Hawking talks about the expanding universe. The universe is getting bigger over time. One of the things he uses to explain his idea is the "Doppler shift". The Doppler shift happens when something moves toward or away from another object. There are two types of things that happen in Doppler shift - "red shifting" and "blue shifting". Red shifting happens when something is moving away from us. This is caused by the wavelength of the visible light reaching us increasing, and the frequency decreasing, which shifts the visible light towards the red/infra-red end of the electromagnetic spectrum. Red-shift is linked to the belief that the universe is expanding as the wavelength of the light is increasing, almost as if stretched as planets and galaxies move away from us, which shares similarities to that of the Doppler effect, involving sound waves. Blue shifting happens when something is moving toward us, the opposite process of red-shift, in which the wavelength decreases and frequency increases, shifting the light towards the blue end of the spectrum. A scientist named Edwin Hubble found that many stars are red shifted and are moving away from us. Hawking uses the Doppler shift to explain that the universe is getting bigger. The beginning of the universe is thought to have happened through something called the "Big Bang". The Big Bang was a very big explosion that created the universe.