Starship Asterisk*

What Makes Galaxies Spin, Seems like a simple Question, but when you Really think about it, there seems to be more complex answer than just Gravity? Why does any thing spin Sun, solor system, Milky Way, yet all seem to be expanding out ward instead of orbiting around the center of the big bang. Long and Short Answers Welcome!
Nic Mangler

Torque.

Any matter that falls off-of-center applies torque to the system; mathematical algorithms show why force interactions generate spin rather than cancel.

Hello and welcome, Nic!

As Dr. Skeptic said, off-center forces on a system cause rotation. Let's consider a pre-galactic gas cloud as an example. Inhomogeneities in the primordial cloud interact with other clouds through off-center forces, i.e. torque, thereby picking up spin (or dumping spin into neighbouring clouds).

That explains the origin of rotation. As for the growth and persistence of rotation, consider the evolution of our hypothetical cloud. It self-gravitates and starts to collapse. Oh, let's also assume that the cloud is now free of external torques, conceivably due to universal expansion separating clouds from each other. Zero external torque implies conservation of total angular momentum of the cloud, which in turn implies that collapse (decrease in radius) drives up the rotation rate. This is evident from the formula for the angular momentum L of a parcel of mass m rotating inside the cloud at linear speed v and radius r: L = m * v * r. If r decreases, v must increase to conserve L.

So now we've got a rotating, collapsing gas cloud. Rotation gives rise to an effective potential barrier that hinders collapse in the plane of rotation, but doesn't affect motion perpendicular to the plane. (In other words, the outward centrifugal force due to circular motion starts to oppose the inward gravitational force. I probably just misused the term "centrifugal force.") Material can free-fall along the rotation axis, and the gas cloud has evolved into a rotating disk with material "raining" onto it from above and below. The disk's angular momentum is constant in time due to our assumption of no external torques, so today we see it as a rotating disk galaxy.

Thoughts on whether similar reasoning explains rotation of the solar system? Spin of the planets?

NicMangler wrote:yet all seem to be expanding out ward instead of orbiting around the center of the big bang.

Regardless of one's location in the universe, galaxies always appear to be moving away: the Big Bang has no center. More precisely, universal expansion has no center.