the past and predicting the fate of the cosmos. To perform this task, they developed powerful techniques to measure the energy output of Type Ia supernovas in extremely remote galaxies.

Type Ia supernovas, the catastrophic explosions of a certain kind of star, are valuable to astronomers because they can serve as “standard candles.” Standard candles are objects with well-known energy output. Imagine walking along a dark desert road and seeing a faint street lamp way off in the distance. If you know the intrinsic power of the lamp, you can deduce from its dimness how far away it is. Type Ia supernovas serve a similar purpose for astronomers eager to map the scale of the universe. By matching the apparent brightness of such stellar blasts to their actual luminosities, astronomers can reliably ascertain their distances. Light curves, indicating the progression of each burst, offer added information. Thus, they are solid celestial yardsticks, useful for measuring the remoteness of the galaxies in which they are situated.

Once astronomers know the distances to the galaxies in a given region of space, they can readily determine the expansion rate of that region. Each galaxy’s spectral lines are shifted by the Doppler effect. By measuring this shift, they can assess the galaxies’ velocities. Finally, by combining this information with the distance data, they can calculate how fast each part of the universe is pulling away.

In astronomy the farther out you look, the deeper into the past you see. Therefore, Perlmutter and his colleagues realized they had the perfect tool for determining how the cosmological expansion rate has altered over the eons. This tool could allow them to assess omega, the universe’s density parameter, and help them decide how much of its dynamics is driven by visible material versus dark matter. A second team, led by Brian Schmidt of Australian National University and Nicholas Suntzeff of Cerro Tololo Inter-American Observatory in Chile, enacted an independent program with a similar purpose. Throughout the 1990s the two groups jockeyed for valuable telescope time and competed in a race for publications.

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