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
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.
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