Light from moving objects will appear to have different wavelengths depending on the relative motion of the source and the observer.
Observers looking at an object that is moving away from them see light that has a longer wavelength than it had when it was emitted (a redshift), while observers looking at an approaching source see light that is shifted to shorter wavelength (a blueshift).
Doppler Shift Simulation
Simulator courtesy of University of Nebraska-Lincoln
In 1929 Astronomer Edwin Hubble measured the redshift
of spectral lines of a number of distant galaxies. He also measured their relative distances by measuring the apparent brightness of a class of variable stars called Cepheids in each galaxy. When he plotted redshift against relative distance, he found that the redshift of distant galaxies increased as a linear function of their distance. The only explanation for this observation is that the universe was expanding.
After 10 years of work on the part of the Harvard-Smithsonian Center for Astrophysics, scientists (using the redshift of galaxies), have released the most complete 3-D map of the local universe (out to a distance of 380 million light-years) ever created. To see this 3-D map,