The goal of the Dark Energy Spectroscopic Instrument (DESI) is to determine the nature of dark energy through the most precise measurement of the expansion history of the universe ever obtained (Levi et al. 2013). DESI was designed to meet the definition of a Stage IV dark energy survey with only a 5 yr observing campaign. The Stage IV definition was developed by the Dark Energy Task Force (DETF; Albrecht et al. 2006) to quantify the uncertainty on the dark energy equation of state parameter w0 and its evolution wa. The DETF Figure of Merit is the reciprocal of the area of the error ellipse in the w0–wa plane. DESI is a project of the US Department of Energy (DOE) Office of Science, and the project used DOE funds combined with contributions from private foundations and partners to build substantial new instrumentation that can meet this definition with a survey of at least 9000 deg2. The more ambitious baseline survey is to obtain spectroscopic measurements of 40 million galaxies and quasars in a 14,000 deg2 footprint in 5 yr. DESI will measure the expansion history or distance–redshift relationship from the local universe to redshift 3.5 through precise measurements of the baryon acoustic oscillation (BAO) scale. The BAO scale is a standard ruler that corresponds to a fixed comoving physical size at all redshifts. The BAO scale originates from perturbations in the early universe that excited sound waves in the primordial photon–baryon fluid prior to recombination (eg, Peebles & Yu 1970; Sunyaev & Zeldovich 1970; Bond & Efstathiou 1984). After recombination occurred at z∼ 1100, the sound speed decreased abruptly and the waves stalled. The result was a small excess of baryonic matter at a fixed physical scale of approximately 150 Mpc. This scale is detectable in the late-time clustering of the universe, and it forms a distinctive pattern in the temperature anisotropies and polarization of the cosmic microwave background that have been exquisitely mapped in a variety of experiments (eg, Hinshaw et al. 2013; Planck Collaboration et al. 2016).

The BAO scale was first measured by Eisenstein et al.(2005) with data from the Sloan Digital Sky Survey (SDSS) and by Cole et al.(2005) with data from the 2dF Galaxy Redshift Survey. Numerous, subsequent studies have measured the BAO scale at a range of redshifts (eg, Jones et al. 2009; Blake et al. 2011; Kazin et al. 2014; Alam et al. 2017; Bautista et al. 2017; du Mas des Bourboux et al. 2017; Hinton et al. 2017). Many of these studies were based on progressively larger and larger samples that culminated in the SDSS Sixteenth Data Release (DR16; Ahumada et al. 2020), which contained more than 2.6