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Inhomogeneous cosmology

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Unsolved problem in physics:
Is the universe homogeneous and isotropic at large enough scales, as claimed by the cosmological principle and assumed by all models that use the Friedmann–Lemaître–Robertson–Walker metric, including the current version of the ΛCDM model, or is the universe inhomogeneous or anisotropic?[1][2]
(more unsolved problems in physics)

An inhomogeneous cosmology is a physical cosmological theory (an astronomical model of the physical universe's origin and evolution) which, unlike the dominant cosmological concordance model, assumes that inhomogeneities in the distribution of matter across the universe affect local gravitational forces (i.e., at the galactic level) enough to skew our view of the Universe.[3] When the universe began, matter was distributed homogeneously, but over billions of years, galaxies, clusters of galaxies, and superclusters coalesced. Einstein's theory of general relativity states that they warp the space-time around them.

While the concordance model acknowledges this fact, it assumes that such inhomogeneities are not sufficient to affect large-scale averages of gravity observations. Two studies[4][5] claimed in 1998-1999 that high redshift supernovae were further away than the distance predicted by calculations. It was suggested that the expansion of the universe was accelerating, and dark energy, a repulsive energy inherent in space, was proposed as an explanation. Dark energy became widely accepted, but remains unexplained. Inhomogeneous cosmology falls into the class of models that might not require dark energy.

Inhomogeneous cosmologies assume that the backreactions of denser structures and those of empty voids on space-time are significant. When not neglected, they distort understanding of time and observations of distant objects. Burchert's equations in 1997 and 2000 derive from general relativity, but allow for the inclusion of local gravitational variations. Alternative models were proposed under which the acceleration of the universe was a misinterpretation of astronomical observations and in which dark energy is unnecessary.[6][7] For example, in 2007, David Wiltshire proposed a model (timescape cosmology) in which backreactions caused time to run more slowly or, in voids, more quickly, thus leading supernovae observed in 1998 to be thought to be further away than they were.[8][9] Timescape cosmology may also imply that the expansion of the universe is in fact slowing.[3]

  1. ^ Lee Billings (April 15, 2020). "Do We Live in a Lopsided Universe?". Scientific American. Retrieved March 24, 2022.
  2. ^ Migkas, K.; Schellenberger, G.; Reiprich, T. H.; Pacaud, F.; Ramos-Ceja, M. E.; Lovisari, L. (8 April 2020). "Probing cosmic isotropy with a new X-ray galaxy cluster sample through the LX-T scaling relation". Astronomy & Astrophysics. 636 (April 2020): 42. arXiv:2004.03305. Bibcode:2020A&A...636A..15M. doi:10.1051/0004-6361/201936602. S2CID 215238834. Retrieved 24 March 2022.
  3. ^ a b Gefter, Amanda (March 8, 2008). "Dark Energy Begone!". New Scientist. pp. 32–35.
  4. ^ Perlmutter, S.; Aldering, G.; Goldhaber, G.; Knop, R. A.; Nugent, P.; Castro, P. G.; Deustua, S.; Fabbro, S.; Goobar, A.; Groom, D. E.; Hook, I. M. (June 1999). "Measurements of Ω and Λ from 42 High-Redshift Supernovae". The Astrophysical Journal. 517 (2): 565–586. arXiv:astro-ph/9812133. Bibcode:1999ApJ...517..565P. doi:10.1086/307221. ISSN 0004-637X. S2CID 118910636.
  5. ^ Riess, Adam G.; Filippenko, Alexei V.; Challis, Peter; Clocchiatti, Alejandro; Diercks, Alan; Garnavich, Peter M.; Gilliland, Ron L.; Hogan, Craig J.; Jha, Saurabh; Kirshner, Robert P.; Leibundgut, B. (September 1998). "Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant". The Astronomical Journal. 116 (3): 1009–1038. arXiv:astro-ph/9805201. Bibcode:1998AJ....116.1009R. doi:10.1086/300499. S2CID 15640044.
  6. ^ Ehlers, Juergen; Buchert, Thomas (1997). "Averaging inhomogeneous Newtonian cosmologies". Astronomy and Astrophysics. 320: 1–7. arXiv:astro-ph/9510056. Bibcode:1997A&A...320....1B.
  7. ^ Buchert, Thomas (January 20, 2000). "On Average Properties of Inhomogeneous Cosmologies". Conference Proceedings, Theoretical Astrophysics Division, National Astronomical Observatory. 9: 306–321. arXiv:gr-qc/0001056. Bibcode:2000grg..conf..306B.
  8. ^ Wiltshire, David L (2007-10-22). "Cosmic clocks, cosmic variance and cosmic averages". New Journal of Physics. 9 (10): 377. arXiv:gr-qc/0702082. Bibcode:2007NJPh....9..377W. doi:10.1088/1367-2630/9/10/377. ISSN 1367-2630. S2CID 13891521.
  9. ^ Wiltshire, David L. (2007-12-20). "Exact Solution to the Averaging Problem in Cosmology". Physical Review Letters. 99 (25): 251101. arXiv:0709.0732. Bibcode:2007PhRvL..99y1101W. doi:10.1103/physrevlett.99.251101. ISSN 0031-9007. PMID 18233512. S2CID 1152275.
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