The document discusses the Doppler effect and how it provides evidence for the Big Bang theory. It explains that the Doppler effect is when the frequency of a wave changes depending on whether the source of the wave is moving towards or away from an observer. It then asks what would happen to sound waves in front of and behind a moving object compared to a stationary object. Finally, it notes that the Doppler effect can cause absorption lines in spectra to shift towards the red or blue side when observing celestial objects, providing evidence that the universe is expanding.