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.
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."
- Ball, Philip (2006-06-21). "Hawking rewrites history... backwards". Nature: news060619–6. doi:10.1038/news060619-6. ISSN 0028-0836. S2CID 122979772.
- Hawking, S. W.; Hertog, Thomas (2006-06-23). "Populating the landscape: A top-down approach". Physical Review D. 73 (12): 123527. arXiv:hep-th/0602091. Bibcode:2006PhRvD..73l3527H. doi:10.1103/PhysRevD.73.123527. S2CID 9856127.