BigBOSS: The Ground-Based Stage IV BAO ExperimentThis Response to the Decadal Survey is submitted by:The Lawrence Berkeley National Laboratory 1 Cyclotron Rd MS 50R-5032, Berkeley, CA 94720 David Schlegel, DJSchlegel@lbl.gov, 510-495-2595 Chris EXECUTIVE SUMMARYThe BigBOSS experiment is a proposed DOE-NSF Stage IV ground-based dark energy experiment to study baryon acoustic oscillations (BAO) and the growth of structure with an allsky galaxy redshift survey. The project is designed to unlock the mystery of dark energy using existing ground-based facilities operated by NOAO. A new 4000-fiber R=5000 spectrograph covering a 3-degree diameter field will measure BAO and redshift space distortions in the distribution of galaxies and hydrogen gas spanning redshifts from 0.2 < z < 3.5. The Dark Energy Task Force figure of merit (DETF FoM) for this experiment is expected to be equal to that of a JDEM mission for BAO with the lower risk and cost typical of a ground-based experiment. This project will enable an unprecedented multi-object spectroscopic capability for the U.S. community through an existing NOAO facility. The U.S. community would have access directly to this instrument/telescope combination, as well as access to the legacy archives that will be created by the BAO key project.The BigBOSS survey will target luminous red galaxies, emission line galaxies, and QSOs. This experiment builds upon the SDSS-III/BOSS project, reusing many aspects of the BOSS spectrograph and computing pipeline designs. The BigBOSS project is enabled by the impressive 3 degree diameter field of view of the 4-m Mayall telescope at KPNO. The focal plane of this telescope will be filled with an automated fiber-positioner capable of targeting 4000 objects simultaneously over a wavelength range from 340 nm to 1130 nm with resolution R=2300-6100. This carefully-designed instrument is capable of measuring redshifts to the brightest [OII] emitters to z=2 with a 4-m aperture. Assuming a majority allocation of the dark time and optimal observing conditions during 30% of all nights, and with approximately onehour exposures, over 5 million targets will be visited per year. We propose to operate for six years at KPNO and then move the instrument to CTIO, the Mayall sister telescope in the southern hemisphere, for a four year run commencing after the Dark Energy Survey (DES) program.The 30-million galaxy sample of BigBOSS-North provides precision baryon acoustic oscillation measurements over 14000 square degrees from 0.2 < z < 2.0 and a million QSOs from 1.8< z <3.5. A continuation with BigBOSS-South completes the survey, bringing the total to 50 million galaxies over 24000 square degrees. BigBOSS will sculpt the redshift distribution to maximize the statistical significance of the dark energy measurement. The target selection will be done using existing and planned imaging surveys. A summary of experiment goals is shown in Table 1.BigBOSS is proposed as a partnership between NSF/NOAO and DOE/OHEP. Details of this partnership will be determined w...

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