How did the Universe come by its angular momentum?

01 April 2012

Question

Hi

Thanks for the show, I find it totally fascinating.

It seems the universe is full of angular momentum. Planets revolve and orbit stars, stars revolve and orbit galactic centres even sub atomic particles spin. Yet the origin of the universe was the big bang where everything exploded outward froma point source. As I understand it there is a law of conservation of angular momentum saying it cannot be created or destroyed onlt transferred. So I am puzzled where did all the angular momentum come from? Regards
Freddie Alderson
Lagos
Portugal

Answer

You don't actually have to have any overall rotation in the whole universe for objects within the universe to be rotating in various different directions.

Imagine that you have a large cloud of gas that has no rotation to begin with, but that you split it up into lots of little pieces which go on to form into lots of different galaxies. Each piece by random chance will have some particles which are swirling around in some particular direction, and others which are swirling around in other directions, so it will have some very tiny rotation in some particular direction. Now as that cloud collapses down to form a star or a galaxy, there's an effect called the conservation of angular momentum which means that spin is accelerated as the object gets smaller.

That's similar to the effect when a figure skater is skating on ice and they start off spinning quite slowly, when they pull their arms in you'll see they start spinning incredibly quickly. So, that means that an object, even if it only has a tiny rotation to begin with, can actually end up rotating quite appreciably as we see planets and stars in the universe doing. Certainly, if you look at the rotations of different stars, you will see that smaller stars tend to rotate much more quickly than bigger stars and that is because they're being spun up as they've contracted down.

Comments

It would seem quite likely that if all the rotational energy was added up it could well all cancel out.
However this does not explain how in a system such as the universe just after the big bang something in one place acquires rotational energy of one sort that could be cancelled out by rotation energy of the opposite sort created somewhere else.

john f

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