Most cosmologists believe that roughly 96 percent of the universe consists of invisible ingredients—dark matter and dark energy—yet there is little consensus about what these things are. The idea that our universe is dominated by two separate mystery components, utterly unlike ordinary matter or energy, strikes many researchers as absurd. Two new papers show how scientists are scrambling to simplify one of the biggest puzzles in science today.
Theoretical physicist Robert Scherrer of Vanderbilt University in Tennessee posits that dark matter and dark energy are two versions of the same thing, a form of energy known as a scalar field. “You can think of it as a fluid energy that fills up all of space,” he says. In Scherrer’s equations, the density of the energy in the field breaks into two pieces: One decreases as the universe’s volume increases, like dark matter, while the other remains constant, like dark energy. In the small early universe, he suggests, dark matter dominated. As the cosmos grew, the balance steadily shifted toward dark energy, causing the expansion of the universe to speed up. “Eventually, the density of dark matter will approach zero, and we will end up with a universe that is almost all dark energy and expanding really, really fast,” he says. “It’s kind of depressing.”
Physicist Neal Weiner of the University of Washington takes a different approach, linking dark energy to a particle we already know about: the neutrino, a nearly massless entity that is created by the nuclear reactions within stars. Weiner and his collaborators speculate that a type of tension, dubbed an acceleron field, links neutrinos together. As they move apart, the tension between them grows, in much the same way that tension increases in a stretched rubber band. “Energy stored in that tension gives rise to dark energy,” he says. If he is correct, existing neutrino detectors might be able to detect the way the acceleron field affects the apparent mass of the neutrino.
Neither hypothesis is likely to be the last attempt to explain the universe’s dark enigmas. Fortunately, a spate of experiments, ranging from ground-based observations of the cosmic structure to a satellite that will study distant supernova explosions, should soon put many of these ideas to the test. “Dark energy is like the Wild West of theoretical cosmology right now; there are a lot of theorists out there on the frontier working on different things,” Scherrer says. “But eventually one thing will turn out to be right, and everyone else will be wrong.”