How do we know the universe is getting bigger?
To understand the evidence that the universe is getting larger, we need to understand the Doppler effect. This is the reason why a siren from a police car or fire truck has a higher pitch as it is approaching and a lower pitch when it is traveling away from us. The two images below are really useful in trying to better understand the Doppler effect. The first image has a stationary red dot which is giving off waves, which is shown by the concentric blue circles. The wavelengths of the waves being given off are represented by the distance between the concentric blue circles. The waves would appear the same to four observers each located at the corners of the square area. The second image shows waves being produced by a moving object. In this case, the waves would appear to be different to observers on the right or left side of the square area. Observers on the left side would observe waves that are stretched out because the object giving off ...