In late August, paleontologists reported finding the fossil of a flattened turtle shell that “was possibly trodden on” by a dinosaur, whose footprints spanned the rock layer directly above. The rare discovery of correlated fossils potentially traces two bygone species to the same time and place. “It’s only by doing that that we’re able to reconstruct ancient ecosystems,” one paleontologist told The New York Times.
The approach parallels the way cosmologists go about inferring the history of the universe. Like fossils, astronomical objects are not randomly strewn throughout space. Rather, spatial correlations between the positions of objects such as galaxies tell a detailed story of the ancient past. “Paleontologists infer the existence of dinosaurs to give a rational accounting of strange patterns of bones,” said Nima Arkani-Hamed, a physicist and cosmologist at the Institute for Advanced Study in Princeton, New Jersey. “We look at patterns in space today, and we infer a cosmological history in order to explain them.”
One curious pattern cosmologists have known about for decades is that space is filled with correlated pairs of objects: pairs of hot spots seen in telescopes’ maps of the early universe; pairs of galaxies or of galaxy clusters or superclusters in the universe today; pairs found at all distances apart. You can see these “two-point correlations” by moving a ruler all over a map of the sky. When there’s an object at one end, cosmologists find that this ups the chance that an object also lies at the other end.