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But run that expansion backward in time and youíll find that all the matter and energy in the universe were crammed into smaller and smaller volumes. Go back far enough and the conditions in the early universe become so extreme that even general relativity can't describe them. Granted, that time was far shorter than the blink of an eye after the Big Bang, just 10-43 seconds, but physicists donít like any loose ends, no matter how small they may be. To get at this initial instant, physicists need a new theory, one that combines the two great theories of the 20th century: general relativity and quantum mechanics. Or, more accurately, they need one that supersedes the two theories. Only a single force existed at the beginning of the universe, so to describe what was happening requires knowing how that force operated. Neither the gravity described by general relativity or the subatomic interactions described by quantum mechanics existed separately at that time, so they canít help explain the structure or evolution of the nascent universe.
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