All matter, whether dark or light, produces gravity that should cause the universe to slow down. Cosmic acceleration implies that the enigmatic “73 percent” element must cause a repulsive effect that counteracts gravity over enormous distances. Strange as it may sound, most cosmologists have come to believe that this invisible element consists of energy present throughout seemingly empty space. Such dark energy might arise from the tangle of fields that fill the vacuum on the subatomic level. The simplest versions of quantum theory actually predict far too much energy, so physicists typically assumed the energy from all these fields somehow canceled out to zero. Now they speculate that (for reasons equally unknown) a residual trace of this quantum energy remains and accounts for the inferred dark energy. Other researchers invoke a hypothetical energy field called quintessence, a play on the ancient Greek term for a heavenly “fifth element,” which would also cause cosmic repulsion but which could decay as the universe expands.
Whichever theory is correct—and both may be wrong—the discovery of dark energy is forcing astronomers to rethink the life history of our universe. Early measurements of cosmic density indicated there is so little matter out there that the expansion could continue to infinity. As more and more dark matter turned up, researchers began to consider that gravity might eventually cause the Big Bang to reverse and initiate another cycle of creation. Now it seems that dark energy, whatever it is, controls the destiny of the universe.
In this new view, we occupy a startlingly peripheral place in the universe. Dark energy accounts for most of its mass, exotic dark matter comes in second place, and ordinary matter—the atoms we are made of—lands in a distant third place, with just 4.4 percent. Even the bulk of ordinary matter is dark. Everything we know, see, and touch is an insignificant part of the whole. It is a sobering thought, but it leads us into a world far more curious and marvelous than we could have ever imagined. For the first time in history, we are able to look past the luminous flecks of foam dotting the surface of the cosmic sea and begin to explore the vast, murky depths.
A Bright Light on Dark Energy Supernova explosions, among the brightest events out there, led to the discovery of dark energy. By monitoring the light from distant supernovas, two research teams measured how the expansion rate of the universe has changed over time. Their remarkable result: The universe had been slowing down for its first few billion years, but then it started speeding up. The standard interpretation is that gravity competes with a repulsive effect produced by dark energy. When the universe was young and dense, gravity held sway. As everything continued to expand and thin out, energy became dominant. Recent Hubble Space Telescope images of a supernova that exploded 8 billion light-years from Earth (below) is filling in details of what happened during the transition period between deceleration and acceleration. That will help distinguish between the two leading theories of dark energy. A proposed space-based Supernova/Acceleration Probe should be able to rule out one theory—perhaps even both of them—during its three-year mission. —C.S.P. |
