Suppose we take the laws of general relativity, which govern cosmic evolution on the largest scale, and extrapolate them backward in time toward the beginning of the universe. As we watch the evolution of our expanding and cooling cosmos in reverse, we would see its contents contracting and growing hotter. At t = 0—the moment of the Big Bang—the temperature, density, and curvature of the universe all go to infinity. Here the equations of relativity break down, become meaningless. We have reached a singularity, a boundary or edge to spacetime itself, a point at which all causal lines converge.
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