Balloon Analogy in Cosmology

The expanding balloon analogy for cosmological models is shown below at two different times. A common misconception is that the balloon is expanding into empty space that is "beyond the Universe" and that it is expanding from a single point in the center of the balloon. But the balloon analogy is a 2-dimensional model, and the center of the balloon and the space around are not part of the 2-dimensional Universe. In our 3-dimensional Universe, these points could only be reached by traveling in a 4th spatial dimension (not the time dimension of 4-D spacetime), but there is no evidence that this dimension exists.

This animation shows an expanding universe model with yellow blobs for galaxies and moving, redshifting photons. The animation starts at a redshift of 3 when the Universe was 4 times smaller than it is now, and finishes at the present. Taken literally, the ratio of the radius of the "balloon" to the Hubble distance c/H_o implies an Omega = 2 currently and Omega = 1.14 at z = 3. Note that the galaxies do not expand: bound systems are not affected by the expansion of the Universe. Also note that the speed of light relative to the nearby galaxies is a constant - if your browser is properly using the duration values in the animated GIF file.

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© 1998 Edward L. Wright. Last modified 8-May-1998