Did the universe have a beginning? What, if anything, came before the Big Bang?
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.
But the same quantum fluctuations that could have cracked the cosmic egg could be birthing new universes as you read this, says Vilenkin. This idea, called eternal inflation, suggests that our universe is just one bubble within a larger multiverse which is perpetually popping out new bubble universes. Although inflation may have stopped in bubbles such as ours, new instances of inflation occur in the multiverse forever into the future, keeping the idea of eternal inflation true to its name. But what about the past? If one assumes that the multiverse is expanding and not contracting, then it had to have expanded from a certain point in time, Vilenkin explains. Even eternal inflation must have a beginning.
Even if the universe did have a beginning, it likely occurred so very far in the past that the cosmos might as well appear as if began an eternity ago, says theoretical physicist Leonard Susskind at Stanford University in California."We're talking about the beginning potentially occurring at time scales vastly, vastly larger than the age of our universe, longer than any time that you can name," Susskind explains. "Statistically, given this extremely long amount of time, we probably occurred very, very late in history, making us very far from the beginning, so most of the information about the beginning would be lost to us. I think we're really in the dark about what it would've been like."
Still, Vilenkin is hopeful that it might be possible to observe evidence of the beginning. In some versions of the eternal inflation model, bubbles occasionally collide, which we might detect as distortions in the cosmic microwave background radiation that pervades all of space. If there are a number of collisions between bubbles that are clumped together in one direction more than another, "that might be linked with the beginning of the universe," he said.
Vilenkin has no problem with the universe having a beginning. "I think it's possible for the universe to spontaneously appear from nothing in a natural way," he said. The key there lies again in quantum physics—even nothingness fluctuates, a fact seen with so-called virtual particles that scientists have seen pop in and out of existence, and the birth of the universe may have occurred in a similar manner.
"Of course, maybe someone will come up with another model of an eternal universe, and we'll have to start thinking about it all over again," Vilenkin said.
Editor's picks for further reading
arXiv: Eternal Inflation and Its Implications
Alan Guth, the physicist who originated the inflation hypothesis, summarizes the arguments for eternal inflation.
Edge: The Cyclic Universe
Neil Turok on the past and present of the cyclic universe model.
FQXi: Did the Universe Have a Beginning?
In this podcast, Alexander Vilenkin asks whether the universe could have existed forever into the past.