Survey finds galaxy clumps stirred up by dark energy
Daniel Clery | Science 11 Aug 2017
The biggest study so far of large-scale cosmic structure—a survey of 26 million galaxies that traced the forces shaping the universe over deep time—has revealed the dominating effect of dark energy, the mysterious pressure expanding space faster and faster. The survey finds about as much dark energy as the gold standard in assessing this cosmic recipe: precision maps of the nearly 14-billion-year-old cosmic microwave background (CMB) that forms the backdrop of the universe. But the new survey and other techniques are now allowing astronomers to watch how dark energy plays out over time. Their goal: to see whether it changes.
Dark energy was discovered 2 decades ago, when astronomers found that supernovae in distant galaxies were farther away than they should have been if the universe were expanding at a stately, steady pace. A bizarre picture of the universe emerged, in which only about 5% of its current mass comes from the atoms and photons we see. Another 25% is “dark matter,” invisible mass that helps pull galaxies together into clusters and filaments but doesn’t otherwise interact with ordinary matter. The remaining 70% is dark energy, doing its part to rebuff gravity’s clumping tendencies.
The CMB maps responsible for that picture, from missions such as Europe’s Planck satellite, provide only a snapshot of the universe in its infancy, before dark energy grew, along with space itself. To chart how dark energy evolved over time in its cosmic tug-of-war with dark matter, astronomers are pursuing four main techniques. They have continued to look back in time for ever-more-distant supernovae, although there are nagging uncertainties about the regularity of the stellar explosions and whether they can serve as reliable beacons. They also have developed a powerful new technique based on baryon acoustic oscillations (BAOs), ripples in the density of normal matter that trace their heritage back to sound waves in the roiling gas of the early universe. The ripples, now preserved as peaks in the density of galaxies at intervals of 490 million light-years, provide a yardstick for measuring how fast space is expanding.
The DES is focusing on two other techniques for gauging how space is stretching. One is a statistical measurement of how “clumpy” galaxy clusters are; the other is weak gravitational lensing, a measure of how the observed shapes of distant galaxies are distorted by the gravity of matter—both visible and dark—between them and Earth. The density and distribution of that matter, in turn, can reveal the stretching of space.
Because weak lensing smears the shape of each galaxy by just a percent or two, such surveys are challenging from the ground, where the atmosphere introduces its own distortions. But DES astronomers equipped the 4-meter Víctor M. Blanco Telescope in Chile with a 570-megapixel camera—large enough to capture an image 14 times the area of the full moon in one 90-second exposure. The project’s first observing season, from August 2013 to February 2014, amassed enough galaxies to see the imprint of dark energy, every bit as potent as the Planck results predicted.
So far, the prevailing cosmological model holds that dark energy is an intrinsic property of empty space, described by adding a fixed term to Albert Einstein’s theory of general relativity called the cosmological constant. This version of dark energy would generate the same force everywhere, throughout the history of the universe. Although the DES’s first results agree with that picture, to a few percent, some astronomers were hoping they would veer off and point to a more exotic explanation. Contenders include a new force field that might vary in time and space, akin to the Higgs field that gives particles mass. Another possibility is that, at cosmological scales, general relativity breaks down and a new theory of gravity is required. Advocates of such theories still hold out hope that the DES and other surveys, as they push farther out across space and time, will point to one of these alternatives.
They may not have to wait long. By next year the DES will have gathered 5 years of data. Investigators will analyze 300 million galaxies and thousands of supernovae going back 7 billion years. An expanded version of BOSS—eBOSS—is underway, and it will be joined next year by the Dark Energy Spectroscopic Instrument on a 4-meter telescope in Arizona. In 2022, the 8.4-meter Large Synoptic Survey Telescope (LSST) in Chile is due to begin supplying data for all four kinds of dark energy probes. And weak lensing surveys will eventually move to space for a clearer view of the distorted galaxy shapes. Euclid will launch in 2020. And later next decade, NASA’s Wide Field Infrared Survey Telescope may take up the search.