In 1998, studies of the brightnesses of Type Ia supernovae showed us that the universe is dominated by something called Dark Energy, which is causing the expansion of the universe to accelerate. Since then, numerous experiments have been proposed (and initiated), all aimed at measuring the precise amount of Dark Energy now contained in the universe.
But exactly how does this Dark Energy work? Einstein wrote his field equations with a Cosmological Constant which defines a precise rule for the cosmic acceleration: the greater the size of the universe, the greater the universal acceleration. In other words, the pressure pushing the universe apart is inversely proportional to density, P ∝ w ρ, with w = -1.
But is this true? Alternatives do exist to the Cosmological Constant model: for instance, if the cosmic acceleration is due to something called "quintessence", then w may vary through space and time. The Hobby-Eberly Telescope Dark Energy Experiment is aimed not only at measuring the amount of Dark Energy contained in the universe, but also determining its evolution. This makes HETDEX unique, as it is one of the only Dark Energy experiments currently planned to test the Cosmological Constant paradigm over ~11 Gyrs of cosmic time. Put mathematically, the question is simple: is w = -1 really a constant?
HETDEX is a collaboration between Penn State, the University of Texas at Austin, Texas A&M, the Universitats-Sterwarte Munich, the Leibniz Institute for Astrophysics of Potsdam, the Max-Planck-Institut fuer Extraterrestrische Physik, the Institut für Astrophysik Göttingen, and the University of Oxford. The project's Principal Scientist is Karl Gebhardt (University of Texas); the Principal Investigator is Gary Hill (University of Texas). At Penn State, the lead scientists are Robin Ciardullo, Caryl Gronwall, Don Schneider, Derek Fox, and Donghui Jeong.