top-down cosmology

{{Short description|Idea of multiple histories into a complete unified theory .}}

In theoretical physics, top-down cosmology is a proposal to regard the many possible 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,{{Cite journal|last=Ball|first=Philip|date=2006-06-21|title=Hawking rewrites history... backwards|url=http://www.nature.com/articles/news060619-6|journal=Nature|language=en|pages=news060619–6|doi=10.1038/news060619-6|s2cid=122979772|issn=0028-0836|url-access=subscription}} 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.Spoon, M. (2021, February 22). How Stephen Hawking Worked. HowStuffWorks. https://science.howstuffworks.com/dictionary/famous-scientists/physicists/stephen-hawking3.htm

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."{{Cite journal|last1=Hawking|first1=S. W.|last2=Hertog|first2=Thomas|date=2006-06-23|title=Populating the landscape: A top-down approach|url=https://link.aps.org/doi/10.1103/PhysRevD.73.123527|journal=Physical Review D|volume=73|issue=12|pages=123527|doi=10.1103/PhysRevD.73.123527|arxiv=hep-th/0602091|bibcode=2006PhRvD..73l3527H|s2cid=9856127}}