19 Mar 2025-------------- Noirlab 2512

 

The DESI collaboration has published a new analysis of dark energy using their first three years of collected data, which spans nearly 15 million galaxies and quasars. Combined with studies of the cosmic microwave background, supernovae, and weak lensing, the analysis hints that dark energy changes over time. Data Release 1, containing data from DESI’s survey validation and first year of observations, has now been made available for scientists and the public to explore.

 

The fate of the Universe hinges on the balance between matter and dark energy — the fundamental ingredient that drives its accelerating expansion. New results from the Dark Energy Spectroscopic Instrument (DESI) collaboration use the largest 3D map of our Universe ever made to track dark energy’s influence over the past 11 billion years. Researchers see hints that dark energy, widely thought to be a 'cosmological constant', might be evolving over time in unexpected ways.

DESI is an international experiment with more than 900 researchers from over 70 institutions around the world. DESI is managed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (LBNL) with primary funding from the Department’s Office of Science. The instrument is mounted on the U.S. National Science Foundation Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, a Program of NSF NOIRLab.

The collaboration shared their findings today in multiple papers that will be posted on the online repository arXiv and in a presentation at the American Physical Society’s Global Physics Summit in Anaheim, California.

“The Universe never ceases to amaze and surprise us,” says Arjun Dey, NOIRLab's DESI Project Scientist and Mid-Scale Observatories' Associate Director for Strategic Initiatives. “By revealing the evolving textures of the fabric of our Universe as never before, DESI and the Mayall telescope are changing our very understanding of the future of our Universe and nature itself.”

Taken alone, DESI’s data are consistent with our standard model of the Universe: Lambda CDM, where CDM is cold dark matter and lambda represents the simplest case of dark energy, where it acts as a cosmological constant. However, when paired with other measurements, there are mounting indications that the impact of dark energy may be weakening over time and that other models may be a better fit.

Those other measurements include the light leftover from the dawn of the Universe (the cosmic microwave background or CMB), distance measurements of exploding stars (supernovae), and observations of how light from distant galaxies is warped by the gravitational influence of dark matter (weak lensing).

So far, the preference for an evolving dark energy has not risen to '5 sigma' — the gold standard in physics that represents the commonly accepted threshold for a discovery. However, different combinations of DESI data with the CMB, weak lensing, and supernovae sets range from 2.8 to 4.2 sigma [1].

The analysis used a technique to hide the results from the scientists until the end, mitigating any unconscious bias about the data. This approach sets a new standard in how data from large spectroscopic surveys are analyzed.

DESI is a state-of-the-art instrument that can capture light from 5000 galaxies simultaneously, enabling it to conduct one of the most extensive surveys of the cosmos ever. The experiment is now in its fourth of five years surveying the sky, with plans to measure roughly 50 million galaxies and quasars (extremely distant yet bright objects with black holes at their cores) and more than 10 million stars by the time the project ends.

The new analysis uses data from the first three years of observations and includes nearly 15 million of the best measured galaxies and quasars. It’s a major leap forward, improving the experiment’s precision with a dataset that is more than double what was used in DESI’s first analysis, which also hinted at an evolving dark energy.

DESI tracks dark energy’s influence by studying how matter is spread across the Universe. Events in the very early Universe left subtle patterns in how matter is distributed, a feature called Baryon Acoustic Oscillations (BAO). That BAO pattern acts as a standard ruler, with its size at different times directly affected by how the Universe was expanding. Measuring the ruler at different distances shows researchers the strength of dark energy throughout history. DESI’s precision with this approach is the best in the world.

The collaboration will soon begin work on additional analyses to extract even more information from the current dataset, and DESI will continue collecting data. Other experiments coming online over the next several years will also provide complementary datasets for future analyses.

“Our results are fertile ground for our theory colleagues as they look at new and existing models, and we’re excited to see what they come up with,” said Michael Levi, DESI director and a scientist at Berkeley Lab. “Whatever the nature of dark energy is, it will shape the future of our Universe. It’s pretty remarkable that we can look up at the sky with our telescopes and try to answer one of the biggest questions that humanity has ever asked.”

“These are remarkable results from an incredibly successful project,” says Chris Davis, NSF program director for NSF NOIRLab. “The potent combination of the NSF Mayall Telescope and DOE’s Dark Energy Spectroscopic Instrument shows the benefits of federal agencies working together on fundamental science that improves our understanding of the Universe.”

Alongside unveiling its latest dark energy results at the APS meeting today, the DESI collaboration also announced that its Data Release 1 (DR1) is now available for anyone to explore. The full set of DR1 files are available to access through the National Energy Research Scientific Computing Center (NERSC), a facility at Berkeley Lab where DESI processes and stores data. For additional convenience, the DR1 main catalogs and the full-depth spectra of DESI’s data release are made available as searchable databases via the Astro Data Lab and SPARCL at the Community Science and Data Center, a Program of NSF NOIRLab. These services are accessible to the entire astronomy community to facilitate data access and analysis. Space fans can also explore some of DESI’s data through an interactive portal: the Legacy Survey Sky Browser.

Stephanie Juneau, NSF NOIRLab astronomer, DESI Data team member, and the Project Scientist for SPARCL, says, “SPARCL provides the community with a user-friendly yet very powerful method to quickly access the spectra from a database, which can be much faster than reading files.”

The dataset is the largest of its kind, with information on 18.7 million objects — roughly 4 million stars, 13.1 million galaxies, and 1.6 million quasars (extremely bright but distant objects powered by supermassive black holes at their cores). 

The new dataset vastly expands DESI’s Early Data Release (EDR), containing roughly 10 times as much data and covering seven times the area of sky. DR1 includes information from the first year of the 'main survey' collected between May 2021 and June 2022, as well as from the preceding five-month 'survey validation' where researchers tested the experiment.

“This new dataset being so large is exciting but it can be challenging to navigate this immense sea of data. So we prepared example tutorials that researchers and students can follow along,” says Juneau.

Although DR1 is just a fraction of what DESI will eventually produce, the 270-terabyte dataset represents a staggering amount of information, including precise distances to millions of galaxies. The release contains more than twice as many extragalactic objects (those found outside our galaxy) as have been collected in all previous 3D surveys combined. 

 

Notes

[1] A statistical significance of 5 sigma means an almost certain likelihood that a result is not a statistical fluctuation. A 3-sigma event has a 0.3% chance of being a statistical fluke, but many 3-sigma events in physics have faded away with more data.

 

[Image]

(A) Nicholas U.Mayall 4-meter Telescope at Kitt Peak National Observatory

(B) (C) Nicholas U.Mayall 4-meter Telescope Interior

(D) Nicholas U.Mayall 4-meter Telescope Mirror

(E) DESI Spectrograph Room

(F) DESI Butterfly Plot (no inset)

(G) Simplified DESI Butterfly Plot

(H) DESI Year-3 Fan Plot (wide)

(I) DESI Year-3 Fan Plot (fulldome still)

(J) DESI Contour Plot (not annotated)

(K) DESI Contour Plot (annotated)