A scientific discovery — particularly one that upends prevailing ideas — may eventually bring fame and a spot in the history books. At first, though, it may generate more skepticism than acclaim. Acceptance comes with confirmation, when several observers or experimenters back up the discovery with more evidence.
That certainly was the case with dark energy. The stunning announcement of its discovery brought hefty doses of both praise and finger wagging. But a growing number of observations support the idea that a mysterious force is causing the universe to expand faster as it ages.
Part of that confirmation came with the discovery itself.
Two competing teams of astronomers were observing a type of exploding stars in distant galaxies to see how quickly the expansion of the universe was slowing down. Yet both teams found that the expansion was getting faster, not slower. The fact that two respected teams reported similar results within months of each other strengthened the case for dark energy from the beginning.
Both teams have since conducted additional studies using larger numbers of supernovae.
In 2000, for example, a team led by Saul Perlmutter used Hubble Space Telescope to look at about two dozen supernovae. Many astronomers were concerned that these stars looked fainter than expected not because they were farther away, but because their host galaxies contained large amounts of dust, which would dim the view of the exploding stars. But the team found that the galaxies were no dustier than expected, suggesting that the supernovae really were farther away as a result of the expanding universe.
“It was very difficult for us to actually believe that there is something radical like dark energy from a single observation.”
Eiichiro Komatsu, University of Texas at Austin
And two years later, Adam Riess' team completed a similar HST survey. Its results addressed concerns that the supernovae in the original studies were somehow different from supernovae seen in the more modern epoch of the universe. Spectra taken with Hubble confirmed that the exploding stars were exactly what astronomers thought they were — "normal" explosions of white-dwarf stars with binary companions.
By then a second technique was adding more evidence to the case for dark energy: the geometry of the universe itself.
The geometry of the universe depends on how much total matter and energy it contains. If there is a lot of matter and energy, then the universe is curved at an angle that scientists describe as "closed." In that case, the universe would eventually collapse in on itself. With relatively little matter and energy, though, the curves the other way, so it is "open" — it would expand forever, although at an ever-slower rate.
But if matter and energy add up to just the right amount, then the universe is neither open nor closed, but flat.
For years, astronomers added up all the galaxies, gas clouds, dark matter, and radiation in the universe, and came up with only about a third of what was needed to produce a flat universe. Shortly after the turn of the 21st century, though, that changed.
Using telescopes aboard balloons and satellites, astronomers probed the "afterglow" of the Big Bang. This glow, known as the cosmic microwave background (CMB), was produced when the universe was less than 400,000 years old. It contains the imprint of the first "clumps" of matter, which eventually gave birth to galaxies and galaxy clusters.
Measuring the structure of the CMB revealed that the geometry of the universe is flat. Since dark matter and normal matter and energy account for only about 30 percent of the total needed to create a flat universe, scientists concluded that a hidden component must account for the balance: dark energy.
A 2002 study that compared the lumpiness of the CMB with how galaxies are distributed in the universe today produced the same result.
And in 2006, another Hubble Space Telescope study found evidence that dark energy was beginning to assert itself as early as nine billion years ago.
All of this evidence has provided solid confirmation for dark energy, igniting a race toward an even bigger discovery: the nature of dark energy.