explaining the origin of dark energy and resolving other cosmological questions. Like the Ekpyrotic model, it involves colliding branes— in this case bouncing off each other.

Let’s run through the cycle that Steinhardt and Turok proposed. First, the branes collide, essentially wiping out all traces of what existed in the universe before. The resulting burst of power replicates what we construe as the Big Bang but with no singularity. Quantum fluctuations in the impacting brane seed the formation of structure in observable space—the galaxies we see today. As the colliding branes move apart, the vacuum energy of the universe changes. The resulting dark energy produces universal acceleration—driving the galaxies farther and farther apart. That’s the phase we’re in today. Then, as the universe evolves, it will exhaust its usable energy. With stars dying out—turning into white dwarfs, neutron stars, and black holes—entropy will build up more and more. However, as galactic recession speeds up, this measure of disorder will become more and more dilute. In due course, for any given region, it will effectively revert to zero.

Meanwhile, the branes will eventually reverse course and come together again. The visible universe will cease its acceleration and begin to slow down, heralding the calm before the storm. Then, there will be unmistakable harbingers of doom. As Steinhardt describes these signs:

Once the universe turns to the decelerating phase you still have roughly another 10 billion years to go. But finally, in the last few seconds you’d see some significant changes in the fundamental constants and that would be the hint that something is about to happen. You would notice that something really strange is happening in the universe. Some tremendous form of energy is building up all of a sudden. It would reach a crescendo, and then, bam, the universe would fill with matter and radiation. That would be the collision. You and I would be vaporized unless we were otherwise protected.

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