Is our universe just one of many? The “multiverse” has occupied the pages of theorists’ notebooks for decades. Now, astronomers are on the brink of testing this hypothesis as they begin the search for evidence of universes beyond our own.
Though the first test, using data from a satellite called WMAP, came up empty-handed, cosmologists are now turning their attention to fresh results from the European Space Agency’s Planck satellite, which is mapping the cosmic microwave background radiation and creating an all-sky temperature map with three times greater resolution than its predecessor. If other universes exist, it is possible that they have collided with our own universe. Physicists believe that such a collision would leave an imprint on the background radiation in the form of a disc-shaped region of very-slightly different temperature than the surrounding background. Planck’s improved resolution will sharpen the edges of any collision-induced disc in the background radiation.
Planck is also studying a property of the background radiation called polarization, which describes the angle at which the electromagnetic waves vibrate in relation to the direction they are traveling. (You encounter polarization every time you slip on polarized sunglasses; because sunlight reflected off the horizontal surface of a road or a body of water becomes polarized, these special lenses can selectively block it out, reducing glare.) Polarization is a sensitive probe of the conditions that prevailed when the photons were released. Though WMAP was not sensitive enough to see any patterns in the polarization of the photons, Planck, with three times more sensitivity, is expected to see such patterns, which just might contain the fingerprint of so-called “bubble collisions.”
What would such a signal look like? Matthew Kleban of New York University and his colleagues have shown that bubble collisions should leave two highly-polarized rings surrounding the temperature disc.
“According to our predictions for the probability of these bubble collisions, we are more likely to see larger discs than smaller discs,” says Kleban. “And it turns out that for larger discs, polarization is actually a very sensitive test. The signal is more distinctive, and it gets stronger as the disc gets bigger.”
So, if we did see such signatures in the Planck data, what would it prove?
“It would be conclusive of the fact that before our observable universe was formed, there was this precursor phase, and you could say with great certainty that that precursor phase still exists, somewhere, and that we are one small little pocket in a much, bigger multiverse,” says Matthew Johnson of the Perimeter Institute for Theoretical Physics. “It would be fairly direct evidence for the existence of a multiverse.”
That would be revolutionary. It would also focus attention on an unorthodox view of quantum mechanics which is already growing in popularity: the many-worlds hypothesis.
Proposed in the 1950s by physicist Hugh Everett, the many-worlds hypothesis takes a radical view of what happens to the wave function—the equation that spells out the probability of finding a quantum system in a particular state—when a measurement is made. In Niels Bohr’s Copenhagen Interpretation, any time we make a measurement, the wave function “collapses,” giving us one outcome from an infinity of possibilities. Everett argued that the wave function never collapses. Rather, every possibility exists in a parallel universe. This suggests a staggeringly large number of other worlds.
But are they the same “other worlds” predicted by eternal inflation? Recent work by Leonard Susskind of Stanford University and Raphael Bousso of the University of California, Berkeley, hints that the many worlds of quantum theory and the multiverse of eternal inflation might be two sides of the same coin. By linking eternal inflation with Everett’s many worlds, Susskind and Bousso hope to establish the physical meaning of the probabilistic predictions that have confounded quantum physicists for decades.
Yet even if bubble universes exist, the odds might be against spotting a collision. “Everyone thinks that we would have to be lucky,” says Susskind. “I would not try to estimate just how lucky, but at least somewhat lucky.” After all, our universe is much, much bigger than what we can see—so the collision may lie beyond our cosmic horizon.
Henry David Thoreau once wrote, “The universe is wider than our views of it.” That is true, of course. But the quest to find evidence of universes beyond our own shows that our “view” of the universe is a window that widens just as far as technology, theory, and the laws of physics can stretch it.