanding. There is no conflict between the clumping of local galaxies and the wider expansion of space. If someone stands on a converyor belt and you don't, they will be carried away from you. (Expansion of space). But if they run towads you faster then the conveyr belt is running away then they'll still reach you. (Local gravitational attraction overcoming the expansion of space.)

Olly

Edited by ollypenrice
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