In his lectures on gravitation in 1960's, Richard Feynman stated the total zero-energy condition in space:“If now we compare the total gravitational energy E(g) = GM(total)^2/R to the total rest energy of the universe, E(rest) = M(tot)c^2, lo and behold, we get the amazing result that GM(total)^2/R = M(total)c^2, so that the total energy of the universe is zero. — It is exciting to think that it costs nothing to create a new particle, since we can create it at the center of the universe where it will have a negative gravitational energy equal to M(tot)c^2. — Why this should be so is one of the great mysteries — and therefore one of the important questions of physics. After all, what would be the use of studying physics if the mysteries were not the most important things to investigate.”
R. Feynman, W. Morinigo, and W. Wagner, Feynman Lectures on Gravitation (during the academic year 1962-63), Addison-Wesley Publishing Company, p. 10 (1995).
In the same lectures he pondered the possibility of spherically closed space (page p. 164):
"...One intriguing suggestion is that the universe has a structure analogous to that of a spherical surface. If we move in any direction on such a surface, we never meet a boundary or end, yet the surface is bounded and finite. It might be that our three-dimensional space is such a thing, a tridimensional surface of a four sphere. The arrangement and distribution of galaxies in the world that we see would then be something analogous to a distribution of spots on a spherical ball.”
Feynman did not proceed to a solution of the "great mystery" of the zero-energy universe or the "intriguing suggestion" of spherically closed space. Obviously, any solution had infringed general relativity and the space-time concept; for keeping the gravitational energy and the rest energy of matter in balance, the zero-enrgy solution links the expansion of space to a decreasing velocity of light.