You have now investigated two major pieces of evidence supporting an inflation model such as the Big Bang. Now we will dig deeper into the concept of redshift. We can see galaxies moving away from us, but how do we know how fast they are moving? For objects within a few billion light years, we can use a well-known phenomenon known as the Doppler effect to help us. The Doppler effect is an apparent change in the frequency of sound or radiation due to motion. You may have experienced this effect while waiting at a train crossing. The pitch (frequency) of the train horn is higher as the horn approaches, and lower as it moves away.