This Interactive allows you to play with the equations for the evolution of the Universe. The units on the graph are Time (in billions of years) on the horizontal axis and the Size of the Universe or "scale factor" on the vertical axis. The scale factor does not have any units on it and is just a measure of how much the distance between two points (say, two galaxies) will have increased up to a given time since the Big Bang. The approximate age of our Universe as determined by the oldest stars is marked with a green line.
The steps to follow to use this Interactive are:
1. Choose a value for the Hubble Constant H0. This will set the timescale of the Universe’s evolution. Small values of the Hubble constant make for Universes that evolve "slowly" while large values give rapidly evolving Universes.
2. Choose a value for the density parameter Omega. This will determine the fate of the Universe. The value Omega = 1 is the dividing line between Universes that expand forever and those that collapse back on themselves.
3. Hit the Big Bang button to launch your Universe.
4. Inside the movable frame you will see a representation of the Universe’s evolution in terms of the expansion or collapse of a uniform distribution of galaxies. Notice how the color of the galaxies change from red when they are expanding away from each other (redshifting) to blue (blueshifting) when they collapse back towards each other in Omega > 1 Universes.
5. You may create as many as 5 different Universes on the same screen, before pressing "Reset" and starting all over again.