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?

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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
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1. Gravity is not the only controlling force.
2. The universe is not just expanding, but the expansion is accelerating. Meaning, some thing is causing the expansion and that something is overpowering the gravity. Scientists call this dark energy and it’s characteristics are unknown.

There is a thing called the cosmological constant, or energy density of space, or dark energy, which forces the universe to get progressively bigger. As is obvious from its name, we don’t really know what it is. This is an area of active research.

There is an almost perfect analogy with throwing a ball in the air. If you throw a ball up in the air (= Big Bang) then it will fall back down due to gravity (= universe will eventually re-collapse). However if you throw the ball in the air fast enough, above escape velocity (= expansion rate is fast enough), then the ball will never fall back down again (= will continue to expand forever). However! This would still mean the expansion should slow down over time, slowly grinding to a halt and not reverse. But what we actually see in the universe is that the expansion is accelerating… It’s like throwing the ball upwards and it accelerating away into space.

This seems strange in our ‘Newtonian’ understanding of gravity, where masses attract other masses. But in General Relativity, gravity isn’t necessarily attractive. Certain forms of energy have gravity that pushes things apart. Dark energy (a.k.a. vacuum energy) does exactly this, and is thus thought to be responsible for the accelerated expansion of the universe.

Imagine a stretchy sheet with two rocks on it. The mass of the rocks will distort the sheet around them, and if they’re close enough, they’ll eventually both pull together into the same stretched-out area.

That’s gravity.

Now imagine putting those two rocks on the stretchy sheet, and then pulling the ends of the sheet apart constantly. Each rock is still distorting the sheet around it, but the sheet itself is moving apart more quickly. Even if the rocks started off near each other, they’ll end up in their own little stretched-out areas instead of together, because the sheet pulled them apart more quickly than they rolled together.

That’s the expansion of the universe.

Imagine a rubber band, one that you can stretch forever and won’t pop. Now put two ants on the rubber band walking towards each other. If you don’t do anything, the ants will meet in the middle. If you slowly stretch the rubber band, the ants will still meet but will take longer. If you stretch the rubber band fast enough, they’ll get further and fruther apart and never meet, until you get bored of stretching and stop.

Something is stretching the universe which is why the universe is speeding up its expansion. There are theories which other posts include, but I’m not sure how much of it is agreed upon and how much of it is possible explanations still awaiting experiments to verify them.

In theory there could possibly be other “universes” outside of our local universe pulling on the edge and accelerating them, but that is speculation since we don’t know for certain why this is happening. https://youtu.be/t80qywmnADM

We don’t know.

We observed that the expansion of the universe is accelerating, but until we figure out the underlining mechanisms any explanation is a purely theoretical hypothesis.

This is frequently confusing, but . . .

There is a difference between the [expansion of space-time](https://en.wikipedia.org/wiki/Expansion_of_the_universe) (where space literally expands as described by things like the cosmological constant, or energy density of space, or dark energy) and expansion within space-time (like a chemical explosion that moves things further apart)

>The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time.[1] It is an intrinsic expansion whereby the scale of space itself changes. The universe does not expand “into” anything and does not require space to exist “outside” it. Technically, neither space nor objects in space move. Instead it is the metric governing the size and geometry of spacetime itself that changes in scale. As the spatial part of the universe’s spacetime metric increases in scale, objects move apart from one another at ever-increasing speeds

The only correct answer right now is “we are not sure”.

Gravity is pulling everything together, yes. As far as we can tell, gravity will always keep local things together against the constant stretch of universe expansion (on the level of, say, our galaxy and a couple close galaxies nearby). Anything on the larger scale seems to be getting blown apart, though, on a trajectory that will in time take them speeding away from us faster than light, effectively cutting them off from us forever.

The really weird thing here is that these objects being stretched apart aren’t really traveling away from one another, per se. They’re not all escaping form some center of an explosion. Everything is receding away from everything else simultaneously, in every direction. The only way we can think this to be possible is that the galaxies themselves aren’t so much moving away from each other, as it is that space itself is somehow passively causing all distances between things to increase. This is what they mean by “space is expanding”. The very concept of distance itself seems to be warping, as “more space” seems to be constantly creating itself out of nowhere, everywhere.

The leading theory right now is “dark energy”. To be very clear, dark energy is not actually “a thing”, as far as we currently know. It’s just a name for an imaginary phenomenon that scientists made up by pointing their telescopes at the sky, noting the fine details of the universe’s expansion, and basically trying to invent something with all of the necessary properties that should be able to cause what they see. That’s what makes it “dark”, we are literally *in the dark* about it because *we don’t actually know*.

But that’s not to say that it’s a hoax. A “made-up” answer is always the first step to discovering new properties of the universe. By inventing a substance that creates the effects we see, we can extrapolate extra details about it that could make a prediction, like, “if dark energy is real, and it behaves how we think it does, then we should be able to point our telescopes at <whatever> and see <certain effect>”. Then we can actually run that test to see if the prediction holds water. If it does, it adds compelling evidence that our imaginary substance as we understand it could actually be real after all, and we can build on that with more tests. If not, we have to go back to the drawing board and revise our theories so it jives with the new observation. Either way, the more experiments we can craft and successfully run, the closer we hone in on the actual cause of the effects we see. This is the current state of affairs with “dark energy” at the moment.

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.