To Do and Notice Draw a sine wave with a wavelength of 1 cm on the unstretched rubber band. Measure the length of your rubber band. Stretch the rubber band to twice its original length. Notice what happens to the wavelength of the wave you have drawn. When you double the length of the rubber band by stretching it, the wavelength of the sine wave also doubles.
Light is a wave that travels through the vacuum of space.
When space expands so does the light that is traveling through the space. If the distance between two clusters of galaxies doubles, the light traveling through the vacuum between these galaxy clusters doubles in wavelength. The universe began with a big bang about 14 billion years ago. It was hotter and denser than it is now. It was so hot that all of the hydrogen atoms were ionized.