Balloon Analogy in Cosmology

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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.

Note that the galaxies (yellow blobs) do not grow, but the distance between galaxies grows, and that the photons move and shift from blue to red as the Universe expands.

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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