DESI has made the largest 3D map of our universe to date. Earth is at the center of this thin slice of the full map. In the magnified section, it is easy to see the underlying structure of matter in our universe. Claire Lamman/DESI collaboration.
 

The Dark Energy Spectroscopic Instrument (DESI) has produced the best 3D map of the universe out to redshift 3 (corresponding to 11 billion years’ lookback time) over an area of 15,000 square degrees. Its 5,000 robotic fibers can measure a million spectra of distant galaxies and quasars per month. The latest results, sent to the arXiv in April 2024, are based on just the first year of data (DESI-Y1), almost 8 million spectra, about twice that of the previous complete spectroscopic survey, the Sloan Digital Sky Survey.

DESI has precisely measured the 3D positions of galaxies and quasars and determined the rate of expansion of the universe as a function of redshift. Its findings suggest that the universe is not only accelerating today, but that the agent responsible for this cosmic acceleration may be something different from a cosmological constant. If confirmed, this could create a crisis in our understanding of the universe based on the standard model of cosmology, the Lambda Cold Dark Matter (LCDM) paradigm.

These potentially revolutionary results are based on three sets of data: the Baryon Acoustic Oscillations (BAO) from DESI-Y1, the Supernova of type Ia from the five-year Dark Energy Survey data (DES-Y5), and the Cosmic Microwave Background (CMB) data from the European Planck mission. Together, they give an almost 5-sigma tension with LCDM predictions. Although the CMB data will not improve in the near future, the SN-Ia data may increase in precision when combined with other SN catalogs, but by far the best improvement will come from DESI itself, when the Y1 BAO results are complemented with BAO-Y3 data, as well as measurements of the Redshift Space Distortions (RSD) associated with the growth of fluctuations that give rise to galaxies and clusters of galaxies.

In the near future, within the next year or so, we will have many more BAO data points as a function of redshift and with significantly better error bars, thanks to at least three times more data that DESI has already collected and is being analyzed. This new precise data should give us confidence in the significance of the deviations from LCDM and may even point toward a particular scenario describing the accelerating universe within a new cosmological paradigm. There are several contenders for extensions of LCDM beyond a cosmological constant. Some consider a new scalar field (called quintessence after the Greek analog of the Aether) with some dynamics that make it move very slowly. Such models come in many forms and have completely unique consequences on the rate of change of the universe’s expansion. When the data becomes convincing enough, we should be able to discard a wide range of possibilities.

The Dark Energy Spectroscopic Instrument (DESI) is installed on the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory near Tucson, Arizona. Credits: KPNO/NOIRLab/NSF/AURA/P. Marenfeld

Alternatively, we may consider a radically new idea based on the principle of holography in quantum gravity, where the quantum degrees of freedom on the boundary of our horizon may be responsible for the present acceleration in the bulk of our space-time, in a theory known as General Relativistic Entropic Acceleration (GREA). Such a theory predicts that the present period of acceleration of the universe is a transitory epoch that will eventually end in flat and empty space-time, billions of years into the future. The precise rate of change of the universe’s expansion is sufficiently distinct from LCDM and quintessence predictions that we may be able to rule it out in a few years, thanks to DESI and other future surveys. In fact, the European Euclid mission was successfully launched on July 1, 2023, and will be releasing its primary results in a year from now. It will be fascinating to see whether Euclid confirms DESI’s results, since they have entirely different systematics (spectroscopic versus photometric and ground versus satellite telescopes). Moreover, in the not-so-distant future, we will also have contributions from the Vera Rubin Observatory LSST survey, the Nancy Roman Space Telescope, as well as the Simons Observatory. All of these next-generation surveys should be able to determine with high precision whether the hints we have today about a crisis in cosmology are real and be able to distinguish between alternative scenarios that may constitute a new paradigm shift in cosmology.

DESI is an international collaboration of more than 900 researchers from over 70 institutions around the world. The instrument was constructed and is operated with funding from the DOE Office of Science, and sits atop the U.S. National Science Foundation’s Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, a program of NSF’s NOIRLab. The DESI collaboration is honored to be permitted to conduct scientific research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.

A complete list of participant institutions and more information about DESI is available at: https://www.desi.lbl.gov.