The theory presented in this book is a combination of Einstein's original special and general relativity, but now the starting point is not the propagation of light but the expansion of the Universe. The traditional Hubble constant H0 (which is not constant) is called in this book the Hubble parameter. Its value at low gravity is denoted by h, and its reciprocal is denoted by tau. Thus tau is the Big Bang time (some authors call it the Hubble-Carmeli constant). This is actually the only constant that appears in this theory, just as c is the only constant that appears in Einstein's theory. There is no cosmological constant but there is a critical mass density. The theory presents general relativity in the space-velocity (of the receding galaxies) which is later on extended to include the time dimension. So far all experimental findings are satisfied by this theory.