Cosmological Redshift Animation written by Paige Junghans, Samuel Knutson, Dr. Melissa Hayes-Gehrke, and Dr. Alberto Bolatto in 2011-2013. We acknowledge support from a CAREER award by the National Science Foundation, grant AST-0955836, and from a Cottrell Scholar award by the Research Corporation for Science Advancement, grant 19968. We also thank Reiner "Tiles" Prokein for the cannon animation images, available at http://www.reinerstilesets.de/.
The cannon in this applet starts out tilted at an angle of one degree to the ground. With each cannonball the cannon fires, this angle increases by one degree. The cannon stops tilting once the angle specified by the user at the beginning of the animation is reached.
Whenever the cannon reaches an angle with the ground that is a multiple of ten degrees, a path is drawn out in red. These red paths illustrate the fact that as the angle increases so does the path length of the cannonball.
As the animation runs, the wavelengths in the lower graph labeled "time of impact" start out significantly larger than the rest wavelength and then slowly decrease in length until the cannon stops tilting. At this point the wavelengths remain a constant length equal to the rest wavelength. This change in wavelength represents the idea that the universe is expanding.
Due to the fact that the universe is expanding, galaxies are getting farther and farther apart. This expansion causes light traveling between galaxies to stretch and results in longer wavelengths. The cannonballs in the animation represent this light traveling between galaxies as the universe expands. The cannonballs fired as the cannon is tilting travel a longer and different path than the cannonballs before. The longer paths result in longer wavelengths just like the longer wavelengths of light stretched as the universe expands. When the cannon stops tilting, the path length of the cannonballs stops changing and the wavelengths are no longer stretched. From this point onward the animation represents a universe with no expansion.
Normally a cannonball shot up into the air would decelerate as it travels upward, reach a maximum height, and then accelerate while traveling downward. However, the cannonballs in this applet move at constant velocities throughout their entire parabolic path because the speed of the expansion of the universe does not decelerate and then accelerate again.