The idea that there is more than one universe, more than one version of reality, a multiverse if you will, is a very compelling one.
Especially if there are an infinite number of universes: there would have to be others identical – or almost identical – to our own. Somewhere out there could be another you, reading a blog nearly as good as this one.
It’s also an attractive idea for physicists, as it naturally leads to the Anthropic Principle, which is one of the easiest ways to explain why our universe is the way it is – and in particular why it seems to be just right to support intelligent life.
Basically, with an infinite number of possible universes, there has to eventually be one capable of supporting life; and of course, we can only exist in that type of universe.
You can decide yourself whether the Anthropic Principle is a truly satisfying explanation, or just an over-complicated attempt at getting out of explaining anything at all. The question for today is, can we ever know if it’s true?
Normally you’d have to say no: the whole idea of these being separate universes is that they’re, well, completely separate to our own. But there’s one multiverse theory in which each universe exists in a bubble, and if these bubbles were to collide, they’d leave telltale signs.
This from an idea called eternal inflation. Our universe is believed to have gone through a period of extremely rapid inflation early in its life (from around 10−36 to 10−32 seconds after the Big Bang). This inflation stopped when the universe reached a stable state.
But what if there was more space beyond the boundaries of our universe that didn’t ever reach stability? Our universe would be a bubble within this eternally inflating region; and if there’s one bubble, why not more? And what’s to stop them colliding with our own universe?
To see the signs of these collisions you’d have to see the edge of the universe, which of course is a very long way away. But fortunately at the start of the universe it used to be a lot closer, and we can still see the results of this in the leftover heat from the Big Bang, the Cosmic Microwave Background (CMB).
Full sky map of the cosmic microwave background. The red, green, orange and light blue areas are the areas most likely to be bubble collisions, but even then they're not very likely. The data comes from the Wilkinson Microwave Anisotropy Probe (WMAP).
A team of physicists from University College, London, and elsewhere, have come up with an algorithm for searching the CMB for circular patterns that could be the result of bubble collisions.
So, did they find any? Well, there were a few possibilities (see the image above), but they weren’t really significant enough to separate from random noise. The best they could really do is put an upper limit on how many collisions there are (less than 1.6, at 68% confidence).
Of course, no paper is complete without saying that further data will help their chances of finding something in future. But still: it’s pretty amazing to think that other universes could collide with our own, and we could detect them. You just wouldn’t want to be standing there when it happens.
And also, it’s worth pointing out that even if no other bubble universes ever collided with our own, that doesn’t mean they’re not out there somewhere. After all, the eternally inflating multiverse has to be pretty big.
Read more about this study in the recently published papers, Feeney SM, Johnson MC, Mortlock DJ & Peiris HV 2011, “First observational tests of eternal inflation”, Physical Review Letters 107, 071301 and Feeney SM, Johnson MC, Mortlock DJ & Peiris HV 2011, “First observational tests of eternal inflation: analysis methods and WMAP 7-year results”, Physical Review D 84, 043507.
Or, read a take-down of the media hype around this study – i.e., all the news articles claiming they did find something - on the blog Not Even Wrong.
