By measuring the light from a distant Type Ia supernova and comparing its observed brightness to the known brightness from similar supernovae in nearby galaxies, we can precisely calculate a supernova's distance from Earth. Keep in mind that the speed of light is constant. So, once we know distance, we also have a relative measure of how long ago each supernova occurred. Perlmutter's team has captured the light of some 50 supernovae. If the team can determine the amount of expansion since the time of each of these events, then they will know the extent to which our expanding universe is running out of momentum.
Spectroscopy provides the answer.
Using powerful telescopes, researchers focus on a supernova and separate its spectrum of light into different wavelengths. They then determine the degree to which the supernova's spectral lines are shifted towards the red end of the spectrum. The Supernova Cosmology Project is comparing the amount of expansion over billions of years to that over shorter more recent periods of time, pinpointing the change in the rate of expansion. The preliminary results are in and they indicate that the universe appears to be slowing down. Additional data will reveal a more precise picture of the future.