The vast space that we call our universe might be just a small speck in a fabric dotted with other universes, maybe an infinite number of them. We could be living in what physicists call a multiverse. It is a tantalizing prospect, but verifying it has remained in the realm of fantasy–until now. For the first time, the theorists have a testable prediction: Collisions with nearby universes could have left a mark in the cosmic microwave background, the fossil radiation of the big bang. Now, the hunt is on to detect the echoes of these collisions, which would provide the first experimental evidence for the existence of other universes.
Let’s back up a bit and see why physicists think there may be universes beyond our own in the first place. Fractions of a second after the big bang, our universe is thought to have expanded exponentially in a phase called inflation. Soon after inflation was proposed by the physicist Alan Guth, now of MIT, other physicists, including Andrei Linde of Stanford University and Alex Vilenkin of Tufts University, realized that once inflation got going, it should never end. According to this idea, now called “eternal inflation,” what we think of as the vacuum of space isn’t actually empty; it contains energy that makes it unstable and prone to form new bubble-like vacuums, much like bubbles of air emerging in boiling water. Each bubble inflates in turn, and new bubbles can form within it. In this view, our universe is just one of a huge and ever-increasing number of bubbles, each capable of giving rise to a new universe.
But how does one turn such esoteric theory into experimentally verifiable fact? Our best shot, according to Matthew Johnson of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and his colleagues, is to seek out evidence that one of these bubble universes collided with our own. Such a collision would create perturbations in the fabric of spacetime which would leave an imprint on the cosmic microwave background radiation.
The background radiation is the universe’s first light, made up of photons that were released throughout the universe a mere 370,000 years after the big bang. The photons contain information about the state of the universe at that instant, and that state would have been influenced by events close to the big bang—such as inflation itself or even a collision with other bubbles.
Johnson and his colleagues calculated that a collision would leave a distinct disc-shaped imprint on the background radiation. The temperature inside the imprint would be ever-so-slightly different from the temperature outside the disc. Properties such as the size of the disc, which could range from a few fractions of a degree to half the sky, and the intensity of the temperature difference between the inside and the outside of the disc, would depend on the exact nature of the collision.
The team then set about predicting how many collisions one should expect to find in the data collected by NASA’s WMAP satellite, a telescope designed specifically to probe the minute variations in the CMB, and what the traces of those collisions would look like to WMAP. Their result may be a bit of a downer for fans of the multiverse—they discovered that the data are most consistent with zero collisions–but that is not the end of the story. It is possible that the instrument simply isn’t sensitive enough to detect the traces of collisions with other universes. In fact, there are a few spots in the data that match the kind of signal expected from bubble collisions, but the result is not statistically significant. Random fluctuations in the background radiation could also create such patterns.
Still, Johnson is pleased. “Before this analysis these ideas seemed untestable,” he says. “It seemed like science fiction, but it’s not. It’s very exciting that you can rigorously do science in this theory. You might be unlucky and not see anything, but that’s kind of beside the point. It’s testable.”
Next week, discover what’s next in the search for signs of the multiverse.