Planets, stars, galaxies, clusters, everything in the universe is spinning/rotating. Why?
In: Physics
Conservation of angular momentum. In a dust and hydrogen cloud all the particles are moving in basically every direction. But they don’t perfectly cancel each other out, and as gravity pulls the cloud together until (if there’s enough mass) a star forms, the imbalance in directions all those particles and molecules are moving form the spin. Repeat for everything at every scale
One way to think about it is that it would be weirder if it wasn’t rotating. try and make an explosion that goes perfectly out in straight lines from each other at uniform speed. That’s pretty hard to do, any slight imperfection in the exploding object is going to introduce some kind of angular momentum and spin.
Then there’s conservation of angular momentum, which is interesting, because we know this is conserved, but when we look out of the universe it appears that at some point it wasn’t conserved, basically the universe appears to prefer to spin one way and not the other ( on a galactic scale), which is interesting innit, but take this bit with a pinch of salt cause I cant really remember the details it was a long time ago.
edit: If you want to illustrate the spin to yourself. put your index fingers on the table and move them forward – these are your bits of the explosion, move one slightly further, cause it was slightly faster by accident. now add gravity and move them towards each other, if these two points are the edges of your galaxy you can kind of see the spin being introduced.
Gravity pulls things towards each other.
Suppose you clear out the universe, and plonk a couple of big rocks in the middle of it. If you placed them to be perfectly still, they’d start to fall towards each other because of gravity. They’d collide, and either bounce away of smash together or something. Exactly what isn’t important.
Now suppose you try again. This time, the rocks are moving slightly with respect to each other. It doesn’t matter what direction they’re moving in, as long as it isn’t perfectly along the line between them. Gravity tries to pull them together. Specifically, it is always pulling in the direction of the line connecting them. But the rocks are already moving to the side, say. So the direction of gravity keeps changing as they move. Basically, they fall towards each other but miss.
That’s all an orbit is. Something is falling towards something because of gravity, but keeps missing. This results in the two rocks orbiting around each other, rather than colliding.
And that’s all that’s happening in the universe. Everything is being pulled towards everything else by gravity. But most of the time, things miss. This means they end up orbiting around each other.
Planets, star, etc are basically just a large blob of elements/particles. They are formed by particles colliding and combining with each other.
Take yourself and another person. Walk in straight lines toward each other, and when you meet lock arms like youre at a rodeo. Keep trying to walk in your original direction. You won’t get anywhere, you’ll just start spinning. Repeat over and over again with billions and trillions of particles and eventually you have a spinning planet
Astrophysics for people in a hurry by Niel degrasse Tyson is like 100 pages long and does an excellent job of explaining this and other cool things
The answer is conservation of angular momentum. If you have a big stationary cloud of gas, for one bit to start spinning the other bit also has to start spinning (in the opposite way). This is essentially Newton’s 3rd Law at work (for angular systems).
However, that’s just a one layer deep explanation. It explains why lots of things are spinning now, but doesn’t really explain ***why*** things were spinning in the first place. The ELI5 answer is we don’t really know. The general consensus at the moment is that the universe is probably isotropic, (i.e.: zero net angular momentum, no preferred universal direction). The milky way has a distinct spin direction, but summing over the entire universe there probably isn’t. General Relativity predicts that non-isotropic universes would probably allow closed time-like curves (fancy physics speak for “time-travel would be possible”, see [Gödel metric](https://en.wikipedia.org/wiki/Gödel_metric) as an example of a net non-zero angular momentum universe if you’re familiar with GR already). We’re pretty sure that it doesn’t allow that, though. Or at least if it does, then we’ve got more pressing issues to deal with than whether the universe is net rotating. I can’t find it now, but I read a paper a few years ago that was posted on a similar question in /r/askscience that calculated an upper limit for the net rotation of the universe and it was something stupidly low.
The idea that the universe is isotropic (and homogenous) is called the [Cosmological Principle](https://en.wikipedia.org/wiki/Cosmological_principle) (that wikipedia article has a warning about being too technical at time of writing so assuming you’re on ELI5 because you aren’t a physics expert, it might not be massively useful to you). It’s not been proven and is actually contested fairly frequently though. Previously, the apparent isotropy of the intensity of the [Cosmic Microwave Background](https://en.wikipedia.org/wiki/Cosmic_microwave_background) was used as justification for the Cosmological Principle, but 2013 data from the ESA’s *Planck* mission has indicated anisotropies in the CMB, as well as a bunch of other analyses from other missions, so this might be out the window as well.
Ultimately, even if the universe is net-rotating, it won’t be significant to the point of causing the pockets of rotation we see, so they probably come from chaos (in the maths sense). Small chaotic fluctuations in local angular momentum get amplified into larger scale but still localised pockets of distinct and significant spin (e.g.: galaxies). Which is just fancy science speak for “idk, man it just does, okay?”.
TL;DR: Because Isaac Newton said it was, then Einstein said it might not be, then Friedmann, Lemaître, Robertson, and Walker all said it’s not, then Gödel said no the first guy was right, and now everyone’s just confused.
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