After reading the background information and working through all exercises of the Hubble's Law Simulator, the student will understand that...
In the 1920's, Edwin Hubble was studying galaxies. He took spectra and from the redshifts of the spectral lines determined the velocities of the galaxies. He then found pulsating variable stars (stars that periodically vary in brightness) known as Cepheid variables in the galaxies. Cepheids obey a period-luminosity relation meaning that larger brighter Cepheids pulsate with larger periods while smaller fainter Cepheids have smaller periods. Thus, observing the pulsation periods of the Cepheids allowed him to estimate their luminosities and ultimately the distances to the galaxies.
When he graphed the galaxy's velocities versus distance he found two interesting things:
Velocity v is equal to a constant H0 (pronounced H-naught) times distance D. H0 is known to have a value of about 70 km/sec/Mpc. So for each Mpc a galaxy is away from us, its velocity of recession increases by 70 km/s. A galaxy 1 Mpc away has a velocity of 70/km/s, a galaxy 2 Mpc away has a velocity of 140 km/s, and so on.
Note that we often talk about galaxies moving away from us, but what is really happening is that space itself is being created which pushes the galaxies away from us. There is more space between us and a distant galaxy than a nearby galaxy. Thus, when space itself expands, it pushes the distant galaxy away from us by a greater amount.
This astronomy "Little Big Picture" was programmed by REU student Nick Robe. It is an early effort of the UNL Astronomy Education Group to provide materials for mobile devices. More astronomy teaching materials can be found on the web at astro.unl.edu.
This simulation makes use of the Dojo tooklit available at dojotoolkit.org.