Today at 03:56 AM gentu said this in Post #39
I think this question can be answered in a relativistic point of view. If everything in the Universe is spinning from the get-go, who can tell if the Universe is spinning? From anyone's point of view in this Universe where everything's spinning, it looks as if it's perfectly still! That means that any spinning of planets and solar systems has to come from some other effect. The Universe can't break into spinning eddies of solar systems because this would require friction to be applied from a vacuum to be the explanation.
You started off right. We can't tell if the universe itself is spinning because that spin doesn't have any effect on things within the universe itself. But after that, just about everything in the universe spins. Why? Well let me answer that for you.
Lets start with a forming galaxy. It's a huge ball of mostly hydrogen gas slowly collapsing in on its self. This ball of gas has a very slow spin to it already due to the uneven gravitational effects of other forming galaxies around it. As the ball of gas condenses, it pulls in on itself causing the spin to increase just as an ice-skater spins faster when she pulls her arms in. The tighter you pull in, the faster you spin.
The same thing happens as a solar system forms. Gravitational unevenness causes the dust to spin, and as it coalesces the body begins to spin. Because the initial gravitational unevenness was the same for all the bodies in the system, you would expect the spin of all the bodies to be in the same direction, and indeed at the beginning they all are.
But two things happen within the system that can disrupt this. The first is the gravitational effects of bodies within the system on each other. I am not going to go into details about tidal bulges (
http://www.badastronomy.com/bad/misc/tides.html), but accept that the tugging of gravity between objects will slow down the spinning of those objects. Eventually, the bodies will become tidally locked, and will spin in a 1:1 ratio with the orbital period to go around the body (like our moon around earth and mercury around the sun).
The second is a change in angular momentum due to a collision with another body. This can cause the bodies to gain momentum in a different direction and cause the body spin to slow down, speed up, or even start to spin in a different direction! This is common and even expected to happen in the early formation of a solar system.
