In theoretical physics, top-down cosmology is a proposal to regard the many possible past histories of a given event as having real existence. This idea of multiple histories has been applied to cosmology, in a theoretical interpretation in which the universe has multiple possible cosmologies, and in which reasoning backwards from the current state of the universe to a quantum superposition of possible cosmic histories makes sense. Stephen Hawking has argued that the principles of quantum mechanics forbid a single cosmic history, and has proposed cosmological theories in which the lack of a past boundary condition naturally leads to multiple histories, called the 'no-boundary proposal', the proposed Hartle–Hawking state. According to Hawking and Thomas Hertog, "The top-down approach we have described leads to a profoundly different view of cosmology, and the relation between cause and effect. Top down cosmology is a framework in which one essentially traces the histories backwards, from a spacelike surface at the present time. The noboundary histories of the universe thus depend on what is being observed, contrary to the usual idea that the universe has a unique, observer independent history." See also Consistent histories Multiverse Quantum cosmology Hartle–Hawking state References Physical cosmology Quantum measurement