National Geographic : 2015 Jan
122 national geographic • january 2015 observable universe, covering the southern night sky up to ten times each month. With such tools, cosmologists hope to recon- struct the history of dark energy’s emergence and influence by directly measuring the cosmic expansion rate throughout the past. At issue may be nothing less than the future of the universe— and of its study. If we live in a “runaway uni- verse” increasingly dominated by dark energy, most galaxies eventually will be driven beyond one another’s sight, leaving far-future cosmolo- gists with little to observe but their immediate neighborhood and the blackness of space. In the nearer future, making sense of dark energy may require radical improvements in the way we conceive of space itself. The voids between the planets and stars were long thought to be sheer nothingness, although Isaac Newton admitted that he couldn’t imagine how gravity could keep the Earth spinning around the sun if the space between them was utterly vacuous. In the 20th century, quantum field theory came to the rescue by demonstrating that space is never really empty but instead is suffused with quantum fields, which are literally everywhere. The protons, electrons, and other particles often described as the building blocks of matter are themselves excitations of quantum fields. Space looks empty when the fields languish near their minimum energy levels. But when the fields are excited, space comes alive with visible matter and energy. The mathematician Luciano Boi compares space to the water in a quiet Alpine pond: invisible when calm but evident when a breeze ripples its surface. “Empty space is not empty,” the American physicist John Archibald Wheeler once said. “It is the seat of the most rich and surprising physics.” Dark energy may prove him to have been prophetic on the largest possible scale. To A Survey of Cosmic Repulsion Dark matter pulls stuff together; dark energy drives it apart. The Dark Energy Survey’s 570-megapixel camera (above), mounted on a tele- scope in Chile, is designed to image 300 million galaxies in five years, including NGC 1365 (right). Peering out eight billion light-years, it could see how fast the cosmos was expanding billions of years ago— when its acceleration by dark energy is thought to have begun.