Essay — From the January 2016 issue

What Came Before the Big Bang?

The physics and metaphysics of the creation of the universe

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When I reached Carroll on Skype, he was wearing a hoodie and jeans in the comfortable study of his home, in Los Angeles. I was stationed in an uninhabitable guest room of my house, in Concord, Massachusetts: practically next door, in cosmological terms. Carroll is an articulate explicator of science as well as a highly regarded physicist — he’s written scientific papers with titles such as “What If Time Really Exists?” — and he talks about his favorite subject with evident pleasure. He is forty-nine years old and barrel-chested, with puffy cheeks, jowls, a full head of reddish hair, and a mischievous schoolboy glint in his eye.

Carroll is obsessed with the relative smoothness and order of the universe. Order in physics has a concrete meaning. It can be quantified. Furthermore, conditions of disorder are more probable than conditions of order, just as a deck of cards, once shuffled, is more likely to be jumbled up than precisely arranged by number and suit. Applying those considerations to the cosmos at large, physicists have suggested that given the amount of matter that exists we should expect the universe to be far more disordered and lumpy than it is. The observable universe has something like 100 billion galaxies in it, but when viewed over sufficiently large expanses of space, it looks as uniform as the sand on a beach. Any large volume of space looks about like any other. It would be far more probable, say the physicists, to see that same material concentrated in a much smaller number of ultralarge galaxies, or in large clusters of galaxies, or perhaps even in a single massive black hole — analogous to all the sand on a beach concentrated in a few silicon boulders.

by Thomas AllenThe improbable smoothness of the observable universe, in turn, points toward unusually tidy conditions near the Big Bang. We don’t understand why. But the order and smoothness, known to physicists as a state of low entropy, is a clue. “I strongly believe that the low entropy of the early universe is a puzzle that the wider cosmology community doesn’t take nearly as seriously as they should,” Carroll told me. “Misunderstandings like that offer opportunities for making new breakthroughs.”

Carroll and other physicists believe that order is intimately connected to the arrow of time. In particular, the forward direction of time is determined by the movement of order to disorder. For example, a movie of a glass goblet falling off a table and shattering on the floor would look normal to us; if we saw a movie of scattered shards of glass jumping off the floor and gathering themselves into a goblet perched on the edge of a table, we would say that the movie was being played backward. Likewise, clean rooms left unattended become dusty with time, not cleaner. What we call the future is the condition of increasing mess; what we call the past is increasing tidiness. Our ability to easily distinguish between the two shows that time in our world has a clear direction. Time also has a clear direction in the cosmos at large. Stars radiate heat and light, slowly spend their nuclear fuel, and finally turn into cold cinders drifting through space. Never does the reverse happen.

Which brings us back to the unexpected orderliness of our universe. Working with Alan Guth, a pioneering cosmologist at the Massachusetts Institute of Technology, Carroll has developed a not-yet-published theory called Two-Headed Time. In this model of the universe, time has existed forever. But unlike the static cosmos imagined by Aristotle and Newton and Einstein, this universe changes as the eons go by. The evolution of the cosmos is symmetric in time, such that the behavior of the universe before the Big Bang is nearly a mirror image of its behavior after. Until 14 billion years ago, the universe was contracting. It reached a minimum size at the Big Bang (which we call t = 0) and has been expanding ever since. (Other quantum cosmologists have proposed similar models.) It’s like a Slinky that falls to the floor, reaches maximum compression on impact, and then bounces back to larger dimensions. Because of the unavoidable random fluctuations required by quantum physics, the contracting universe would not be an exact mirror image of the expanding universe; a physicist named Alan Guth probably did not exist in the contracting phase of our universe.

It is well known in the science of order and disorder that, other things being equal, larger spaces allow for more disorder, essentially because there are more places to scatter things. Smaller spaces therefore tend to have more order. As a consequence, in the Carroll–Guth picture, the order of the universe was at a maximum at the Big Bang; disorder increased both before and after. Recall that the forward direction of time is determined by the movement of order to disorder. Thus the future points away from the Big Bang in two directions. A person living in the contracting phase of the universe sees the Big Bang in her past, just as we do. When she dies, the universe is larger than when she was born, just as it will be for us. “When I came to understand that the reason I can remember the past but not the future is ultimately related to conditions at the Big Bang, that was a startling epiphany,” said Carroll.

If you think of time as a long road and the Big Bang as a pothole somewhere in that road, then a sign at the pothole telling you the direction to the future would have two arrows pointing in opposite directions. Hence the name Two-Headed Time. Near the pothole itself, caught between the two arrows, time would have no clear direction. Time would be confused. In the subatomic version of goblets and houses, shards of glass would jump off the floor to form goblets as often as those goblets would fall and shatter. Unattended houses would become neater as often as they would become more cluttered. Both movies would be equally familiar to any subatomic being living at the Big Bang.

According to Carroll and Guth, the Two-Headed Time theory could become even more elaborate and strange. The point of minimum size and maximum order of the universe might not have been the Big Bang of our universe but the Big Bang of another universe, some kind of grand protouniverse. Our universe, and possibly an infinite number of universes, could have been spawned from this parent universe, and each of the universes could have its own Big Bang. The process of spawning new universes from a parent universe is called eternal inflation. The idea was developed by quantum cosmologists in the early 1980s. In brief, an unusual energy field (but one permitted by physics) in the protouniverse acts like antigravity and causes exponentially fast expansion. This unusual energy field has different strengths in different regions of space. Each such region expands to cosmic proportions, and the energy field becomes ordinary matter, forming a new universe that is closed off and completely out of contact with the protouniverse that sired it.

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, a physicist and novelist, teaches at MIT. His essay “Our Place in the Universe” appeared in the December 2012 issue of Harper’s Magazine.

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