Editing Hartle–Hawking state (section)

== Technical explanation ==

More precisely, the Hartle-Hawking state is a hypothetical [[Vector (geometric)|vector]] in the [[Hilbert space]] of a theory of [[quantum gravity]] that describes the [[wave function of the universe]]. 

It is a [[Functional (mathematics)|functional]] of the [[metric tensor]] defined at a (''D''&nbsp;&minus;&nbsp;1)-dimensional [[compact surface]], the universe, where ''D'' is the [[spacetime]] dimension. The precise form of the Hartle–Hawking state is the path integral over all ''D''-dimensional geometries that have the required [[induced metric]] on their boundary. According to the theory, [[time]], as it is currently observed, diverged from a three-state dimension after the universe was in the age of the [[Planck time]].<ref>{{Cite book |last=Barrow |first=John D. |url=https://archive.org/details/originofuniverse0000barr_d5b7/ |title=The origin of the universe |date=1994 |publisher=New York : BasicBooks |isbn=978-0-465-05354-4}}</ref>

Such a wave function of the universe can be shown to satisfy, approximately, the [[Wheeler–DeWitt equation]].