Using a cosmic looking glass that would dazzle Sherlock Holmes, an international team of astronomers has taken the first major step in a five-year observing project to help crack a case you could call The Sign of Dark Energy.
The team has released the first stunning images from a $40 million camera that over the next five years is expected to capture and help characterize 300 million galaxies, 100,000 galaxy clusters, and some 4,000 exploding stars, or supernovae – all in the quest to figure out the nature of dark energy and how it has affected the evolution of the cosmos.
Where gravity exerts a pull on other objects in space, dark energy pushes objects apart, causing the universe to expand at an increasing rate.
The name dark energy was coined shortly after the phenomenon was discovered – in no small part to convey its mysterious nature.
Some theorists posit that dark energy is the cosmological constant first proposed, then withdrawn, by Albert Einstein. Einstein invoked it as he worked out the implications of his theory of general relativity for the evolution of the universe. His equations pointed to the eventual collapse of the universe under its own gravity, but the universe at the time was widely thought to be static. So he added a cosmological constant to his calculations to keep the universe from collapse.
A cosmological constant would permeate the universe in ways that would induce it to expand at an increasing rate.
Others have proposed that dark energy varies with time and may even be a fifth fundamental force of nature, dubbed quintessence. Since it varies, it could be a repulsive force or one that attracts, depending on other conditions in the universe.
Whatever the explanation, "we believe dark energy became the dominant force driving the large-scale evolution of the universe several billion years ago," says Joshua Frieman, a researcher at the Fermi National Accelerator Laboratory in Batavia, Ill., and the director of the Dark Energy Survey project. "So we want to go back somewhat farther than that to see whether the properties of dark energy have changed over time – since that would give us clues to its nature."
The Dark Energy Camera, bolted to the back end of a 4-meter telescope high in the Chilean desert, aims to hunt for dark energy through its influence on some of the universe's largest visible structures, peering back through time to a period when the universe was roughly 5 billion years old. During its five-year run, the camera will survey about 1/8th of the sky.
The discovery of dark energy is a classic case of scientists pursuing one objective stumbling on something profound in the process.
Two teams of scientists working independently first reported the discovery of dark energy’s effect in 1998 and 1999. The research teams had aimed to measure the expansion rate of the cosmos at different periods in its history.
To do that, they hunted for light from a particular type of exploding star as a tool to help them clock the rate at which the universe is expanding following the Big Bang, a sudden release of energy that spawned the visible universe some 13.8 billion years ago.
The species of supernovae the teams hunted, so-called type 1a supernovae, reach the same peak intrinsic brightness wherever they explode. For a time, a supernova's light can outshine the light from its host galaxy. Because light dims at a known rate as it travels, measurements or estimates of peak brightness of a supernova's light when it reaches Earth can reveal the distances to the galaxy hosting the explosion.
By measuring the spectra of a supernova, researchers can see how far the chemical fingerprints in the spectra have shifted toward the red end of the spectrum – a measurement that yields the pace at which the universe's expansion is carrying the supernova's host galaxy away from the observer.
Armed with distances and speeds, it's possible to take snapshots of the pace of expansion at various points in the past.
The problem: When they compared expansion rates from nearby supernova with those from distant ones, they discovered that somewhere along the way, the universe's expansion rate had been kicked up a notch. The earliest supernovae the teams picked up were 25 percent dimmer than they should have been.
Prior to the discovery of dark energy, cosmologists estimated that the universe had just enough matter and energy in it so that gravity would continue to slow the expansion, although the expansion rate would never reach zero.
In 2011, the discovery earned portions of a Nobel Prize in Physics for three researchers collectively representing both teams.
Researchers now say that several billion years ago, the universe had expanded to a point where gravity at the cosmic level became weak enough for dark energy – present from the beginning – to take over and speed the expansion.
The observation of an accelerating universe has drawn support from other, independent ways to observe the change. And it's overhauled the cosmos's recipe.
Prior to dark energy's discovery, some 5 to 10 percent of the universe was thought to consist of so-called baryonic matter – built from protons and neutrons. The rest was thought to consist of so-called dark matter, a form that astronomers couldn't directly detect but was inferred from its gravitational affects on galaxies and clusters of galaxies.
Now, the dominant ingredient appears to be dark energy, making up 74 percent of all the matter and energy in the cosmos. Twenty-two percent of the universe consists of dark matter, with baryonic matter – making up stars, planets, and people –filling out the remaining 4 percent.
The Dark Energy Camera is exquisitely sensitive to light at the red end of the spectrum, since at the distances the team is exploring, the expansion of the universe has reddened the light, in effect stretching it to longer wavelengths.
The Dark Energy Survey, which involves 120 scientists from five countries, will be interrogating several aspects of the cosmos for answers to researcher's questions about it, explains Dr. Frieman in an email exchange.
For instance, taking the measure of additional supernovae across time and space will refine estimates of how the pace of expansion has changed, he says.
In addition, over the past few billion years, the pace at which new clusters of galaxies are forming has slowed. Taking the measure of clusters at different points in the universe's history will allow scientists to estimate the pace of the construction slowdown, giving further insights into the nature of dark energy.
Other telescopes can peer farther back in time than the telescope that will gather light for the camera, but by nature they also cover far smaller patches of the sky.
"The very large area of our survey is necessary to get the information we want," Frieman says.
Among the Dark Energy Camera's "first light" images, taken Sept. 12, is a shot of a galaxy some 60 million light-years from Earth. The galaxy is a member of the Fornax cluster.