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Hartle–Hawking state
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{{Short description|Proposal concerning the state of the universe prior to the Planck epoch}} [[File:Big Bang Singularity and Hartle-Hawking State.jpg|thumb|400x400px|''[[Big Bang]]'' and Hartle–Hawking State diagram.]] The '''Hartle–Hawking state''', also known as the '''no-boundary wave function''' is a proposal in [[theoretical physics]] concerning the state of the [[universe]] prior to the [[Planck epoch]].<ref>{{Cite journal | volume = 28 | pages = 2960 | year = 1983 | doi = 10.1103/PhysRevD.28.2960 | last1 = Hartle | journal = Physical Review D | first1 = J. | title = Wave function of the Universe | last2 = Hawking | first2 = S. | issue = 12|bibcode = 1983PhRvD..28.2960H | s2cid = 121947045 }}</ref><ref>{{Cite journal |last=Lehners |first=Jean-Luc |date=June 2023 |title=Review of the no-boundary wave function |url=https://linkinghub.elsevier.com/retrieve/pii/S0370157323001904 |journal=Physics Reports |language=en |volume=1022 |pages=1–82 |doi=10.1016/j.physrep.2023.06.002|arxiv=2303.08802 }}</ref><ref>{{Cite journal |last=Weinberg |first=Steven |date=1989-01-01 |title=The cosmological constant problem |url=https://link.aps.org/doi/10.1103/RevModPhys.61.1 |journal=Reviews of Modern Physics |language=en |volume=61 |issue=1 |pages=1–23 |doi=10.1103/RevModPhys.61.1 |hdl=2152/61094 |issn=0034-6861|hdl-access=free }}</ref> It is named after [[James Hartle]] and [[Stephen Hawking]]. ==History== According to the Hartle–Hawking proposal, the universe has no origin as we would understand it: before the [[Big Bang]], which happened about 13.8 billion years ago, the universe was a [[Gravitational singularity|singularity]] in both space and time. Hartle and Hawking suggest that if we could travel backwards in time towards the beginning of the universe, we would note that quite near what might have been the beginning, time gives way to space so that there is only space and no time.<ref>{{cite web |last=Hawking |first=Stephen |date=1996 |title=The Beginning of Time |url=https://www.hawking.org.uk/in-words/lectures/the-beginning-of-time |url-status=live |archive-url=https://web.archive.org/web/20141006200729/http://www.hawking.org.uk/the-beginning-of-time.html |archive-date=6 October 2014 |access-date=10 March 2014 |website=Hawking.org.uk}}</ref> == 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'' − 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]]. == See also == {{Portal|Physics}} * [[Imaginary time]] * [[Multiple histories]] * [[Signature change]] ==References== {{Reflist}} {{Stephen Hawking}} {{DEFAULTSORT:Hartle-Hawking state}} [[Category:Physical cosmology]] [[Category:Stephen Hawking]]
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