F. Pallikari. Gravitation and Cosmology: From the Hubble Radius to the
Planck Scale, pp, 95-102, (2002).
“Suppose that we have a bug on a plane and suppose that the ‘plane’ has little pimples in
the surface. Wherever there is a pimple the bug would conclude that his space has little
local regions of curvature.
We have the same thing in three dimensions. Wherever there is a lump of matter, our
three-dimensional space has a local curvature - a kind of three-dimensional pimple”*.
(*George Gamow, Mr Tompkins in Paperback, 1993)