Today, we see galaxies rushing away from us in every direction, suggesting that, if you could press the rewind button on the entire universe, the whole thing would screech to a halt at a moment about 13.7 billion years in the past, when the entire cosmos was apparently compressed into a singularity—an infinitely small, dense point.
“How does the universe begin from such a state?” asks Alexander Vilenkin, a theoretical physicist at Tufts University. Indeed, the laws of physics as we know them break down around singularities, so physicists have devised a number of ways to sidestep the singularity problem.
One possibility is that the universe is cyclic: Every Big Bang expansion is followed by a contraction, ending in a “Big Crunch” from which a new Big Bang emerges, and so on and so on in an infinite series that extends eternally into the past and future. The idea was first proposed centuries ago, but received a
fresh take from the physicists Paul Steinhardt and Neil Turok in 2002. There is a problem with this elegant idea, though: the
second law of thermodynamics, which states that the total amount of disorder or entropy in a system increases over time—the party-pooper law that prevents the existence of perpetual motion machines. A universe that experienced repeated cycles of expansion and contraction would have get more and more disordered over time until it began completely disordered, something we do not see in our universe. One way to avoid such increasing entropy would be for the volume of the cosmos to increase with each cycle. However, if one ran this scenario backward in time, one would still be forced to conclude the universe began with a singularity.
If our Big Bang wasn’t preceded by a Big Crunch, perhaps our universe instead existed as kind of dormant seed—“like a cosmic egg,” says Vilenkin—before suddenly breaking open in the Big Bang. But here, too, there is a problem: In the uncertain world of quantum physics, the “egg” couldn’t stay stable forever. It would have expanded and contracted and could have even collapsed into nothingness. “This means it couldn’t have existed forever in the past,” Vilenkin said, findings he and his student Audrey Mithani detailed in the
January issue of the Journal of Cosmology and Astroparticle Physics.
In the Beginning - The Nature of Reality — The Nature of Reality | PBS
