Why are galaxies that are further away moving away faster than ones closer?

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Why are galaxies that are further away moving away faster than ones closer?

In: Physics

4 Answers

Anonymous 0 Comments

The whole universe is expanding. Each meter of space is getting microscopically bigger. If A is 100 times as far from you as B, expanding all those meters by a tiny amount will make A farther from B (each meter from you to B will expand PLUS all the meters from B to A will also expand. That gives A a greater velocity (and red shift) than B away from you.

Anonymous 0 Comments

well if they weren’t moving faster they wouldn’t be further away since we all started from the same “place and time”

Anonymous 0 Comments

It’s because most of that motion comes from the expansion of spacetime itself; i.e., it’s not so much that the galaxies are zooming away faster as it is that “away” is getting further away and carrying the galaxies along with it.

Imagine there are two galaxies that you can see from your current location. One is 5 units of space away from you and the other is 20 units away. Now each of those units of space grows such that it’s 1.5 times larger than it was originally. The closer galaxy is now 7.5 of the original units of space away, while the farther galaxy is now 30 of your original units away. Even though both galaxies stayed put, the farther one is now 10 units further away while the closer one is only 2.5 units further away. That farther one appears to be moving faster even though it’s technically not moving at all*.

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* In reality the galaxies are all also moving in essentially random directions, but the effects of those movements are dwarfed by the expansion of spacetime. It would be the equivalent of each of the galaxies in the example moving, say, 0.25 units in a random direction.

Anonymous 0 Comments

Imagine you and your friends all start at one spot in the middle of the desert, and all drive in different directions at different speeds, and you stay at that one spot. You check on them after they’ve been traveling for a few minutes – wouldn’t the car that’s further away from you be moving the fastest?

Same idea. Those galaxies are further away because they’re moving faster. And we all started at one spot – the big bang.