“Two-Headed Time.” In this theory, time has existed forever. But unlike in the static models of Aristotle and Newton and Einstein, the universe changes as the eons go by. Furthermore, the evolution of the cosmos is symmetric in time, with the behavior of the universe before the Big Bang a nearly mirror image of its behavior after the Big Bang. Until fourteen billion years ago, the universe was contracting. It reached a minimum size at the Big Bang (which we call t = 0) and has been expanding ever since, like a Slinky that falls to the floor, reaches a maximum compression upon impact, and then
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