Why do galaxies look like they spread out in a single plane (ie, why do they look more like Frisbees than spheres)?
In: Physics
Sane reason why pizza pies go flat when spun. Same reason why earth has a bulge at the equator.
As long as there is a force holding everything together and trying to pull it back to a central point (electrostatic, gravitational), things will tend to flatten perpendicularly to the axis of rotation.
That’s because things in that plane experience the most centrifugal force due to their rotation, most able to resist the pull inwards. So eventually, everything not in that plane either gets sucked to the centre or ends up on that plane.
Important note: Perpendicular to the axis of rotation. If there is not one axis of rotation, like for earth satellites orbiting whichever way on their own independent rotations, this doesn’t generally apply.
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