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Our universe may show bruises from smacking four other universes
We live inside a universe, that much is (more or less) certain. What's less certain is what exists outside our universe, but scientists now think that they might have spotted evidence of other universes, four of which seem to have smashed into us.
The multiverse theory (or one of them, anyway) suggests that just outside of our universe, other universes are appearing and disappearing, each in their own bubble of space-time. Generally, these universes don't get close enough to interact, but sometimes a universe will appear right next to ours, and when that happens, we get smacked.
Getting smacked by an entire universe would definitely leave a mark. Specifically, it would leave a disk-like pattern in our universe's cosmic microwave background radiation, and this is what a group of cosmologists from University College London have been looking for. Somewhat incredibly, they managed to find some of these bruises. Four of them. And it's ten times more likely that the four marks are universe collision signatures than that they are anything else that we know of.
Still, the overall statistical significance of the cosmic microwave background marks is not that great, and more data are necessary before we can definitively prove this whole bumper-car multiverse theory one way or another. Fortunately, a new microwave satellite is currently collecting data at three times the resolution of what was used in this study, and by sometime in 2013, we should get a definitive answer as to whether or not there are a whole bunch more universes floating around out there.