 In practice, cosmologists often take the physical state at the expected boundary of the domain of applicability of GR as the “initial state”. (For example, this might be taken as the state specified on a spatial hypersurface at a very early cosmic time. However, the domain of applicability of GR is not well understood, given uncertainty about quantum gravity.) Projecting observed features of the universe backwards leads to an initial state with three puzzling  features:[35] 