26.5 The expanding universe  (Page 7/27)

The expansion of the universe, by the way, does not imply that the individual galaxies and clusters of galaxies themselves are expanding. Neither raisins nor the ants in our analogy grow in size as the loaf expands. Similarly, gravity holds galaxies and clusters of galaxies together, and they get farther away from each other—without themselves changing in size—as the universe expands.

Key concepts and summary

The universe is expanding. Observations show that the spectral lines of distant galaxies are redshifted, and that their recession velocities are proportional to their distances from us, a relationship known as Hubble’s law. The rate of recession, called the Hubble constant, is approximately 22 kilometers per second per million light-years. We are not at the center of this expansion: an observer in any other galaxy would see the same pattern of expansion that we do. The expansion described by Hubble’s law is best understood as a stretching of space.

For further exploration

Articles

Andrews, B. “What Are Galaxies Trying to Tell Us?” Astronomy (February 2011): 24. Introduction to our understanding of the shapes and evolution of different types of galaxies.

Bothun, G. “Beyond the Hubble Sequence.” Sky&Telescope (May 2000): 36. History and updating of Hubble’s classification scheme.

Christianson, G. “Mastering the Universe.” Astronomy (February 1999): 60. Brief introduction to Hubble’s life and work.

Dalcanton, J. “The Overlooked Galaxies.” Sky&Telescope (April 1998): 28. On low-brightness galaxies, which have been easy to miss.

Freedman, W. “The Expansion Rate and Size of the Universe.” Scientific American (November 1992): 76.

Hodge, P. “The Extragalactic Distance Scale: Agreement at Last?” Sky&Telescope (October 1993): 16.

Jones, B. “The Legacy of Edwin Hubble.” Astronomy (December 1989): 38.

Kaufmann, G. and van den Bosch, F. “The Life Cycle of Galaxies.” Scientific American (June 2002): 46. On galaxy evolution and how it leads to the different types of galaxies.

Martin, P. and Friedli, D. “At the Hearts of Barred Galaxies.” Sky&Telescope (March 1999): 32. On barred spirals.

Osterbrock, D. “Edwin Hubble and the Expanding Universe.” Scientific American (July 1993): 84.

Russell, D. “Island Universes from Wright to Hubble.” Sky&Telescope (January 1999) 56. A history of our discovery of galaxies.

Smith, R. “The Great Debate Revisited.” Sky&Telescope (January 1983): 28. On the Shapley-Curtis debate concerning the extent of the Milky Way and the existence of other galaxies.

Websites

ABC’s of Distance: http://www.astro.ucla.edu/~wright/distance.htm. A concise summary by astronomer Ned Wright of all the different methods we use to get distances in astronomy.

Cosmic Times 1929: http://cosmictimes.gsfc.nasa.gov/online_edition/1929Cosmic/index.html. NASA project explaining Hubble’s work and surrounding discoveries as if you were reading newspaper articles.

Edwin Hubble: The Man Behind the Name: https://www.spacetelescope.org/about/history/the_man_behind_the_name/. Concise biography from the people at the Hubble Space Telescope.