Redshift Surveys: The Universe in Three Dimensions

The Universe has three 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.

Redshifts and the Third Dimension

The standard way to obtain distances on larger scales is to use the Hubble law and the redshift measured 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, typically by examining narrow slices of the celestial sphere.

Example: Las Campanas Survey
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 three-dimensional picture of large-scale structure to be constructed. This information suggests structure on even larger scales than that of superclusters. The following image shows one such redshift effort, the Las Campanas redshift survey.

The figure corresponds to a superposition of three slices in declination covering a total of 9 degrees in the Northern Hemisphere and 6 degrees in the Southern Hemisphere. We are in the center and the distance from the center is the recessional velocity (redshift), which is related to distance from us by the Hubble law. The dots give the positions on the celestial sphere, and the distance (specified by the recessional velocity) of almost 24,000 galaxies. The structure in this survey has the character of soap bubbles, with filaments surrounding large open spaces called voids that contain little luminous matter.