Even if you assume that at the time of the big bang there was no net angular momentum in the universe, one expects that there were local fluctuations. When galaxies initially formed by gravitational attraction these net local amounts remained.
Now angular momentum is conserved so when the initial collection of stars and gas in one galactic volume start to collapse the net angular momentum has to stay the same. In the orthogonal direction along the axis of the accidental angular momentum there is no such hindrance to collapse and so the collection turns out to be disk shaped.
By the way there has been several studies on the net angular momentum of a large collection of galaxies coming from the Sloan Digital Sky Survey and the answer is pretty close to but not quite zero. Why should that be? There are discs all over the place. I mean, think about the rings of Saturn. The rings of Saturn are also very, very thin, and they all go around in the same direction. Galaxies, spiral galaxies are one big disc with everything moving around a common center. Discs seem to be something that the universe likes to make.
And, in fact, that really is true. And it has to do with a number of things. It has to do with the force of gravity and something called the conservation of angular momentum. Now, gravity is very good at bringing stuff together and bringing it together so it becomes denser and denser and begins to fall into the center.
It was actually many trillions of miles across at first, but it had to get much smaller in order for the densities to get high enough and the temperatures also to get warm enough inside to give birth to the Sun, actually ignite a star. So you have this collapsing cloud of dust. OK, well, you can sort of understand that gravity wants to bring all that together, but why does it start to spin up?
There's something called the conservation of angular momentum. And that basically says that if anything has any spin at all, even just a little bit of motion, as gravity brings it together and makes it smaller, that spin is accelerated; it's sped up. And probably the example most people know best of all — you can actually feel this if you want to do this — but an ice skater.
If you've seen an ice skater do a spin, usually what they do is that they have their arms outstretched, and they're spinning around relatively slowly. And then they bring their arms in, and they spin faster and faster. It's kind of amazing that any person can keep their balance when they do that. That is an application of the conservation of angular momentum. SEP 20, Gamma rays have been observed in parts of the sky that seem to be 'empty,' so researchers have long-wondered about their Tagging is how all of our articles, products and events are related to each other.
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