In 1998, two teams of astronomers —one led by the Australian National University’s Brian Schmidt— reached the same conclusion: the expansion of the Universe is not slowing down, as most people had assumed, it is accelerating. The discovery triggered a flurry of activity to understand more about dark energy, the hypothetical driving force pushing the Universe apart and counteracting gravity. It has also brought the Nobel Prize in Physics to Schmidt, Adam Riess, and Saul Perlmutter.
To calculate the Universe’s rate of expansion, both teams were studying Type Ia supernovae as a means of measuring distances across the cosmos. The further away the star, the fainter the stellar explosion appears. By combining those distances with the supernovae’s redshifts, where light from receding stars is shifted towards the red end of the spectrum, the astronomers could gauge how fast the Universe was expanding at different stages of its life.