Red-Shift and the Expanding Universe
When white light passes through a gas, electrons in the gas atoms absorb specific frequencies of light. As a result, the spectrum of light emerging from the gas shows dark lines at those absorbed frequencies. These are known as absorption lines. Different gases produce unique patterns of absorption lines.
We observe absorption lines in the light received from distant galaxies. These lines are caused by gases in the outer layers of stars absorbing certain frequencies of light. However, in the spectra from most galaxies, these absorption lines are shifted from their expected positions, usually towards the red end of the spectrum. This phenomenon is known as red-shift.
Red-shift means that the wavelengths of light we receive from these galaxies are longer than they would be if the galaxies were stationary. According to the Doppler Effect, an increase in wavelength indicates that the source of the waves is moving away from the observer. Therefore, red-shift provides evidence that galaxies are receding from us.
Furthermore, the greater the red-shift, the faster the galaxy is moving away. Observations show that more distant galaxies have greater red-shifts, meaning they are moving away faster. This relationship suggests that the universe is expanding.
Red-shift is considered strong evidence for the expansion of the universe.