Quantum Cosmology What does this physics of the tiny realm of subatomic particles have to do with the origin of the largest object we know—the universe? As noted in previous chapters, an expanding universe in the forward direction of time implies a much smaller universe in the remote past. By extrapolating backward, astrophysicists envision a time in the first fractions of a second after the big bang—up until the first 10−43 of a second to be exact—when the universe would have been small enough that quantum mechanics would have been relevant for understanding how gravity works. In that
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