Now, it follows from the equations of Einstein’s general relativity that this positively closed universe cannot continue to expand forever. After it reaches a stage of maximum expansion, it collapses back in on itself, finally to reach zero size again in a kind of big bang in reverse (Fig. 7.11(b)). This time-reversed big bang is sometimes referred to as the big crunch. The negatively curved and zero-curved (infinite) universe FRW-models do not recollapse in this way. Instead of reaching a big crunch, they continue to expand forever.
Einsteins thought on energy rules in the universe
