Themes > Science > Astronomy > The Galaxies > Groups, Clusters, and Superclusters of Galaxies > Redshift Surveys: the Universe in 3 Dimensions


The Universe has 3 spatial dimensions and in order to understand its geometry we need distance information to supplement the angular information obtained from locating galaxies on the celestial sphere.
Las Campanas redshift survey. The right ascension is given around the partial circles, and the figure corresponds to a superposition of 3 slices in declination (39-45 degrees). We are in the center, and the distance from the center is the recessional velocity, which is related to distance from us.

Redshifts and the Third Dimension

The standard way to obtain distances on larger scales is to use the Hubble law and the redshift measured from the Doppler effect on spectral lines. This is difficult, however, because to accumulate sufficient light to measure the redshift of distant objects accurately requires long observations.

Thus, deep space redshift surveys (that is, those out to very large distances) to date have been confined to limited regions, either by examining narrow slices of the celestial sphere, or by concentrating on a very small region of the celestial sphere but trying to observe objects as far away as possible in that region (pencil surveys).

Although such surveys are difficult and time-consuming, they have over the last decade or so begun to be available for limited portions of the sky and have begun to allow a full 3-dimensional picture of large-scale structure to be constructed. This information suggests structure on even larger scales than that of superclusters. The adjacent image shows one such redshift survey.

The 3D Structure of the Universe

From detailed studies of this sort the following picture begins to emerge from redshift surveys out to approximately 200 Mpc.
  • The 3-dimensional distribution of luminous matter has a "soap-bubble" appearance, with the visible galaxies mostly on the surface of these soap bubbles. This can be seen in the preceding image. It is perhaps even more apparent in this black and white version of the image.
  • The superclusters appear as elongated strands where different soap bubbles come together.
  • The clusters appear as bright spots on the strands corresponding to the superclusters.
  • The soap bubbles surround large voids of approximately 100 Mpc size that contain little easily seen matter.
  • The Universe is consistent with a soap-bubble structure, but NOT a ``spaghetti structure'' (3-dimensional data are required to make this distinction clearly; these situations can have a similar appearance when projected onto two dimensions.)
Such observations place strong constraints on our later discussions of the origin of structure in the Universe, since any viable theory of such structure must explain why the Universe has this appearance.


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