Imagine holding a spring (or a slinky) tightly between your hands. When you move your hands apart the spring stretches; when you move your hands closer together the spring compresses. What happens to the distance between each coil of the spring, when you stretch and compress the spring?

When dealing with light, the distance between each wave crest is called the wavelength. We can make an analogy between the coils of the spring and successive wave crests of light. Longer wavelengths (crests further apart)correspond to redder light, and shorter wavelengths (crests brought closer together) to bluer light. Just as moving your hands closer together or further apart changes the distance between the coils of a spring, moving toward or away from another object in the Universe causes light emitted from it to appear at shorter or longer wavelengths.

We know that the Universe is expanding away from us, in all directions. What does this imply about the light emitted from distance objects?

When we say that the Universe itself is expanding, we mean that every part of space is expanding away from every other part of space. If the Universe triples in size then a length of three original units will stretch to nine units, while a length of six original units will stretch to eighteen units. Objects which are furthest from us thus appear to be moving away most quickly (compare a twelve unit increase to a measly six!), because a large distance grows more than a small one does.

Imagine drawing dots on the surface of a balloon, to represent galaxies in space. What happens to the distances between the dots when you blow up the balloon? Which dots move the most quickly away from each other – those that are close together, or those that are widely separated?