Weyl Curvature Hypothesis in Light of Quantum Backreaction at Cosmological Singularities or Bounces

The Weyl curvature constitutes the radiative sector of the Riemann curvature tensor and gives a measure of the anisotropy and inhomogeneities of spacetime. Penrose’s 1979 Weyl curvature hypothesis (WCH) assumes that the universe began at a very low gravitational entropy state, corresponding to zero...

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Autor principal:	Bei-Lok Hu
Formato:	article
Lenguaje:	EN
Publicado:	MDPI AG 2021
Materias:	weyl curvature hypothesis early universe cosmology singularity and bounce cyclic universe quantum fields backreaction effects Elementary particle physics QC793-793.5
Acceso en línea:	https://doaj.org/article/61b531b028d44715b20140e416033411
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spelling	oai:doaj.org-article:61b531b028d44715b20140e4160334112021-11-25T19:09:39ZWeyl Curvature Hypothesis in Light of Quantum Backreaction at Cosmological Singularities or Bounces10.3390/universe71104242218-1997https://doaj.org/article/61b531b028d44715b20140e4160334112021-11-01T00:00:00Zhttps://www.mdpi.com/2218-1997/7/11/424https://doaj.org/toc/2218-1997The Weyl curvature constitutes the radiative sector of the Riemann curvature tensor and gives a measure of the anisotropy and inhomogeneities of spacetime. Penrose’s 1979 Weyl curvature hypothesis (WCH) assumes that the universe began at a very low gravitational entropy state, corresponding to zero Weyl curvature, namely, the Friedmann–Lemaître–Robertson–Walker (FLRW) universe. This is a simple assumption with far-reaching implications. In classical general relativity, Belinsky, Khalatnikov and Lifshitz (BKL) showed in the 70s that the most general cosmological solutions of the Einstein equation are that of the inhomogeneous Kasner types, with intermittent alteration of the one direction of contraction (in the cosmological expansion phase), according to the mixmaster dynamics of Misner (M). How could WCH and BKL-M co-exist? An answer was provided in the 80s with the consideration of quantum field processes such as vacuum particle creation, which was copious at the Planck time (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mn>10</mn><mrow><mo>−</mo><mn>43</mn></mrow></msup></semantics></math></inline-formula> s), and their backreaction effects were shown to be so powerful as to rapidly damp away the irregularities in the geometry. It was proposed that the vaccum viscosity due to particle creation can act as an efficient transducer of gravitational entropy (large for BKL-M) to matter entropy, keeping the universe at that very early time in a state commensurate with the WCH. In this essay I expand the scope of that inquiry to a broader range, asking how the WCH would fare with various cosmological theories, from classical to semiclassical to quantum, focusing on their predictions near the cosmological singularities (past and future) or avoidance thereof, allowing the Universe to encounter different scenarios, such as undergoing a phase transition or a bounce. WCH is of special importance to cyclic cosmologies, because any slight irregularity toward the end of one cycle will generate greater anisotropy and inhomogeneities in the next cycle. We point out that regardless of what other processes may be present near the beginning and the end states of the universe, the backreaction effects of quantum field processes probably serve as the best guarantor of WCH because these vacuum processes are ubiquitous, powerful and efficient in dissipating the irregularities to effectively nudge the Universe to a near-zero Weyl curvature condition.Bei-Lok HuMDPI AGarticleweyl curvature hypothesisearly universe cosmologysingularity and bouncecyclic universequantum fieldsbackreaction effectsElementary particle physicsQC793-793.5ENUniverse, Vol 7, Iss 424, p 424 (2021)
institution	DOAJ
collection	DOAJ
language	EN
topic	weyl curvature hypothesis early universe cosmology singularity and bounce cyclic universe quantum fields backreaction effects Elementary particle physics QC793-793.5
spellingShingle	weyl curvature hypothesis early universe cosmology singularity and bounce cyclic universe quantum fields backreaction effects Elementary particle physics QC793-793.5 Bei-Lok Hu Weyl Curvature Hypothesis in Light of Quantum Backreaction at Cosmological Singularities or Bounces
description	The Weyl curvature constitutes the radiative sector of the Riemann curvature tensor and gives a measure of the anisotropy and inhomogeneities of spacetime. Penrose’s 1979 Weyl curvature hypothesis (WCH) assumes that the universe began at a very low gravitational entropy state, corresponding to zero Weyl curvature, namely, the Friedmann–Lemaître–Robertson–Walker (FLRW) universe. This is a simple assumption with far-reaching implications. In classical general relativity, Belinsky, Khalatnikov and Lifshitz (BKL) showed in the 70s that the most general cosmological solutions of the Einstein equation are that of the inhomogeneous Kasner types, with intermittent alteration of the one direction of contraction (in the cosmological expansion phase), according to the mixmaster dynamics of Misner (M). How could WCH and BKL-M co-exist? An answer was provided in the 80s with the consideration of quantum field processes such as vacuum particle creation, which was copious at the Planck time (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mn>10</mn><mrow><mo>−</mo><mn>43</mn></mrow></msup></semantics></math></inline-formula> s), and their backreaction effects were shown to be so powerful as to rapidly damp away the irregularities in the geometry. It was proposed that the vaccum viscosity due to particle creation can act as an efficient transducer of gravitational entropy (large for BKL-M) to matter entropy, keeping the universe at that very early time in a state commensurate with the WCH. In this essay I expand the scope of that inquiry to a broader range, asking how the WCH would fare with various cosmological theories, from classical to semiclassical to quantum, focusing on their predictions near the cosmological singularities (past and future) or avoidance thereof, allowing the Universe to encounter different scenarios, such as undergoing a phase transition or a bounce. WCH is of special importance to cyclic cosmologies, because any slight irregularity toward the end of one cycle will generate greater anisotropy and inhomogeneities in the next cycle. We point out that regardless of what other processes may be present near the beginning and the end states of the universe, the backreaction effects of quantum field processes probably serve as the best guarantor of WCH because these vacuum processes are ubiquitous, powerful and efficient in dissipating the irregularities to effectively nudge the Universe to a near-zero Weyl curvature condition.
format	article
author	Bei-Lok Hu
author_facet	Bei-Lok Hu
author_sort	Bei-Lok Hu
title	Weyl Curvature Hypothesis in Light of Quantum Backreaction at Cosmological Singularities or Bounces
title_short	Weyl Curvature Hypothesis in Light of Quantum Backreaction at Cosmological Singularities or Bounces
title_full	Weyl Curvature Hypothesis in Light of Quantum Backreaction at Cosmological Singularities or Bounces
title_fullStr	Weyl Curvature Hypothesis in Light of Quantum Backreaction at Cosmological Singularities or Bounces
title_full_unstemmed	Weyl Curvature Hypothesis in Light of Quantum Backreaction at Cosmological Singularities or Bounces
title_sort	weyl curvature hypothesis in light of quantum backreaction at cosmological singularities or bounces
publisher	MDPI AG
publishDate	2021
url	https://doaj.org/article/61b531b028d44715b20140e416033411
work_keys_str_mv	AT beilokhu weylcurvaturehypothesisinlightofquantumbackreactionatcosmologicalsingularitiesorbounces
_version_	1718410218644176896

Weyl Curvature Hypothesis in Light of Quantum Backreaction at Cosmological Singularities or Bounces

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