ESA Science & Technology03-Aug-2005 11:24:29

Galaxies and the Expanding Universe

Hubble's Law

In 1929 US astronomer Edwin Hubble first noticed that distant galaxies were moving away from us. He also realized that the further away they were, the faster they were moving and that this relationship was constant.

A copy of Hubble's original data with the velocity of recession plotted against the known distance.  The gradient of this plot is known as the Hubble Parameter, or Hubble Constant is defined in the following equation

Limitations of Hubble€™s Law

By using Hubble's equation, it is possible to determine a galaxy's distance from us, once we have measured its velocity of recession. This is calculated by determining by how much the observed light has been shifted into the red part of the spectrum. There are, however, several factors that make this measurement problematic:

  1. Galaxies often have intrinsic motions which influence the observed velocity.
  2. Gravitational movements caused by two galaxies orbiting each other, or by galaxies orbiting within a cluster of galaxies.

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