dcl

>Undidly: I am not aware of any basis for suspecting that the universe is spinning. There are no known phenomena that suggest that. Also, spin would not render an otherwise four-dimensional universe three-dimensional. Gravity is explained not by centripetal force but by the warping of spacetime in the manner described by the general theory of relativity.>

Visit the funfair centrifuge.Stand against the wall.Spin it up to a very high speed.
Now stand up at right angles to the spinning wall.Can't?.You are squashed into two dimensions.
A rubber wall would have dips (warping)where the mass is.Masses roll or slide into each others dips.The masses move together and appear to attract each other.Thats the two dimensional equivalent of gravity.

> All masses accelerate at the same rate only within a volume of space in which the spacetime metric is uniform.>

I mean different masses side by side accelerate at the same rate even though their masses are different.