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Some of my colleagues are doing experiments to test the multiverse.
This isn't an easy task. For one thing, by its definition, a multiverse is a collection of universes that include some that are outside our own -- specifically, separate universes that are impossible for us to interact with. So how do we test the theory that there are actually other universes, if they are by definition unobservable?
It turns out there's an idea that's been floating around for a while, first explored by Hiranya Peiris (one of the most rock-star astrophysicists I know), my Perimeter Institute colleague Matt Johnson (who is also awesome and fantastically creative). With a small group of others, they put together a proposal for a kind of observation that could provide evidence of other universes. The idea (discussed in this very nice Quanta Magazine article) is that if multiple universes were produced in the first moments of the cosmos, during the process of cosmic inflation, it's possible that one could have been born close enough to ours that we would have literally bumped into each other. That kind of cosmic collision would leave a kind of "bruise" in the cosmic microwave background (CMB), the background light of the Big Bang (the image above is a visualization of the CMB, projected so the whole sky is in an oval shape). So far, no such signature has been conclusively detected, though there are some parts of the CMB that seem to have patterns that could be consistent with a multiverse collision.
The source of the uncertainty here is not so much the data, but the theory. In this case, it's not even entirely clear what we should be looking for. The process by which these "bubble universes" could be created, and the rate at which they might appear, are both subject to calculations that are complicated enough to leave big question marks in the predictions. The basic idea is that these universe bubbles might be formed in a process where the cosmos undergoes a transition from one vacuum state to another. In this context, a vacuum state is sort of the configuration of the universe: the basic properties of physics in that space. A transition between vacuum states can be utterly cataclysmic (see the universe-ending scenario of vacuum decay), but such a transition could also be how the initial conditions of our own universe were set up.
Vacuum transitions occur when a certain kind of quantum tunneling event happens in one point in space, nucleating a bubble of the new vacuum, which then expands at about the speed of light. Both the nucleation of these bubbles, and the dynamics of their collisions, are hot topics, subject to a huge amount of effort in theory and numerical simulations.
So what do you do when the theory is hugely uncertain, and at the same time the experiment is at best impossible and at worst cosmically cataclysmic?
In this case, Peiris, Johnson, and their colleagues are exploring the topic through analog experiments: experiments with systems that seem to exhibit the same kind of dynamics, but in a safe, convenient table-top form. Although they're primarily theorists, they're working with experimental colleagues to use a Bose-Einstein condensate -- a kind of exotic matter in which quantum phenomena (like quantum interference) can be observed at macroscopic scales -- to watch how quantum phase transitions can imitate vacuum bubble nucleation and growth. The experiments are still in progress, but they're an exciting new way to learn about the early universe (and perhaps cosmic doom as well). Check out the collaboration webpage here for more info on these and other analog experiments.
Part of why I'm bringing this all up is that I was recently contacted by journalist Miriam Frankel of New Scientist Magazine to give comments about this whole research program. The only part of the conversation that made it into the article was the following:
Verifying an experimental analogue with theory, whilst also trying to verify the theory with the experiment, is incredibly difficult. “But this is pretty much how all of science is done, and the best we can do when our observational data of what happened in the early universe is so limited,” says Katie Mack, a cosmologist at the Perimeter Institute, who feels the experiment is an important one.
The more I think about it, the more I feel that this back-and-forth between experiment and theory is a really key (and perhaps under-appreciated) feature of scientific exploration. When we learn science in school, experiments are presented as a way to test theories, as though the theories appear out of nothing (like bubbles in the vacuum), and then they're either confirmed or struck down by the data. But the practical reality is that theories ultimately are created as attempts to explain experiments (or observations), and they don't always arise fully formed and complete. In physics and cosmology, when we compare data to a theory prediction, we always attach error bars to the data points to represent the uncertainty in the measurements, but much of the time, we blur out the theory line too, to account for the fact that we're not 100% sure what the theory really predicts. This bubble nucleation project is just one example in which the calculations are incredibly complicated and based on physics that is not yet understood, so it may be that we don't fully understand the theory until we do the experiment, at the same time as we're trying to use the experiment to see if we're on the right track with theory. I was recently talking with a CERN physicist who said something similar about experiments with heavy ions at CERN -- the theory of the strong force interaction is just way too complicated for the predictions to be made without experimental input. We're navigating and exploring the landscape based on a map we haven't yet drawn. But somehow, hopefully, we're finding our way.
If you want to read more about the vacuum bubble experiments, you can check out the New Scientist article linked in the Cosmic Conversations section below. (But note that it requires a New Scientist subscription.)
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