Scientists propose new projects to unravel dark-energy secrets
||Scientists have risen to the challenge to design an experiment that will make measurements of millions of galaxies to probe dark energy in new ways. Image: Sloan Digital Sky Survey
About 5 billion years ago the universe underwent a crucial transition. The gravitational tug that pulled together the matter in the universe was overwhelmed by a different, repulsive phenomenon. As a result, the universe began to expand at an accelerating rate.
Scientists have given that phenomenon a name: dark energy. However, they can say with confidence only what it does, not what it is, where it comes from or why it's pushing galaxies apart at an ever more rapid speed.
The Department of Energy recently declared the need to construct a powerful new device that scientists could use to address fundamental questions about dark energy.
Scientists have proposed two different projects to fulfill this need. The projects aim to study the three-dimensional distribution and motions of galaxies before and after the transition epoch between the matter-dominated and dark-energy-dominated eras.
"It turns out that the rate at which galaxies formed in the early universe as well as the expansion rate of the universe itself are sensitive to this stuff we call dark energy," says Fermilab physicist Josh Frieman, director of an existing dark energy experiment called the Dark Energy Survey. "So, those two things are what we're really trying to measure."
Scientists can look into the past by studying the light from distant galaxies. This will give scientists a better understanding of what role dark energy has played, Frieman says.
Scientists' main tools for studying how galaxies have been distributed over billions of years are imaging surveys and spectroscopic surveys. Imaging surveys compile a collection of images of the sky, while spectroscopic surveys go one step further, dispersing the light from certain objects within an image into individual wavelengths called a spectrum. They do this by attaching an instrument called a spectrograph to a telescope.
Scientists use imaging surveys to map out the sky and pinpoint galaxy locations. They follow these up with spectroscopic surveys. Both survey types are important because imaging surveys can accurately measure the two-dimensional positions and shapes of galaxies on the sky and spectroscopic surveys can precisely determine the three-dimensional positions.
A galaxy's spectrum contains information on its chemical composition and—more importantly to dark energy researchers—its distance from Earth. With spectroscopic surveys, scientists can construct three-dimensional maps of the sky. To do this, they have proposed two new spectroscopic surveys, one in the northern hemisphere and the other in the southern hemisphere.