Credit: iStockphoto
Cosmologists are not alone in postulating multiple universes. String theory suggests that our universe contains extra dimensions – most of which are curled up and so tiny we can't see them – that underlie our particular set of physical laws. Many scientists wondered why our way of curling up the extra dimensions should be the only possible way.
In string theory, another universe could have as many as 10 dimensions of space, with seven curled up. The types and masses of fundamental particles, as well as the varieties and strengths of fundamental forces, could vary in an uncountable number of ways.
String theory predicts eternal inflation that "will create lots of different universes," says Aguirre, "or regions where inflation stops, but they'll correspond, each of them, to different ways that those extra dimensions are curled up, and have different properties for the universe that they form."
From the inside, each universe appears infinite. So the woman who missed the train in her universe can never know about the woman who caught it in the other; in a sense, she's trapped in her bubble.
But, like the foam in a sink full of soapy water, these bubbles could run into each other. For a long time scientists thought that, while collisions almost certainly happen, the chances of seeing one would be low. Or if you did see one, it would be fatal. "The bubble running into our bubble would come into our bubble and destroy us," Aguirre says. "And since that obviously hasn't happened, there's some reason."
With those things in mind, Guth and two colleagues calculated how likely it would be to see a bubble collision in our observable region of the universe – a tiny part of one bubble. They published their study in the journal Physical Review D and posted it online in December 2006.
"We concluded that most observers in bubble universes live very far away from the collision regions and will not see any signatures of the collisions," says co-author Alex Vilenkin, of Tufts University in Medford, Massachusetts. He says they assumed that new bubbles form at a slow rate, so "bubble collisions are not very frequent."
That means the woman on the train would never know if her bubble crashed into the bubble of the woman who missed the train, because the first woman is too far away from the place where the bubbles hit. Even if the collision destroyed part of one of the bubbles, the unaffected region each woman sees would still look infinite from her standpoint. Then again, a collision might wipe out the other bubble.
Aguirre and his collaborators took another view. "Why should one of those collisions necessarily be so devastating?" he wondered. And what if a gentle collision happened close enough that you could see it?
In another study, Aguirre's team tried to imagine what the collision would look like. and came up with "a disc on the sky," which "might be infinitesimally tiny in some cases, or the whole sky in other cases."
But a question remained about when bubble collisions would be fatal to the observer. "And when might they just 'ping' the bubble the observer is in, not really disrupting it too much, but leaving some kind of signature?" Aguirre says, an indicator that someone in the bubble could see and say, "Oh look, there's another universe."
This is the scenario he postulated in a study with postdoctoral researcher Matthew Johnson of the California Institute of Technology in Pasadena, which they posted online on arXiv.org in December 2007.
They found in many instances a collision wouldn't necessarily be fatal, but might be seen as a disturbance of the microwave background. What it would look like, Aguirre says, isn't something they've been able to calculate. But if it was just one bubble, or even if it was many bubbles, it would appear to come primarily from one direction.
Imagine just one bubble bumping up against another, the situation cosmologist Kleban and his collaborators considered in their study, posted at arXiv.org just days before Aguirre posted his. "If you're inside one of them," says Kleban, "obviously there is one direction where one bubble came from, and you'll see something special in that direction."
