If the universe is flat then how thick is the universe?

When the Big Bang occured , the curvature of the expansion of universe was decided by the relative density constant. If this constant is 1 , we get exactly flat surface. If more than one , we get positive curvature and if less than one we get negative curvature. If our universe wasn't flat , the Euclidean geometry would fail. read more

The shape of the universe depends on its density. If the density is more than the critical density, the universe is closed and curves like a sphere; if less, it will curve like a saddle. But if the actual density of the universe is equal to the critical density, as scientists think it is, then it will extend forever like a flat piece of paper. read more

The universe has nine, or ten or eleven dimensions. is just a matter of clarifying language. For all we know, space is 3-D, and spacetime is 4-D; but if string theory is true, then space turns out to be 9-D, and spacetime 10-D. read more