The DESI project

Andreu Font-Ribera


In 2015, IFAE joined the Dark Energy Spectroscopic Instrument (DESI), an international project led by LBNL (USA). Together with groups at ICE (Institut de Ciències de l’Espai), CIEMAT (Centro de Investigaciones Energéticas, Medio Ambientales y Tecnológicas) and Universidad Autónoma de Madrid (UAM), IFAE designed and built the Guiding, Focus and Alignment cameras (GFA), a key hardware component of DESI.

INTRODUCTION

In May 2021, DESI started a five-year campaign to measure accurate redshifts of 40 million galaxies and quasars over 14 000 square degrees, a third of the sky. To do this, it is using a new multi-fiber spectrograph at the Mayall 4-m telescope at Kitt Peak National Observatory, in Arizona (US), equipped with 5000 robotically actuated fibers. By the end of 2021, DESI had already collected the largest spectroscopic dataset to date, and in 2023 DESI already made public the Early Data Release (DESI EDR).

2024 Activities

On April 4th, 2024, DESI made public the measurements of Baryon Acoustic Oscillations (BAO) from the first year of observations (DESI DR1). We used 3D maps of the Universe consisting of millions of galaxies and quasars and measured a particular feature in their distribution, known as the BAO feature (an imprint of acoustic waves in the early universe). In particular, DESI presented six BAO measurements at different epochs using different types of galaxies (publication [1]) and a seventh measurement at earlier times using a different map, the Lyman-alpha forest (publication [2], co-led by Andreu Font-Ribera). The Lyman-alpha forest is a series of absorption features present in the spectrum of very distant quasars, and it allows us to reconstruct a 3D map of the distribution of gas in the Universe. Using these BAO measurements in combination with external datasets (from measurements of supernovae and the cosmic microwave background), DESI provided the tightest constraints on neutrino masses and on the nature of dark energy (publication [3]). Interestingly, the data seem to prefer a model where dark energy is not constant, but rather has a density that changes with time.

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Figure 1: BAO measurements from DESI 2024

On November 9th, 2024, DESI made public a second batch of publications. Using the same galaxy catalogs from DESI DR1, we extracted information not only from the BAO position, but from the “full-shape” of the correlation function of galaxy positions (publications [4], [5] and [6]). One of the supporting papers of these analyses, looking at the impact of varying image quality in the galaxy maps, was led by Hui Kong (publication [7]). These measurements are complementary to those of BAO, since they allow us to measure the impact of dark energy (or modified gravity) on the growth of structure over cosmic time.
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Figure 2: Galaxy maps from DESI DR1

References

  • [1] DESI Collaboration (2024a); “DESI 2024 III: Baryon Acoustic Oscillations from Galaxies and Quasars:“; https://arxiv.org/abs/2404.03000
  • [2] DESI Collaboration (2024b); “​​DESI 2024 IV: Baryon Acoustic Oscillations from the Lyman Alpha Forest“; https://arxiv.org/abs/2404.03001
  • [3] DESI Collaboration (2024c); “DESI 2024 VI: Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations“; https://arxiv.org/abs/2404.03002
  • [4] DESI Collaboration (2024d); “​​DESI 2024 II: Sample Definitions, Characteristics, and Two-point Clustering Statistics“; https://arxiv.org/abs/2411.12020
  • [5] DESI Collaboration (2024e); “​​DESI 2024 V: Full-Shape Galaxy Clustering from Galaxies and Quasars“; https://arxiv.org/abs/2411.12021
  • [6] DESI Collaboration (2024f); “​​DESI 2024 VII: Cosmological Constraints from the Full-Shape Modeling of Clustering Measurements“; https://arxiv.org/abs/2411.12022
  • [7] Kong et al. (2025); “Forward modeling fluctuations in the DESI LRGs target sample using image simulations“; Journal of Cosmology and Astroparticle Physics, Volume 2025, Issue 01, id.146, 55 pp.; 10.1088/1475-7516/2025/01/146