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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2021-11-01
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| Online Access: | https://www.mdpi.com/2218-1997/7/11/424 |
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| author | Bei-Lok Hu |
| author_facet | Bei-Lok Hu |
| author_sort | Bei-Lok Hu |
| collection | DOAJ |
| 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. |
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| issn | 2218-1997 |
| language | English |
| last_indexed | 2024-03-10T05:00:10Z |
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| spelling | doaj.art-61b531b028d44715b20140e4160334112023-11-23T01:50:26ZengMDPI AGUniverse2218-19972021-11-0171142410.3390/universe7110424Weyl Curvature Hypothesis in Light of Quantum Backreaction at Cosmological Singularities or BouncesBei-Lok Hu0Maryland Center for Fundamental Physics and Joint Quantum Institute, University of Maryland, College Park, MD 20742-4111, USAThe 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.https://www.mdpi.com/2218-1997/7/11/424weyl curvature hypothesisearly universe cosmologysingularity and bouncecyclic universequantum fieldsbackreaction effects |
| spellingShingle | Bei-Lok Hu Weyl Curvature Hypothesis in Light of Quantum Backreaction at Cosmological Singularities or Bounces Universe weyl curvature hypothesis early universe cosmology singularity and bounce cyclic universe quantum fields backreaction effects |
| title | 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_short | 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 |
| topic | weyl curvature hypothesis early universe cosmology singularity and bounce cyclic universe quantum fields backreaction effects |
| url | https://www.mdpi.com/2218-1997/7/11/424 |
| work_keys_str_mv | AT beilokhu weylcurvaturehypothesisinlightofquantumbackreactionatcosmologicalsingularitiesorbounces |