The Rosetta Stone for cosmologists is the cosmic microwave background (CMB), remnant radiation from the surface of last scatter some 380 000 years after the Big Bang. Through increasingly precise microwave-band measurements, most recently by the Planck satellite, scientists have refined their basic model of the universe. Known as ΛCDM, the model posits the existence alongside normal matter of cold (that is, nonrelativistic) dark matter and dark energy in the form of the cosmological constant. Yet for all the useful clues embodied in the CMB, there is a complementary cosmological story to be told by analyzing more modern structures, particularly galaxy clusters.
Enter the Dark Energy Survey, a galactic census of unprecedented detail conducted with a 570-megapixel camera mounted on a 4-meter optical telescope in Chile (see the article by Josh Frieman, Physics Today, April 2014, page 28). In the first phase of the project, conducted over five-plus...
