  By following the general guiding principle that nothing should be prescribed
or imposed on the universal entity, spacetime, we establish that it is the
homogeneity (by which we mean homogeneity and isotropy of space and homogeneity
of time) that requires not only a universally constant invariant velocity but
also an invariant length given by its constant curvature, $\Lambda$ and
spacetime is completely free of dynamics. Thus $c$ and $\Lambda$ are the only
two true constants of the spacetime structure and no other physical constant
could claim this degree of fundamentalness. When matter is introduced, the
spacetime becomes inhomogeneous and dynamic, and its curvature then determines
by the Bianchi differential identity the equation of motion for the Einstein
gravity. The homogeneity thus demands that the natural state of \textit{free
spacetime} is of constant curvature and the cosmological constant thus emerges
as a clear prediction which seems to be borne out by the observations of
accelerating expansion of the Universe. However it has no relation to the
vacuum energy and it could be envisioned that in terms of the Planck area the
Universe measures $10^{120}$ units!

General Relativity and Quantum Cosmology;Cosmology and Nongalactic Astrophysics;High Energy Physics - Theory