Adam Riess thought there must have been a mistake.
In late 1997, he was analyzing observations of exploding stars to help determine whether the universe would expand forever, or would someday reverse course and collapse in on itself. His observations seemed to show that not only would the universe continue to expand, but also that it was expanding faster as it got older — something that no one expected.
Riess, who is now an astronomer at Johns Hopkins, double-checked his figures, and found there was no mistake. Something was pushing down on the universe's gas pedal, making it expand faster. For want of a better name, that 'something' was identified as dark energy.
Riess was a member of one of two teams that made the same basic discovery. The other was headed by Saul Perlmutter of the University of California, Berkeley. Their discovery stunned scientists because it indicated that their understanding of the universe was far from complete — that some unknown force was counteracting gravity and pushing galaxies away from each other. Astronomers, physicists, and others continue to try to identify the cause of the accelerating universe.
Both teams were examining a class of exploding stars known as Type Ia supernovae. These stars are white dwarfs — the hot, dense, dead cores of stars that were once similar to the Sun — with close stellar companions. The white dwarf 'steals' hot gas from its companion. The gas piles up, forming a superhot layer atop the white dwarf. When the gas pushes the star past a critical mass, it sets off a runaway nuclear explosion that blasts the star to bits. For a while, the supernova can outshine an entire galaxy of normal stars, so it's easy to see these exploding stars even in distant galaxies.
The ability to see Type Ia supernovae far across the universe makes them good "standard candles" — a way to measure distances to other galaxies.
All Type Ia supernovae brighten and fade in a predictable way. Measuring how long it takes a supernova to brighten and then fade reveals its true brightness. By comparing a star's true brightness to how bright it looks in our sky, astronomers can find its distance.
One more step completes the picture. Astronomers measure how fast the star is moving away from Earth by measuring its redshift — a stretching of its light waves by the expansion of the universe itself.
By putting together the distances and speeds of many supernovae, the two teams hoped to show how the expansion of the universe had changed over the eons.
Most astronomers expected one of two outcomes.
If the universe contained enough stars, gas, planets, and other matter, and this material were packed tightly enough, then gravity would slow the expansion and eventually reverse it. In that case, the universe would end with a Big Crunch — the opposite of the Big Bang in which it was born.
If, on the other hand, the universe contained less matter, then the universe would continue to expand forever, although the expansion would grow slower with time.
Riess was analyzing observations made by the High-z Supernova Search Team, while Perlmutter led the Supernova Cosmology Project. Both teams observed several supernovae over several years. They used small telescopes to look at large patches of sky at different times, then compared the pictures to see if a supernova had appeared in any of the galaxies in their field of view. When a supernova appeared, they used larger telescopes to study the explosions in detail. Improved technology yielded supernovae that were up to 9 billion light-years away, which is much farther than earlier searches.
After collecting their data, Riess ran a computer program that calculated how much mass was required to account for how quickly the universe was slowing down. The computer's answer was a negative number — in other words, negative mass. There's no such thing as negative mass, though, so Riess basically flipped the answer over. He realized that the computer was telling him that the expansion of the universe wasn't slowing down, but instead was getting faster.
Perlmutter's team reached the same conclusion, and the teams published their findings in 1998. Science magazine named their discovery the 'Breakthrough of the Year' for 1998, and hundreds of other scientists jumped into the fray — beginning the effort to understand dark energy.