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 currently widely accepted 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 have coalesced, and must, according to Einstein's theory of general relativity, 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 in our observations. When two separate studies[4][5] claimed in 1998-1999 that high redshift supernovae were further away than our calculations showed they should be, it was suggested that the expansion of the universe is accelerating, and dark energy, a repulsive energy inherent in space, was proposed to explain the acceleration. Dark energy has since become widely accepted, but it remains unexplained. Accordingly, some scientists continue to work on models that might not require dark energy. Inhomogeneous cosmology falls into this class.

Inhomogeneous cosmologies assume that the backreactions of denser structures, as well as those of very empty voids, on space-time are significant enough that when not taken into account, they distort our understanding of time and our observations of distant objects. Following Thomas Buchert's publication of equations in 1997 and 2000 that derive from general relativity but also allow for the inclusion of local gravitational variations, a number of cosmological models were proposed under which the acceleration of the universe is in fact a misinterpretation of our astronomical observations and in which dark energy is unnecessary to explain them.[6][7] For example, in 2007, David Wiltshire proposed a model (timescape cosmology) in which backreactions have caused time to run more slowly or, in voids, more quickly, thus giving the supernovae observed in 1998 the illusion of being 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.