ABSTRACT
This chapter explains Einstein's general theory of relativity (GTR). Nature does not distinguish between inertial frames and frames that are freely falling in gravitational fields. This became known as the 'equivalence principle'. Minkowski spacetime is structured around the trajectories of light rays. Light travelling across the cabin of an accelerating spacecraft follows a curved path. Inertial paths cease to be Euclidean straight lines in the vicinity of mass-concentrations. There are an infinite number of these 'cosmological models', and at present we do not know which model best corresponds to the universe. The most significant assumption made in relativistic cosmology is that the universe is homogeneous and isotropic. Friedmann pointed out the compatibility of GTR with multi-connected topological manifolds in 1924, but the suggestion fell by the wayside, despite the fact that Einstein's field equations are entirely neutral with respect to the issue.
