[Paper Review] The DESI Experiment, a whitepaper for Snowmass 2013

M. E. Levi, C. Bebek+12 more|arXiv (Cornell University)|Aug 4, 2013

Astronomy and Astrophysical ResearchPhysics and Astronomy3 references370 citations

TL;DR

The DESI experiment proposes a massively multiplexed spectroscopic survey using the Mayall 4-meter telescope to obtain redshifts for 18 million emission-line galaxies, 4 million luminous red galaxies, and 3 million quasars, enabling 1%-level measurements of the cosmic distance scale via baryon acoustic oscillations (BAO) and redshift-space distortions (RSD) across 35 redshift bins, with a projected dark energy figure of merit (FOM) of 2723—over 100x higher than BOSS—making it a transformative Stage-IV dark energy experiment.

ABSTRACT

The Dark Energy Spectroscopic Instrument (DESI) is a massively multiplexed fiber-fed spectrograph that will make the next major advance in dark energy in the timeframe 2018-2022. On the Mayall telescope, DESI will obtain spectra and redshifts for at least 18 million emission-line galaxies, 4 million luminous red galaxies and 3 million quasi-stellar objects, in order to: probe the effects of dark energy on the expansion history using baryon acoustic oscillations (BAO), measure the gravitational growth history through redshift-space distortions, measure the sum of neutrino masses, and investigate the signatures of primordial inflation. The resulting 3-D galaxy maps at z<2 and Lyman-alpha forest at z>2 will make 1%-level measurements of the distance scale in 35 redshift bins, thus providing unprecedented constraints on cosmological models.

Motivation & Objective

• To advance dark energy cosmology by achieving 1%-level precision in measuring the cosmic distance scale using baryon acoustic oscillations (BAO) and redshift-space distortions (RSD) across 35 redshift bins.
• To complement existing dark energy surveys like DES (weak lensing) and LSST (supernovae) by focusing on spectroscopic large-scale structure, filling a critical gap in the U.S. dark energy program.
• To extend BAO measurements to redshifts z > 2 using the Lyman-alpha forest in quasar spectra, enabling the first high-precision measurement of the expansion history at z > 2.
• To constrain fundamental cosmological parameters, including the sum of neutrino masses, primordial non-Gaussianity, and the growth rate of structure, via RSD and Alcock-Paczynski tests.
• To maintain U.S. leadership in cosmology by ensuring continuity in dark energy measurements during the hiatus between the completion of DES and the start of LSST.

Proposed method

• DESI uses a fiber-fed, multi-object spectrograph on the Mayall 4-meter telescope to obtain redshifts for up to 5,000 galaxies simultaneously, enabling a massive survey of 18 million emission-line galaxies, 4 million luminous red galaxies, and 3 million quasars.
• The survey targets three primary redshift tracers: emission-line galaxies (ELGs), luminous red galaxies (LRGs), and quasars (QSOs), with Lyman-alpha forest absorption in quasar spectra used to probe structure at z > 2.
• BAO measurements are made by detecting the characteristic scale of acoustic oscillations imprinted in the large-scale distribution of galaxies and quasars, with error estimates derived from the projected correlation function and scale dilation.
• RSD measurements are performed by analyzing the anisotropy in the redshift-space correlation function, which encodes information on the growth rate of cosmic structure via the parameter fσ₈.
• The survey uses a combination of angular and redshift coverage (14,000 to 18,000 deg²) and high target density to maximize survey volume and signal-to-noise, with systematics mitigated through reconstruction techniques.
• Cosmological constraints are derived using a Fisher matrix forecast framework, incorporating Planck priors and modeling systematic effects such as bias evolution and non-linear redshift-space distortions.

Experimental results

Research questions

• RQ1 What is the precision with which DESI can measure the baryon acoustic oscillation (BAO) scale across 35 redshift bins up to z = 1.7?
• RQ2 How accurately can DESI measure the growth rate of cosmic structure via redshift-space distortions (RSD) at 0.2 < z < 1.6?
• RQ3 To what extent can DESI extend BAO measurements to redshifts z > 2 using the Lyman-alpha forest in quasar spectra?
• RQ4 What constraints can DESI place on the sum of neutrino masses and primordial non-Gaussianity from combined BAO, RSD, and Alcock-Paczynski tests?
• RQ5 How does DESI’s dark energy figure of merit (FOM) compare to previous experiments like BOSS and future surveys like LSST?

Key findings

• DESI is projected to achieve a dark energy figure of merit (FOM) of 2723 when surveying 14,000 deg² with high density (14k-HD), representing a 12x improvement over BOSS and a 100x improvement over Stage-II experiments.
• BAO distance scale errors are projected to be 0.35–1.1% per Δz = 0.2 bin out to z = 1.7, with an aggregate precision of 0.17% across the full redshift range.
• Lyman-alpha forest measurements via quasar absorption lines yield BAO distance errors of ~1% per bin at 1.9 < z < 4, with an aggregate precision of 0.37%.
• RSD measurements of the fσ₈ parameter achieve per-bin errors below 2% over 0.2 < z < 1.6, with an aggregate precision of 0.35%.
• With 18,000 deg² of survey area, DESI can achieve a FOM of 3369, significantly exceeding the Stage-IV goal of a 10x improvement over Stage-II experiments.
• The survey will provide the first 1%-level constraints on the distance scale across 35 redshift bins, enabling unprecedented tests of cosmological models, including dark energy, neutrino masses, and inflationary signatures.