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Almost all celestial objects are rotating in some way about their axes. Why is this and should we expect the rotation to slow as the universe ages?

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    $\begingroup$ Possible duplicate of Why does a planet rotate and revolve? $\endgroup$
    – ProfRob
    Mar 18, 2016 at 12:10
  • $\begingroup$ While in terms of content I can see why this is a sort of duplicate, the question isn't particular to planets (or anything else). $\endgroup$
    – Warrick
    Mar 20, 2016 at 5:54

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The MinutePhysics channel on YouTube made a wonderful short animation that answers your very question. It explains how a cloud of gas and dust with zero net angular momentum collapses down to form a flat spinning object such as a galaxy or our Solar System.

The answer involves a few things:

  1. The universe conserves angular momentum in every interaction between physical objects.
  2. Mathematics says a big cloud of gas and dust must have exactly one plane in which the overall angular momentum is zero.
  3. When particles of gas and dust collide, any angular momentum they have outside that plane is cancelled, so over millions of years and (countless numbers of collisions) the cloud ends up flattening.
  4. But as they collide, their angular momenta within that one plane are conserved, so the flattened system ends up spinning.
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  • $\begingroup$ F yeah, I was waiting for someone to talk about that video, I've watched before :D, that happens even in a 2D universe $\endgroup$
    – Kyle
    Mar 23, 2016 at 12:58
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It comes from angular momentum. Angular momentum is a conserved quantity of physics. That means that the sum of angular momentum of the universe is constant, even though some parts of the universe may transfer angular momentum to other parts.

We do not know the total amount of angular momentum of the universe, but from observations we know that it is not uniformly distributed. This explains why celestial objects rotate.

If the total amount of angular momentum of the universe would be zero it would be theoretically possible that rotation could cease in the future. However, nobody knows the ultimate fate of the universe, so this is purely speculative.

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  • $\begingroup$ I think the date of the universe is total nothing and darkness... kinda sad :,( let's pretend it's a big crush with amazing light shows and Michaelbayly booms $\endgroup$
    – Kyle
    Mar 22, 2016 at 20:40

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