 There have been two distinct approaches to recovering some empirical content in this  situation.[54]  First, there may be traces of the early formation of the pocket universes, the remnants of collisions between neighboring “bubbles”, left on the CMB sky (Aguirre &amp; Johnson 2011). Detection of a distinctive signature that cannot be explained by other means would provide evidence for the multiverse. However, there is no expectation that a multiverse theory would generically predict such traces; for example, if the collision occurs too early the imprint is erased by subsequent inflationary expansion.