Our Bubbly Universe: Physicists Work to Test Multiverse Theory
Is our universe just one of many? Scientists have long theorized that a "multiverse" is possible, but have never been able to prove it or even discover a method of proving it. That might be about to change, as researchers at the Perimeter Institute, an international collaboration for theoretical physics research, are attempting to create a test for the multiverse theory.
According to the "eternal inflation" theory of the universe, in the seconds after the Big Bang, the universe expanded exponentially, causing gravitational waves. The universe has continued to expand since then, and continues to expand today, but at a much less accelerated rate. According to eternal inflation, the universe will continue to expand in perpetuity in some regions of the universe. Most eternal inflation models logically lead to a multiverse theory, in which many universes (or sections of spacetime with every conceivable set of physical laws) co-exist much like bubbles.
The multiverse theory has many prominent proponents, including Stephen Hawking, Steven Weinberg, and Brian Greene. However, the inflation theory of the universe is not universally accepted, and there are many respected physicists who claim that the multiverse theory enters the realm of metaphysics or even pseudoscience, particularly since it has historically been untestable.
Now, Perimeter Institute faculty member Matthew Johnson and his team are attempting to bring the multiverse theory from the field of metaphysics and philosophy of science into the purely scientific realm by creating a testable hypothesis. "We're trying to find out what the testable predictions of this picture would be, and then going out and looking for them," he said.
At its heart, Johnson's research involves using computer simulations to confirm hypotheses about the consequences of two "bubble" universes colliding with each other: "We simulate the whole universe. We start with a multiverse that has two bubbles in it, we collide the bubbles on a computer to figure out what happens, and then we stick a virtual observer in various places and ask what that observer would see from there." Their research is the first to create quantitative predictions that can be definitively proven using the scientific method.
He clarifies that it is possible, even easy, to create a simulation of the entire universe because only the most fundamental physical laws are relevant to his hypothesis: "Simulating the universe is easy. We're simulating things only on the largest scales. All I need is gravity and the stuff that makes these bubbles up. We're now at the point where if you have a favourite model of the multiverse, I can stick it on a computer and tell you what you should see."
While the research is still in its infancy, it has already broken ground in the sense that it has allowed for testable predictions that could eventually confirm or invalidate the multiverse theory. The simulation has already managed to effectively eliminate several conceptions of the multiverse theory: "We're now able to say that some models predict something that we should be able to see, and since we don't in fact see it, we can rule those models out," said Johnson. For example, one model tested by the simulation asserts that, in the case of a collision, a disc-shaped "bruise" on the cosmic microwave background will appear, but this disc is nowhere to be found. As a result, certain collision-based models are unlikely.