This expansion can be seen easily in three dimensions if it is built with tinker toys. Using the round yellow blocks as galaxies, connect several together in a random three dimensional pattern using the shortest rods. Then expand, or have the students expand, the universe by connecting these same blocks (or different ones if you want to prepare the three models ahead of time) with the next shortest rods. Then expand again using the longest rods. This is a good model to show the expansion from all points in three dimensions, especially since the "galaxies" stay the same size during the expansion. This is because gravity and other forces hold planets, galaxies and clusters and objects within them together in spite of this universal expansion. For example, people do not stretch because of the expansion of the universe 32.
There are some things that do stretch, however. One important example of this is electromagnetic radiation, or light. Light occurs and is usually represented as waves like these which stretch, cool and grow more red:
A Stretching Light Wave
As you can see, as the wave stretches, the wavelength (the measurement of the length of the wave from peak to peak) increases. This is what happens to electromagnetic radiation in the expanding universe. Hot or High energy waves which occur on the blue end of the spectrum have shorter wavelengths which get stretched to cooler, redder, longer waves. There is an interesting example of this effect which astronomers have discovered and called the Cosmic Background Radiation. This is the very cool, stretched-out radiation left over from the beginning of the universe, or the Big Bang.
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