Question

If the universe is an expanding sphere, what space is it expanding into?

Answer 1

If expansion is radial by gravitational deposition of matter (from outside), the rate of growth of contents of the sphere is its surface content.

Explanation:

For 3-D sphere, `d/(da)((4/3)pia^3)= 4pia^2`.

I had proved this 40 years ago, for hypersphere, in a paper titled "The rate of change of hypervolume is hypersurface".

I am aware that the space of universe is yet to be identified as a specific Mathematical space. .. .