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Hubble's law

An analogy for explaining Hubble's law, using raisins in a rising loaf of bread in place of galaxies. If a raisin is twice as far away from a place as another raisin, then the farther raisin would move away from that place twice as quickly.
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Hubble's law, also known as the Hubble–Lemaître law,[1] is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving away. For this purpose, the recessional velocity of a galaxy is typically determined by measuring redshift, a shift in the light it emits toward the red end of the visible light spectrum. The discovery of Hubble's law is attributed to work published by Edwin Hubble in 1929.[2]

Hubble's law is considered the first observational basis for the expansion of the universe, and today it serves as one of the pieces of evidence most often cited in support of the Big Bang model.[3][4] The motion of astronomical objects due solely to this expansion is known as the Hubble flow.[5] It is described by the equation v = H0D, with H0 the constant of proportionality—the Hubble constant—between the "proper distance" D to a galaxy (which can change over time, unlike the comoving distance) and its speed of separation v, i.e. the derivative of proper distance with respect to the cosmic time coordinate. (See Comoving and proper distances § Uses of the proper distance for discussion of the subtleties of this definition of velocity.)

The Hubble constant is most frequently quoted in km/s/Mpc, which gives the speed of a galaxy 1 megaparsec (3.09×1019 km) away as 70 km/s. Simplifying the units of the generalized form reveals that H0 specifies a frequency (SI unit: s−1), leading the reciprocal of H0 to be known as the Hubble time (14.4 billion years).

The Hubble constant can also be stated as a relative rate of expansion. In this form H0 = 7%/Gyr, meaning that at the current rate of expansion it takes one billion years for an unbound structure to grow by 7%.

Although widely attributed to Edwin Hubble,[6][7][8] the notion of the universe expanding at a calculable rate was first derived from general relativity equations in 1922 by Alexander Friedmann. Friedmann published a set of equations, now known as the Friedmann equations, showing that the universe might be expanding, and presenting the expansion speed if that were the case.[9] Before Hubble, German astronomer Carl Wilhelm Wirtz had, in two publications dating 1922 [10] and 1924,[11] already deduced with his own data that galaxies that appeared smaller and dimmer had larger redshifts and thus that more distant galaxies recede faster from the observer. Then Georges Lemaître, in a 1927 article, independently derived that the universe might be expanding, observed the proportionality between recessional velocity of, and distance to, distant bodies, and suggested an estimated value for the proportionality constant; this constant, when Edwin Hubble confirmed the existence of cosmic expansion and determined a more accurate value for it two years later, came to be known by his name as the Hubble constant.[3][12][13][14][2] Hubble inferred the recession velocity of the objects from their redshifts, many of which were earlier measured and related to velocity by Vesto Slipher in 1917.[15][16][17] Combining Slipher's velocities with Henrietta Swan Leavitt's intergalactic distance calculations and methodology allowed Hubble to better calculate an expansion rate for the universe.[18]

Though the Hubble constant H0 is constant at any given moment in time, the Hubble parameter H, of which the Hubble constant is the current value, varies with time, so the term constant is sometimes thought of as somewhat of a misnomer.[19][20]

  1. ^ "IAU members vote to recommend renaming the Hubble law as the Hubble–Lemaître law" (Press release). IAU. 29 October 2018. Retrieved 2018-10-29.
  2. ^ a b Hubble, E. (1929). "A relation between distance and radial velocity among extra-galactic nebulae". Proceedings of the National Academy of Sciences. 15 (3): 168–173. Bibcode:1929PNAS...15..168H. doi:10.1073/pnas.15.3.168. PMC 522427. PMID 16577160.
  3. ^ a b Overbye, Dennis (20 February 2017). "Cosmos Controversy: The Universe Is Expanding, but How Fast?". New York Times. Retrieved 21 February 2017.
  4. ^ Coles, P., ed. (2001). Routledge Critical Dictionary of the New Cosmology. Routledge. p. 202. ISBN 978-0-203-16457-0.
  5. ^ "Hubble Flow". The Swinburne Astronomy Online Encyclopedia of Astronomy. Swinburne University of Technology. Retrieved 2013-05-14.
  6. ^ van den Bergh, S. (August 2011). "The Curious Case of Lemaitre's Equation No. 24". Journal of the Royal Astronomical Society of Canada. 105 (4): 151. arXiv:1106.1195. Bibcode:2011JRASC.105..151V.
  7. ^ Nussbaumer, H.; Bieri, L. (2011). "Who discovered the expanding universe?". The Observatory. 131 (6): 394–398. arXiv:1107.2281. Bibcode:2011Obs...131..394N.
  8. ^ Way, M.J. (2013). "Dismantling Hubble's Legacy?" (PDF). In Michael J. Way; Deidre Hunter (eds.). Origins of the Expanding Universe: 1912-1932. ASP Conference Series. Vol. 471. Astronomical Society of the Pacific. pp. 97–132. arXiv:1301.7294. Bibcode:2013ASPC..471...97W.
  9. ^ Friedman, A. (December 1922). "Über die Krümmung des Raumes". Zeitschrift für Physik (in German). 10 (1): 377–386. Bibcode:1922ZPhy...10..377F. doi:10.1007/BF01332580. S2CID 125190902.. (English translation in Friedman, A. (December 1999). "On the Curvature of Space". General Relativity and Gravitation. 31 (12): 1991–2000. Bibcode:1999GReGr..31.1991F. doi:10.1023/A:1026751225741. S2CID 122950995.)
  10. ^ Wirtz, C. W. (April 1922). "Einiges zur Statistik der Radialbewegungen von Spiralnebeln und Kugelsternhaufen". Astronomische Nachrichten. 215 (17): 349–354. Bibcode:1922AN....215..349W. doi:10.1002/asna.19212151703.
  11. ^ Wirtz, C. W. (1924). "De Sitters Kosmologie und die Radialbewegungen der Spiralnebel". Astronomische Nachrichten. 222 (5306): 21–26. Bibcode:1924AN....222...21W. doi:10.1002/asna.19242220203.
  12. ^ Lemaître, G. (1927). "Un univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques". Annales de la Société Scientifique de Bruxelles A (in French). 47: 49–59. Bibcode:1927ASSB...47...49L. Partially translated to English in Lemaître, G. (1931). "Expansion of the universe, A homogeneous universe of constant mass and increasing radius accounting for the radial velocity of extra-galactic nebulae". Monthly Notices of the Royal Astronomical Society. 91 (5): 483–490. Bibcode:1931MNRAS..91..483L. doi:10.1093/mnras/91.5.483.
  13. ^ Livio, M. (2011). "Lost in translation: Mystery of the missing text solved". Nature. 479 (7372): 171–173. Bibcode:2011Natur.479..171L. doi:10.1038/479171a. PMID 22071745. S2CID 203468083.
  14. ^ Livio, M.; Riess, A. (2013). "Measuring the Hubble constant". Physics Today. 66 (10): 41–47. Bibcode:2013PhT....66j..41L. doi:10.1063/PT.3.2148.
  15. ^ Slipher, V.M. (1917). "Radial velocity observations of spiral nebulae". The Observatory. 40: 304–306. Bibcode:1917Obs....40..304S.
  16. ^ Longair, M. S. (2006). The Cosmic Century. Cambridge University Press. p. 109. ISBN 978-0-521-47436-8.
  17. ^ Nussbaumer, Harry (2013). "Slipher's redshifts as support for de Sitter's model and the discovery of the dynamic universe" (PDF). In Michael J. Way; Deidre Hunter (eds.). Origins of the Expanding Universe: 1912–1932. ASP Conference Series. Vol. 471. Astronomical Society of the Pacific. pp. 25–38. arXiv:1303.1814.
  18. ^ "1912: Henrietta Leavitt Discovers the Distance Key". Everyday Cosmology. Retrieved 18 February 2024.
  19. ^ Overbye, Dennis (25 February 2019). "Have Dark Forces Been Messing With the Cosmos? – Axions? Phantom energy? Astrophysicists scramble to patch a hole in the universe, rewriting cosmic history in the process". The New York Times. Retrieved 26 February 2019.
  20. ^ O'Raifeartaigh, Cormac (2013). "The Contribution of V.M. Slipher to the discovery of the expanding universe" (PDF). Origins of the Expanding Universe: 1912-1932. ASP Conference Series. Vol. 471. Astronomical Society of the Pacific. pp. 49–62. arXiv:1212.5499.

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