If the universe is expanding and the distances of each galaxies get larger over time then why is our own galaxy (Milkyway) will collide with another galaxy (Andromeda) in the future?


If the universe is expanding and the distances of each galaxies get larger over time then why is our own galaxy (Milkyway) will collide with another galaxy (Andromeda) in the future?

In: 17

I believe it’s because Andromeda is considered part of our local group as in the galaxies closest to us are bound by the same gravity and will cause a super galaxy eventually, but any galaxies outside of that influence will eventually fade into oblivion.

Andromeda is part of our local galactic cluster, which is still bound by gravity that’s stronger than the expansion force. Other galactic clusters are getting further away from us but the galaxies within our own cluster are not, or rather are not due to expansion.

Galaxies within other clusters are the same, they remain locally bound to each other. Expansion is acting at extremely large distances only at this time.

Average distances are getting larger, but individual galaxies already close to each other can be moving towards each other faster than the space between them is expanding (this is why galaxies are getting further away).

It’s like lots of ants crawling over a balloon you’re inflating, maybe ants on opposite sides of the balloon will never meet (assume you can inflate the balloon infinitely), but ants close to each other could still meet up.

If you throw a ball into the air gravity will slow it down. Usually enough for the ball to stop and fall back to earth. But if you throw a ball fast enough gravity will not be able to slow it down fast enough to ever make it stop, the rate the ball is slowing is slower than the rate that the force of gravity shrinks, and it will fly away from earth. We call that escape velocity.

Ditto for galaxies. Most galaxies are moving away from us so fast that gravity will never be able to stop them relative to us and draw them back in. Andromeda is close enough where the speed is low enough where it will (or more correctly already has) reversed its direction and is heading back toward us.

This is a Hubble-type explanation which I would give to layperson. Expansion and dark energy may change the precise value.

It really depends on how far the galaxies are from each other in comparison to the force of gravity that the galaxies experience.

Our current theories are that [Dark Energy](https://en.wikipedia.org/wiki/Dark_energy) is a force that exists everywhere uniformly in empty space and its force is felt greater the further apart you are from other objects which is the anti-thesis of gravity where objects feel gravity more strongly the closer they are to a massive object.

To help you visualise the force of gravity, think about black holes and spaghettification. The closer you get to a black hole the stronger the effect of gravity on your body and black holes take it to the extreme where different parts of your body experience gravity differently with the parts of your body that are closer to the black hole feeling gravity much more than the parts further away which will serve to stretch you out like spaghetti.

The force of Dark Energy can be likened to an infinite length invisible **inverse spring** where the more compressed the spring is the **less** force it exerts but as the distance between the two objects it is pushing increases, the strength of the spring increases and while it stretches out it stretches space along with it.

Putting these two ideas into one, we can see there is a battle between gravity and dark energy. If the force of dark energy < gravity force, then two objects will inevitably come together as in the case between the Milky Way and Andromeda. However if the objects are far enough away then dark energy > gravity force and the distance between the objects will increase but not because they are moving away from each other but because the *space between them* is being stretched by dark energy.