The past and the future are tightly linked in conventional quantum mechanics. Perhaps too tightly. A tweak to the theory could let quantum possibilities increase as space expands.

A tweak to quantum mechanics theory could let quantum possibilities increase as space expands. The expansion of space spells trouble for quantum mechanics, by presenting particles with a growing smorgasbord of options for where to be. Andrew Strominger and Jordan Cotler of Harvard University argue that a more relaxed principle called isometry can accommodate an expanding universe while still satisfying the stringent requirements that first made unitary a guiding light.