Step One: EMPTY UNIVERSE

Each panel represents a three-dimensional membrane. The observable universe exists within the panel on the right; the other membrane is invisible to us. At the end of one cosmic cycle—after about a trillion years of accelerating expansion—matter is so spread out that space is essentially empty. This stage corresponds to the distant future of our universe. All is not static, however: The membranes still contain energy, and a force of attraction gradually draws them together again.

Inflation seemed like a necessary complexity. Without it, the universe would look very different—for instance, galaxies on one side of the universe would be distributed differently from galaxies on the other side, which they don't appear to be. As inflation caught on, however, some cosmologists grumbled about epicycles. Then the Big Bang got even more complicated. Starting about five years ago, astronomers measuring the expansion rate of the universe discovered that billions of years after the Big Bang—long after inflation had died out—cosmic expansion started speeding up again. Theorists invoked another unknown energy field, called dark energy, to account for that cosmic acceleration. "This wasn't really predicted at all," says Steinhardt. "We can fit it into the model, but we don't know what this so-called dark energy is. The standard model is definitely becoming more encumbered with time. It may still be valid, but the fact that we have to keep adding things is a bad sign."

Astronomical evidence clearly indicates that the observable universe has been expanding for the past 13.7 billion years. In the inflationary Big Bang model, the universe was hot and dense at the outset, and then immediately went through a period of hyperexpansion. Steinhardt and his colleagues considered a very different possibility: What if the universe actually started out cool and vacuous?


 If that were the case, the idea of branes colliding in a hidden dimension might provide a simpler explanation for the ongoing expansion. To find out whether the idea made sense, the pair took on the daunting task of mastering the equations of superstring theory and applying them to their theory. For simplicity, the researchers assumed that the branes were flat and parallel to each other. They also assumed that the branes contained no matter. That didn't mean the branes were voids: Quantum theory asserts that even the total vacuum of empty space is seething with "virtual"  subatomic particles that constantly wink in and out of existence. In aggregate, these virtual particles add up to a huge amount of latent energy—which, according to Einstein's theory of special relativity, is equivalent to an astounding amount of mass. So a crash between two empty branes would still be a collision of gigantic proportions.

Step Two: FIERY COLLISION

As the two membranes draw nearer to each other, they ripple and distort so that the surfaces come together in different places and at different times. As the membrane surfaces crash into each other, vast amounts of energy are released (white zone). Called ekpyrosis—the Greek word for conflagration—the colossal collision gives birth to a baby universe in our membrane (right). The force of the impact causes space to expand rapidly and also pushes the two membranes apart.