The Expanding Balloon Analogy

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Click in the box to restart the Universe!

Even though this animation shows a spherical Universe, the scale factor a(t) follows the law for a critical density Universe which expands forever. Eventually you will have totally blue box with no galaxies in it. A mouse click will restart the model with a new set of galaxies.

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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