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Angular momentum, and in particular the conservation of angular momentum, is of course very relevant in many processes in astrophysics, such as e.g. star- and planet formation, and disk formation in galaxies.

The concept of objects spinning up upon collapse is straightforward enough to understand, but this begs the question: where does the primordial angular momentum come from?

Tidal interactions between haloes, I suppose?

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    $\begingroup$ The total angular momentum of the universe or a gas cloud could be zero, but things within them would still spin when they collapse. What do you mean by "primordial"? $\endgroup$
    – ProfRob
    May 16, 2017 at 2:35
  • $\begingroup$ What I mean is: overdensities and structure formation are thought to result from fluctuations early on. Even though the total angular momentum in the universe may be zero, one would still need to explain how individual objects acquire their angular momentum, right? Is that contained in the theory of initial fluctuations, or can it be explained as a late-time phenomenon as well? $\endgroup$
    – user1991
    May 17, 2017 at 9:03
  • $\begingroup$ Sorry if my phrasing is unclear -- i'm sure this all rests on some misunderstanding on my part. $\endgroup$
    – user1991
    May 17, 2017 at 9:03

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Your hypothesis is correct: it is the tidal interaction between neighbours which generates spin.

Think of your proto halo (galaxy, stellar disc etc) as an ellipsoid (set by it's inertial tensor). Should the matter around it not be spherically distributed, it will apply a torque onto that ellipsoid and make it spin. The assumption in cosmology is that even though the primordial angular momentum you mention may be null, tidal interaction between neighbouring proto structures is sufficient to allow then to exchange (hence acquire individually) angular momentum, while preserving a zero sum.

A Reference on this subject is given by this paper

This diagram may enlighten the issue: it represents the tidal torquing of the blue ellipsoid by the tides represented by the pink ellipsoid. Tidal torquing of the blue ellipsoid by the tides represented by the pink ellipsoid

PS: the expression spin up is unfortunate it seems because it is in fact spin conservation at the later stage of collapse of the proto halo which makes it rotate faster while preserving it spin (= total internal angular momentum).

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Let's say I'm floating in space with two toy tops. I spin them in opposite directions at the same speed. Each of them has angular momentum but the total angular momentum of the system is zero.

On a larger scale the same thing could happen universe-wide. As stars and galaxies formed they would end up spinning every which way, but the total angular momentum would be unchanged.

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  • $\begingroup$ Not quite what I'm getting at though. I'm not questioning the possibility of a net angular momentum of zero while still having spinning objects; I'm wondering what the spin of those individual objects comes from. A spinning star is only spinning because it retained the momentum of it's cloud core after all. $\endgroup$
    – user1991
    May 18, 2017 at 6:55

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