What lies beyond all we can see? The question may seem unanswered. Still, some cosmologists have an answer: our universe is a bloated bubble. Outside there are more bubble universes, all immersed in an eternally expanding and energized sea – the multiverse.
The idea is polarizing. Some physicists embrace the multiverse to explain why our bubble looks so special (only certain bubbles can harbor life), while others reject the theory for not making testable predictions (since it predicts every imaginable universe). But some researchers believe they just weren’t smart enough to figure out the exact consequences of the theory.
Now, various teams are developing new ways to infer exactly how the bubbles in the multiverse are and what happens when those bubble universes collide.
“It’s a long way off,” said Jonathan Braden, a cosmologist at the University of Toronto who is involved in the effort, but, he said, it’s a search for evidence “for something you thought you could never test.
The multiverse hypothesis arose out of efforts to understand the birth of our own universe. In the large-scale structure of the universe, theorists see signs of an explosive growth spurt during the infancy of the cosmos. In the early 1980s, as physicists were studying how space might have started – and stopped – inflating, a disturbing picture emerged. The researchers realized that while space may have stopped inflating here (in our bubble universe) and there (in other bubbles), quantum effects should continue to inflate most of the space, an idea known as eternal inflation.
The difference between bubble universes and their surroundings comes down to the energy of the space itself. When space is as empty as possible and cannot waste any more energy, it exists in what physicists call a “true” state of vacuum. Think of a ball lying on the ground – it can no longer fall. But systems can also have “false” states of vacuum. Imagine a ball in a bowl on a table. The ball can roll a little while remaining more or less in place. But a big enough shake will land it on the ground – in true vacuum.
In the cosmological context, space can also get stuck in a false state of vacuum. A grain of false vacuum will occasionally relax into the true vacuum (possibly through a random quantum event), and that true vacuum will inflate outward in the form of an inflated bubble, feasting on the excess. energy from the false vacuum, in a process called false vacuum decay. It is this process that may have kicked off our cosmos. “A vacuum bubble could have been the first event in the history of our universe,” said Hiranya Peiris, a cosmologist at University College London.
But physicists are struggling to predict the behavior of vacuum bubbles. The future of a bubble depends on countless minute details that add up. Bubbles also change rapidly – their walls approach the speed of light as they fly outward – and exhibit quantum mechanical randomness and ripple. Different assumptions about these processes give conflicting predictions, with no way of telling which might look like the real thing. It’s like “you’ve taken a lot of things that are just really hard for physicists to handle and put them all together and said, ‘Go ahead and find out what’s going on,’ Braden said.
Since they cannot produce true vacuum bubbles in the multiverse, physicists have searched for digital and physical analogs.
One group recently squeezed out empty bubble-like behavior from a simple simulation. Researchers, including John preskill, a distinguished theoretical physicist at the California Institute of Technology, began with “the [most] baby version of this problem you can think of, ”as co-author Ashley Milsted put it: a line of about 1,000 number arrows that could point up or down. The place where a mostly upward chain of arrows met a mostly downward chain of arrows marked a bubble wall, and by flipping arrows, researchers could move the bubble walls and collide. In certain circumstances, this model perfectly mimics the behavior more complicated systems by nature. The researchers hoped to use it to simulate false vacuum decay and bubble collisions.
At first, the simple setup did not act realistically. When the bubble walls crashed, they bounced back perfectly, without any of the complex reverberations or particle releases expected (in the form of inverted arrows waving across the line). But after adding a few math flourishes, the team saw colliding walls that ejected energetic particles – with more particles appearing as the collisions became more violent.