Redshift

The team must determine its redshift, which is a measure of how much the universe expanded as the light traveled from the supernova to our telescope. If the universe is expanding, the wavelength of light from the supernova is stretched out and appears more red. If the universe were contracting, the wavelength of light would be compressed and the light would appear more blue. When Kirshner's team looks at the spectrum of a supernova, they can measure this change in a wavelength. There are characteristic patterns of absorption lines in the star spectrum due to the elements present in that star. When the spectral lines shift towards the red end of the spectrum, we infer that the wavelength has stretched. This is called a "redshift." Measuring a supernova's redshift determines how much the universe has expanded since it exploded. The bigger the shift, the greater the cumulative expansion since the light left the supernova.