Red-shift
Scientists believe that red-shifts of very far away galaxies are the most important observation which proves the Big Bang Theory. Red-shift is the Doppler Effect occurring in light. When an object moves away from earth, it looks red, this happens because the wavelength stretches. The red is the lowest wavelength on the visible spectrum which is why that colour occurs. The more red-shift there is, the faster the object is moving away. Scientists proved, by measuring red-shift, that the universe is expanding and they can also work out the speed at which the object is moving. With observation and measurements of the expansion of the universe, scientists have calculated that the universe was at its smallest point (ie a "singularity") approximately 13.7 billion years ago. Because most things become colder as they expand, the universe is assumed to have been very hot when it started.