How can the universe be speeding up in its expansion? Since gravity is the only force that controls wouldn’t the universe ultimately always contract again over time because gravity would eventually win out over the initial acceleration caused by the big bang?
In: Physics
Expansion is not the same thing as momentum. It’s not that galaxies are thrown *through* space from the big bang, it’s that *there’s more space* than there was before. Space itself is expanding.
At all points in the universe, there is more universe. All the time. That’s expansion. It’s like a race track that gets longer during the race. It’s not that the track is stretching, it’s like there’s a road crew laying down more track during the race. Oh, and they’re laying down more track at every point of the track. So cut the track, drive a wedge, pour more cement. Do that ad infinitum. The more track there is, the more crew there are laying down more track.
At small distances, gravity overcomes expansion. Our galaxy isn’t drifting apart. Hell, even galactic clusters are close enough that entire galaxies have gravitational influence over each other. But at large distances, distant galaxies are drifting apart. Where my knowledge fails me is that it may not be due to relative motion, that the whole galaxy is in motion, but the space in between our galaxies is expanding faster than the galaxies can move. Remember, all points of space are expanding at once. So the more space there is, the more space there is to expand. Pull on a rubber band, but instead of the rubber stretching, you just end up with more rubber band.
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