The Dark Energy Spectroscopic Instrument (DESI) allows astronomers to undertake a spectroscopic galaxy survey 20 times faster than the Sloan telescope, which was used for the Sloan Digital Sky Survey. DESI started survey operations on May 17, 2021 and is designed to study Dark Energy at a level not previously available. The start of the survey was accompanied by a press release from the WCA looking ahead to the science to come. DESI is an international project that has turned the Mayall telescope at Kitt Peak National Observatory into the most powerful wide-field spectroscopic facility in the world. Percival leads the DESI group at the WCA, is a founding member of DESI, and has served in many management positions for the project. The survey will now continue over a five-year period aiming to get spectra for on the order of 30 million galaxies.
The problem that DESI will address is one of the most important questions in modern physics: what Dark Energy is, the term given for the unknown physics driving the current acceleration of the expansion rate of the Universe. Dark Energy represents around 75% of the energy content of the Universe today, and together with Dark Matter it dominates the Universe’s matter-energy density. To help to understand Dark Energy, DESI will use two techniques: Baryon Acoustic Oscillations (BAO) to understand the expansion rate, and Redshift Space Distortions (RSD) to understand the growth of structure within it.
Baryon Acoustic Oscillations are a pattern seen in spacetime where galaxies are preferentially located in spheres clustered around other galaxies. These patterns have the same comoving size, allowing them to be used as standard rulers to measure the expansion rate.
Redshift-Space Distortions refer to changes between the true field of galaxies and that recovered when the Hubble expansion is assumed, when translating observed redshifts to distances. The coherent offsets trace the peculiar velocities of galaxies resulting from structure growth and allow scientists to measure test General Relativity. For the WCA team, the goal of this survey is to use BAO, RSD and other techniques to test the standard model of cosmology, and whether Dark Energy matches Einstein’s cosmological constant Lambda.