On August 31, a team of scientists working on the Dark Energy Survey (DES) turned on the world’s most powerful digital camera and pointed it upwards, beginning the process of systematically imaging the entire sky. The resulting imagery will be assembled into the most detailed map of the Universe every created, so that cosmologists can begin picking away at the problem of Dark Energy to answer that most troubling of questions in astronomy: Why is the expansion of the universe not slowing down under the influence of its own gravity? Why is is actually speeding up?The imagery for the survey is being collected by the Dark Energy Survey camera (DECam), at the heart of which lies an array of sixty-two 2048 X 4096 CCD detectors. DECam is mounted at the prime focus of the Victor M. Blanco four meter telescope in the Chilean Andes. The detector and telescope together form an optical system equivalent to a 570 MegaPixel camera with an 8000mm f/2 lens. Of course, it’s actually rather more powerful than that since the CCD detectors are of a special research grade, tuned for maximum sensitivy at the redder wavelengths of distant galaxies.
“The DES will explore some of the most important questions about our existence,” said James Siegrist, associate director for High Energy Physics at the U.S. Department of Energy’s Office of Science. “In five years’ time, we will be far closer to the answers, and far richer in our knowledge of the universe.”
“With the start of the survey, the work of more than 200 collaborators is coming to fruition,” said DES Director Josh Frieman of the U.S. Department of Energy’s Fermi National Accelerator Laboratory. “It’s an exciting time in cosmology, when we can use observations of the distant universe to tell us about the fundamental nature of matter, energy, space and time.”
Our current laws of physics predict that, since every object in the universe exerts a gravitation attraction (however weak) on every other object, the Universe must want to contract, and that the expansion of the universe should therefore be slowing down. The only question was to determine whether the universe was massive enough to eventually stop and reverse its expansion, or if it would simply grow forever. The shocking, and incredibly important discovery from the Hubble Space Telescope was that there was a third alternative, something impossible under the physics that we knew: the expansion of the universe is actually speeding up, as if some force is acting to push everything apart from everything else, against gravity. Current models label this mystery force Dark Energy, and have come up with several forms that it could take. The Cosmological Constant (something Einstein came up with to prevent the universe collapsing, and which he then abandoned when it was discovered that the universe is expanding) is a force which is extremely weak, but of constant strength. Gravity weakens over distance, following the inverse square law, which is why the immense gravity of the distant Sun has a much smaller effect on the earth’s tides than the relatively puny gravity of the nearby moon. The cosmological constant, by contrast, exerts the same very weak force regardless of the distance between objects.
The problem with all variations on dark energy is that we don’t know for certain that they actually exist. Scientists can come up with all the theories in the world, and prove them to be sound mathematically, but eventually they have to test them against the real world, and the DES aims to collect the evidence that will help them to do just that. “We’re looking at this big galaxy map of the universe as a way of finding evidence for dark energy and characterizing its nature with cosmic epoch,” said Ofer Lahav of University College London and head of the DES Science Committee. “An even more challenging goal for DES is to tell if what causes the acceleration of the universe is indeed dark energy, or something entirely different.”
Over the next five years, DES will map out full colour images of 300 million galaxies, 100 000 galaxy clusters, and will likely capture the light of around 4000 supernovae explosion from billions of years ago, back when the universe was half its current size. Obviously, this data won’t show dark energy directly, any more than you could photograph a gravitational or magnetic field. Instead, cosmologists will be able to see how galaxies have changed in shape over time, and precisely measure how the universe has expanded and how large scale structures like galactic superclusters have evolved over time. By comparing these data to the predictions of various competing theories on the nature of dark energy, scientists will be able to either prove, refine, or disprove their theories and eventually solve the mystery.