  We describe an initiative to build and use the Dark Energy Spectrometer
(DESpec), a wide-field spectroscopic survey instrument for the Blanco 4 meter
telescope at Cerro Tololo InterAmerican Observatory (CTIO) in Chile. A new
system with about 4000 robotically positioned optical fibers will be
interchangeable with the CCD imager of the existing Dark Energy Camera (DECam),
accessing a field of view of 3.8 square degrees in a single exposure. The
proposed instrument will be operated by CTIO and available for use by the
astronomy community. Our collaboration proposes to use DESpec to conduct a
wide, deep spectroscopic survey to study Dark Energy. In a survey of about 350
nights, the DESpec collaboration proposes to obtain spectroscopic redshifts for
about 8 million galaxies over 5000 square degrees selected from the Dark Energy
Survey (DES). This Dark Energy Spectroscopic Survey will advance our knowledge
of cosmic expansion and structure growth significantly beyond that obtainable
with imaging-only surveys. Since it adds a spectroscopic third dimension to the
same sky as DES, DESpec will enable increasingly precise techniques to
discriminate among alternative explanations of cosmic acceleration, such as
Dark Energy and Modified Gravity.
