Multiple universes are accessed, in fiction, through portals in space or mystical necklaces or sometimes just in dreams, but always when characters break the rules of space time. In reality, alternate universes are not in other dimensions. They're just far, far away. And the reason they are alternate universes is that they can't be reached no matter what. But can they be tested for?
A work to be published in the Physical Review D posits a test, and some interesting conclusions, for multiverse theory. Multiple, or alternate, universes might, less excitingly, be called 'very far away places in this universe.' The universe is expanding, and has been since the Big Bang. As it expands, certain bubbles of it, also expanding, are pulled so far away so fast that their light can never reach this universe again. Essentially, they are parts of the universe that humans from this universe can never see, never feel any effects from, and never reach. They are, for all practical purposes, different universes that could be developing in entirely different ways.
But they, as well as every other universe including this one, interact with the Cosmic Microwave Background, or CMB. They leave patterns on the cosmic microwave background, which physicists have worked out would look 'disc shaped.' There is already a greatly detailed survey of the CMB done by the Wilkinson Microwave Anisotropy Probe. Scientists have had a computer check the CMB for the 'signature' of alternate universes, and have found four.
Or have they? The scientists have found the signatures of alternate universe 'bubbles' in areas where the multiverse theory is more likely than any other to explain why the Cosmic Microwave Background looks the way it does in that area, but when looking at the entire universe, four patches of weirdness does not a multiverse make. Even the scientist in charge of the team checking for the multiverse, Dr. Hiranya Peiris, openly states that the current data isn't enough. Still, it might be fun to think what's out there. World Without Shrimp? Vogons?
Via the BBC.