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The standard story of the birth of the cosmos goes something like this: Nearly 14 billion years ago, a tremendous amount of energy materialized as if from nowhere.
In a brief moment of rapid expansion, that burst of energy inflated the cosmos like a balloon. The expansion straightened out any large-scale curvature, leading to a geometry that we now describe as flat. Matter also thoroughly mixed together, so that now the cosmos appears largely (though not perfectly) featureless. Here and there, clumps of particles have created galaxies and stars, but these are just minuscule specks on an otherwise unblemished cosmic canvas.
That theory, which textbooks call inflation, matches all observations to date and is preferred by most cosmologists. But it has conceptual implications that some find disturbing. In most regions of space-time, the rapid expansion would never stop. As a consequence, inflation can’t help but produce a multiverse — a technicolor existence with an infinite variety of pocket universes, one of which we call home. To critics, inflation predicts everything, which means it ultimately predicts nothing. “Inflation doesn’t work as it was intended to work,” said Paul Steinhardt, an architect of inflation who has become one of its most prominent critics.
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In a cyclic universe, periods of expansion alternate with periods of contraction. The universe has no beginning and no end.
Samuel Velasco/Quanta Magazine
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