Why are galaxies that are further away moving away faster than ones closer?
In: Physics
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.
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