The solution to the problem of dark energy will require a fundamental shift in our understanding of the Universe, and acceptance of any model will require significant corroboration by observations. The Hobby-Eberly Telescope (HET) Dark Energy Experiment, HETDEX, offers a technically clean and observationally robust approach to measuring the effects of dark energy. HETDEX includes an upgrade of the HET for a wider field of view, deployment of the Visible Integral-field Replicable Unit Spectrograph (VIRUS), and completion of a wide field survey which will use 1400 hours of observation to obtain nearly a million redshifts of Lyman-alpha emitting galaxies. Significant parts of the project are already underway with support from a variety of sources, including substantial private donations.
The survey will determine the local Hubble expansion at redshift 2.4 and measure the angular diameter distance out to that redshift, both to an accuracy of 0.8%, and determine the growth of structure to 2%. Nearly all other dark energy surveys target lower redshifts, while HETDEX was designed to provide high-redshift information on the evolution of dark energy, as well as being sufficiently powerful to measure dark energy effects even at late times. HETDEX further will measure the curvature of the Universe to about 0.1%, an order of magnitude better than current projects, and a strong complement to other current and planned studies. Although HETDEX is optimized for measures of the expansion history, there is significant ancillary science from the survey, such as a collection of spectra of a quarter of a million stars and 1.4 million other galaxies. Given its capabilities, VIRUS will remain a, if not the, primary instrument for HET for many years.
The significant insights on dark energy, the large database with substantial ancillary science, and the unique and powerful integral field spectrograph on the HET will have a broad impact throughout the astronomical community. The attraction of the dark energy enigma enables HETDEX to include significant public outreach involving the McDonald Observatory Visitors Center and Stardate radio, including an interactive display for visitors and the involvement of K-12 teachers, as well as a popular webpage at www.hetdex.org.
