If this applet doesn't work on your browser, try a simpler version.
The little "worms" crawling across the balloon are photons. Their color changes due to the redshift, and I have attempted to make this a smooth variation but most browsers use a rather discontinuous default set of colors so both the colors and the brightness will jump.
The white S's which rotate are the galaxies in the model.
Since this analogy uses a spherical spatial section, it corresponds to a closed Universe which recollapses. The balloon will expand to just touch the edges of the box, and then shrink toward the Big Crunch. At the Big Crunch, the JAVA applet will restart with a new set of galaxies. But we have no way of knowing whether a closed Universe will re-expand after the Big Crunch, since the Big Crunch, like the Big Bang, is a singularity.
If Ho = 65 km/sec/ Mpc, and Omegao = 2, then the height of the box corresponds to 60 billion light years, and the time from Big Bang to Big Crunch is 94.5 billion years. The R and t labels at the bottom of the applet give the radius of the model in billions of light years and the time since the Big Bang in billions of years for these assumed cosmological parameters.
Note that the galaxies do not expand. The stars in a galaxy are in orbit in the potential of the galaxy. The only effect the expansion of the Universe has on these orbits is due to the reduction in the background density. Since galaxies are one million times denser than the average density of the Universe, this effect is very small. The average density of the Solar System is a quintillion times higher than the average density of the Universe, so the effects in the Solar System are miniscule. The electrostatic forces in an atom are 1067 times larger than the gravitational force due to the background density of the Universe, so the effects on atoms are infinitesimal.
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