  The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) will outfit the 10
m HET with a new wide field and an array of 150 integral-field spectrographs to
survey a 420 sq. deg. area in the north Galactic cap. Each fiber-coupled unit
spectrograph will cover 350-550 nm, simultaneously. This instrument, called
VIRUS, will produce ~34,000 spectra per exposure, and will open up the
emission-line universe to large surveys for the first time. The survey will
detect 0.8 million Lyman-alpha emitting (LAE) galaxies with 1.9<z<3.5 and more
than a million [OII] emitting galaxies with z<0.5.
  The 3-D map of LAE galaxies in 9 cubic Gpc volume will be used to measure the
expansion history at this early epoch using baryonic acoustic oscillations and
the shape of the power spectrum. The aim of HETDEX is to provide a direct
detection of dark energy at z~3. The measurement will constrain the evolution
of dark energy and will also provide 0.1%-level accuracy on the curvature of
the Universe, ten times better than current.
  The prototype of the VIRUS unit spectrograph (VIRUS-P) is a powerful
instrument in its own right. Used on the McDonald 2.7 m, it covers the largest
area of any integral field spectrograph, and reaches wavelengths down to 340
nm. VIRUS-P is being used for a pilot survey to better measure the properties
of LAE galaxies in support of HETDEX. We report initial results from this
survey.
