A field that permeates the universe and gives rise to a new force, or "fifth force," between massive objects may be a candidate for dark energy and an explanation for why the expansion of the universe is accelerating. This field, called the symmetron field, is so named because it has a symmetry in regions of high density, while in regions of low density, such as a vacuum, the symmetry is broken and the field mediates the new force.
Currently, the symmetron concept is purely theoretical. But in a new study, physicist Amol Upadhye at Argonne National Laboratory in Argonne, Illinois, has calculated that a previously unexplored symmetron regime near the dark energy scale will give rise to a fifth force at submillimeter distances. He proposes that short-range gravity experiments can search for the fifth force at these distances and possibly reveal if dark energy is in fact a symmetron field.
"Much of my work has focused on chameleon dark energy theories, and I really only started thinking about symmetrons last summer," Upadhye said. "Modern experimental techniques and technologies have advanced enough to search for new physics at distances of interest for dark energy theories." As Upadhye explained, a symmetron field could fulfill the role of dark energy by acting as a negative pressure. "Dark energy in general can be described by a constant (or slowly varying) vacuum energy density, such as that due to a field whose potential is minimized at a small, positive value," he said. "In the presence of such an energy density, Einstein's equation of General Relativity (GR) predicts that the universe will expand at an accelerating rate. In GR, pressure gravitates; positive pressures contribute to the decelerating expansion of the universe. Dark energy acts as a negative pressure which leads to an accelerating expansion.
"The simplest model of a dark energy is Einstein's cosmological constant, a constant vacuum energy density which explains all available data. The big question in cosmology is whether or not the dark energy is just a cosmological constant. Alternative theories predict that the vacuum energy density evolves with time, or that new ('fifth') forces exist between known particles.