Editing Roger Penrose (section)

=== An earlier universe ===
[[File:WMAP 2010.png|thumb|300px|[[Wilkinson Microwave Anisotropy Probe|WMAP]] image of the (extremely tiny) anisotropies in the [[cosmic background radiation]]]]

In 2010, Penrose reported possible evidence, based on concentric circles found in [[WMAP|Wilkinson Microwave Anisotropy Probe]] data of the [[CMB|cosmic microwave background]] sky, of an earlier universe existing before the [[Big Bang]] of our own present universe.<ref name="Gurzadyan">{{cite arXiv |eprint=1011.3706 |class=astro-ph.CO |first1=V. G. |last1=Gurzadyan |first2=R. |last2=Penrose |title=Concentric circles in WMAP data may provide evidence of violent pre-Big-Bang activity". volume "v1 |date=2010}}</ref> He mentions this evidence in the epilogue of his 2010 book ''[[Cycles of Time]]'',<ref>Roger Penrose, ''Cycles of Time'', Vintage; Reprint edition (1 May 2012)</ref> a book in which he presents his reasons, to do with [[Einstein's field equations]], the [[Weyl curvature]] C, and the [[Weyl curvature hypothesis]] (WCH), that the transition at the Big Bang could have been smooth enough for a previous universe to survive it.<ref>{{Cite journal|last=Stoica|first=Ovidiu-Cristinel|date=November 2013|title=On the Weyl Curvature Hypothesis|journal=Annals of Physics|volume=338|pages=186–194|doi=10.1016/j.aop.2013.08.002|arxiv=1203.3382|bibcode=2013AnPhy.338..186S|s2cid=119329306}}</ref><ref>{{cite conference |author=Penrose |first=Roger |date=1979 |editor-last=Hawking |editor-first=S. W. |editor2=Israel |editor2-first=W. |title=Singularities and Time-Asymmetry |conference= |publisher=[[Cambridge University Press]] |pages=581–638 |book-title=General Relativity: An Einstein Centenary Survey}}</ref> He made several conjectures about C and the WCH, some of which were subsequently proved by others, and he also popularized his [[conformal cyclic cosmology]] (CCC) theory.<ref>{{Cite web|date=21 August 2018|title=New evidence for cyclic universe claimed by Roger Penrose and colleagues|url=https://physicsworld.com/a/new-evidence-for-cyclic-universe-claimed-by-roger-penrose-and-colleagues/|access-date=7 October 2020|website=Physics World|language=en-GB|archive-date=1 November 2020|archive-url=https://web.archive.org/web/20201101015848/https://physicsworld.com/a/new-evidence-for-cyclic-universe-claimed-by-roger-penrose-and-colleagues/|url-status=live}}</ref> In this theory, Penrose postulates that at the end of the universe all matter is eventually contained within black holes, which subsequently evaporate via [[Hawking radiation]]. At this point, everything contained within the universe consists of [[photons]], which "experience" neither time nor space. There is essentially no difference between an infinitely large universe consisting only of photons and an infinitely small universe consisting only of photons. Therefore, a singularity for a [[Big Bang]] and an infinitely expanded universe are equivalent.<ref>{{Cite web|url = https://physicsworld.com/a/new-evidence-for-cyclic-universe-claimed-by-roger-penrose-and-colleagues/|title = New evidence for cyclic universe claimed by Roger Penrose and colleagues|date = 21 August 2018|access-date = 7 October 2020|archive-date = 1 November 2020|archive-url = https://web.archive.org/web/20201101015848/https://physicsworld.com/a/new-evidence-for-cyclic-universe-claimed-by-roger-penrose-and-colleagues/|url-status = live}}</ref>

In simple terms, Penrose believes that the singularity in [[Einstein's field equation]] at the Big Bang is only an apparent singularity, similar to the well-known apparent singularity at the [[event horizon]] of a [[black hole]].<ref name="Curiel 2020"/> The latter singularity can be removed by a change of [[coordinate system]], and Penrose proposes a different change of coordinate system that will remove the singularity at the big bang.<ref>{{Cite book|last=Penrose|first=Roger|url=https://books.google.com/books?id=9XKYDwAAQBAJ&q=roger+penrose+%22change+of+coordinate%22&pg=PA55|title=Fashion, Faith, and Fantasy in the New Physics of the Universe|date=5 September 2017|publisher=Princeton University Press|isbn=978-0-691-17853-0|language=en|access-date=12 October 2020|archive-date=7 December 2021|archive-url=https://web.archive.org/web/20211207185551/https://books.google.com/books?id=9XKYDwAAQBAJ&q=roger+penrose+%22change+of+coordinate%22&pg=PA55|url-status=live}}</ref> One implication of this is that the major events at the Big Bang can be understood without unifying general relativity and quantum mechanics, and therefore we are not necessarily constrained by the [[Wheeler–DeWitt equation]], which disrupts time.<ref>{{Cite journal|last=Kiefer|first=Claus|date=13 August 2013|title=Conceptual Problems in Quantum Gravity and Quantum Cosmology|journal=ISRN Mathematical Physics|volume=2013|pages=1–17|doi=10.1155/2013/509316|language=en|doi-access=free|arxiv=1401.3578}}</ref><ref>{{Cite arXiv |last=Vaas |first=Rüdiger |title=The Inverted Big-Bang <!-- unsupported parameter |url=https://arxiv.org/pdf/physics/0407071.pdf --> |year=2004 |eprint=physics/0407071}}</ref> Alternatively, one can use the Einstein–Maxwell–Dirac equations.<ref>{{Cite web |last1=Finster |first1=F. |last2=Smoller |first2=J. A. |last3=Yau |first3=S. -T. |title=The Einstein–Dirac–Maxwell Equations – Black Hole Solutions |url=https://cds.cern.ch/record/403056/files/9910030.pdf |url-status=live |archive-url=https://web.archive.org/web/20201007132312/https://cds.cern.ch/record/403056/files/9910030.pdf |archive-date=7 October 2020 |access-date=7 October 2020}}</ref>