"Despite this excitement, Kleban warns that there are a number of reasons why we might not see any signs of universe collisions. First, our universe might truly be the only one. Or the signal from a collision may be too faint because the crash occurred too far away. Even if they are out there, finding signatures of other universes is a long shot, Kleban acknowledges. "The best reason for hope," he says, "is that we can't conclude it's hopeless." And that is saying something, when you are searching beyond the edge of everything we know."-http://discovermagazine.com/2012/dec/29-when-universes-collide#.ULZ8RtPjknV-No evidence of cosmic wave background circles of collisions