Acoustic Rabi oscillations between gravitational quantum states and impact on symmetron dark energy

The standard model of cosmology provides a robust description of the evolution of the Universe. Nevertheless, the small magnitude of the vacuum energy is troubling from a theoretical point of view(9). An appealing resolution to this problem is to introduce additional scalar fields. However, these have so far escaped experimental detection, suggesting some kind of screening mechanism may be at play. Although extensive exclusion regions in parameter space have been established for one screening candidate-chameleon fields(10,17)-another natural screening mechanism based on spontaneous symmetry breaking has also been proposed, in the form of symmetrons(11). Such fields would change the energy of quantum states of ultracold neutrons in the gravitational potential of the Earth. Here, we demonstrate a spectroscopic approach based on the Rabi resonance method that probes these quantum states with a resolution of Delta E=2x10(-15)eV. This allows us to exclude the symmetron as the origin of dark energy for a large volume of the three-dimensional parameter space.