Researchers are studying very distant supernovae to determine whether the expansion of the universe was decelerating long ago but now is accelerating. Between May and June 1997, Hubble observed the three supernovae to measure their brightness and create a light curve. Researchers then used the light curve to make an accurate estimate of the distances to the supernovae. They combined the estimated distance with the measured velocity of the supernova's host galaxy to determine the expansion rate of the universe in the past (5 to 7 billion years ago) and compare it with the current rate. SN 1997ck (right) exploded when the universe was half its current age, erupting 7.7 billion years ago making it the most distant supernovae ever discovered; the other two supernovae – SN 1997cj (left) and SN 1997ce (center) – exploded about 5 billion years ago.