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The DESI collaboration published a new analysis of dark energy using their first three years of collected data, which spans nearly 15 million galaxies and quasars. Combining their data with other experiments shows signs that the impact of dark energy may be weakening over time — and the standard model of how the universe works may need an update.
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A long-exposure image captures circular star trails above telescope domes illuminated in red light on a dark mountain. A golden glow on the horizon marks distant city lights.
Credits
KPNO/NOIRLab/NSF/AURA/B. Tafreshi

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, once thought to be a “cosmological constant,” might be evolving over time in unexpected ways.
 

The Dark Energy Spectroscopic Instrument has made the largest 3D map of the universe to date. Fly through millions of galaxies mapped using coordinate data from DESI. Credit: DESI collaboration and Fiske Planetarium, CU Boulder


DESI is an international experiment with more than 900 researchers from over 70 institutions around the world and is managed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). 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 obtained results are very interesting”, says Andreu Font-Ribera, a scientist at the Institut de Física d’Altes Energies (IFAE) and a member of the DESI team that has developed this study. “It seems that we are on the verge of a change of paradigm for our models of the Universe, and this is very exciting”.

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 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), exploding stars (supernovae), and how light from distant galaxies is warped by gravity (weak lensing).

“In my opinion, it is still too early to claim categorically that we have discovered an evolving dark energy”, says Eusebio Sánchez, scientific researcher at CIEMAT, who has participated in the data analysis. “However, the fact that different independent projects are observing similar results make this situation especially interesting”.

So far, the preference for an evolving dark energy has not risen to “5 sigma,” the gold standard in physics that represents the 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. (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.) The analysis used a technique to hide the results from the scientists until the end, mitigating any unconscious bias about the data.

“These new data could be indicating that the Universe is more complicated that we thought so far”, comments Sergi Novell Masot, PhD student at ICCUB and a member of the Institut d’Estudis Espacials de Catalunya (IEEC),  who has recently published a complementary study of the DESI maps. ”However, before obtaining final conclusions,  we need to understand the supernovae and CMB data that combined with DESI results seem to point towards this direction”.

DESI is one of the most extensive surveys of the cosmos ever conducted. The state-of-the-art instrument can capture light from 5,000 galaxies simultaneously.
 

DESI has made the largest 3D map of our universe to date and uses it to study dark energy. Earth is at the center in this animation, and every dot is a galaxy. Credit: DESI collaboration and KPNO/NOIRLab/NSF/AURA/R. Proctor


Spanish groups had an important role in its construction and are collaborating in its operation. DESI is mounted on the U.S. National Science Foundation’s Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory in Arizona. 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) by the time the project ends.

Multiple domed buildings on a mountainside. The sky is scattered with clouds.
From its mountaintop location in Arizona, DESI maps the universe. Credit: Marilyn Sargent/Berkeley Lab

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.

“If it is confirmed, this would be one of the most important results in cosmology of the last few decades, since it opens the door to new ideas beyond the standard model, ΛCDM», comments Juan García-Bellido, a researcher at the IFT-UAM/CSIC, who has collaborated in this measurement. “If results get higher significance with future measurements we could explore ideas like new theories of gravitation or quintessence, that predict an evolving acceleration for the Universe expansion”.

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.

“We are in an very exciting moment, since for a long time we thought the Universe behaves in a certain way, but now, with more precise data, we realize that there are aspects that we do not fully understand yet”, says Laura Casas, a PhD student at the Institut de Física d’Altes Energies (IFAE) in Barcelona, who has led the validation of the Lyman-alpha forest analysis, the imprint of intervening clouds of hydrogen in the light from distant quasars. “Although there is still much to research, the hints about evolving dark energy are a fascinating finding”.

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. 

“The observational results we are obtaining about the evolution of the Universe open a wide spectrum for possible theories that can explain these observations”, comments Francisco Javier Castander, a researcher at the ICE-CSIC and IEEC, who has contributed to the experiment. “Independently of the dark energy nature, its properties will determine the Universe’s future. It is very rewarding to verify that the instrument we have built  allows us to make detailed observations of the sky, and then answer one of the biggest questions that humanity has ever asked”.

Red text overlays the faded DESI contour plot. Title: “Is DARK ENERGY changing??” The x-axis is labeled: “How dark energy behaves today.” The y-axis is labeled: “How the behavior of dark energy changes” and “Lower values mean density increases more as space grows.” A star at the intersection of the dashed lines indicates “Where predictions should fall if the standard idea of dark energy is correct (i.e. lambda, energy density is constant).” The green oval is labeled: “Expectations when combining data from: supernovae, DESI, Cosmic Microwave Background.” The inner dark green oval is labeled: “68% of predictions based on data fall here.” The outer light green oval is labeled: “95% fall inside this line.” Text at the bottom reads: “The combination of our best measurements don’t fit the ‘best’ model of the universe… Something is WEIRD!”
An annotation of DESI's contour plot. The plot describes the “behavior” of dark energy – how its density changes as space expands. Credit: Claire Lamman/DESI collaboration

Videos discussing the experiment’s new analysis are available on the DESI YouTube channel. 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. With information on millions of celestial objects, the dataset will support a wide range of astrophysical research by others, in addition to DESI’s cosmology goals. 

 

The Dark Energy Spectroscopic Instrument Collaboration

 

DESI is supported by the DOE Office of Science and by the National Energy Research Scientific Computing Center, a DOE Office of Science national user facility. Additional support for DESI is provided by the U.S. National Science Foundation; the Science and Technology Facilities Council of the United Kingdom; the Gordon and Betty Moore Foundation; the Heising-Simons Foundation; the French Alternative Energies and Atomic Energy Commission (CEA); the National Council of Humanities, Sciences, and Technologies of Mexico; the Ministry of Science and Innovation of Spain; and by the DESI member institutions.

The DESI collaboration is honored to be permitted to conduct scientific research on I’oligam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.

The spanish institutions that participate in DESi are the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), the Instituto de Ciencias del Espacio (ICE-CSIC/IEEC), the Institut de Ciències del Cosmos de la Universitat de Barcelona (ICCUB), the Institut de Física d'Altes Energies (IFAE), the Instituto de Física Teórica (IFT-UAM/CSIC), the Instituto de Astrofísica de Andalucía (IAA) and the Instituto de Astrofísica de Canarias (IAC).


Contact persons

 

CIEMAT: Dr. Eusebio Sánchez, Investigador Científico, eusebio.sanchez@ciemat.es
ICCUB-IEEC: Dr. Héctor Gil, Investigador Ramón y Cajal, hectorgil@icc.ub.edu
ICE-CSIC/IEEC: Dr. Francisco Castander, Profesor de Investigación, fjc@ice.csic.es
IFAE:    Dr. Andreu Font-Ribera, Investigador Ramón y Cajal, afont@ifae.es
IFT-UAM/CSIC: Dr. Juan García-Bellido, Catedrático, juan.garciabellido@uam.es

 

Distributed by the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), the Institut de Ciències del Cosmos de la Universitat de Barcelona (ICCUB), the Instituto de Ciencias del Espacio (ICE-CSIC), the Institut d’Estudis Espacials de Catalunya (IEEC), the Institut de Física d’Altes Energies (IFAE) and the Instituto de Física Teórica (UAM-CSIC) on behalf of the DESI Collaboration.