Two Cosmological Surveys Strengthen Hints That Standard Model of Cosmology Is Wrong

The idea that the Universe is expanding emerged from multiple observations conducted by various scientists working independently throughout the early 1900s. The discovery culminated with Edwin Hubble’s breakthrough paper published in 1929.

For decades, the Lambda Cold Dark Matter (ΛCDM) model has served as the foundation of modern cosmology. This model describes a Universe composed of 5% ordinary matter, 25% dark matter, and 70% dark energy — a mysterious force responsible for cosmic acceleration. According to ΛCDM, dark energy is represented by the cosmological constant (Λ), meaning its energy density remains unchanged over time.

However, new findings from the Dark Energy Spectroscopic Instrument (DESI) Survey (see press releases from NOIRLab and Berkeley Lab) and the Dark Energy Survey (DES) (see press releases from NOIRLab and CIEMAT) challenge this assumption, hinting at a time-varying nature of dark energy.

This research represents a major step toward understanding dark energy.

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 for construction and operation from the DOE’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. DESI, which began its survey in 2021, is a state-of-the-art instrument that can capture light from 5000 galaxies simultaneously. By precisely mapping millions of galaxies and quasars, the DESI collaboration has created the largest ever 3D map of our Universe.

The U.S. National Science Foundation Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, a Program of NSF NOIRLab, which hosts the Dark Energy Spectroscopic Instrument (DESI).

Credit: DESI Collaboration/DOE/KPNO/NOIRLab/NSF/AURA/L. Tyas

The DES is an international collaboration comprising more than 400 scientists from over 25 institutions, led by the DOE’s Fermi National Accelerator Laboratory. The DES was an imaging survey that was conducted using the 570-megapixel DOE-fabricated Dark Energy Camera (DECam), mounted on the NSF Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile, a Program of NSF NOIRLab. DES began its survey in 2013, and completed it in 2019. By taking data on 758 nights, DES scientists mapped an area almost one-eighth of the entire sky. The project employs multiple observational techniques, including supernova measurements, galaxy clustering analysis, and weak gravitational lensing, to study dark energy.

Both surveys track dark energy’s influence by studying how matter is spread across the Universe [1]. Events in the very early Universe left subtle patterns in how matter is distributed, a feature called Baryon Acoustic Oscillations (BAO). BAO refers to a standard cosmic ruler formed by sound waves in the early Universe, with peaks spanning approximately 500 million light-years. Astronomers can measure these peaks across several periods of cosmic history to see how dark energy has stretched the scale over time.

The U.S. National Science Foundation Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile, a Program of NSF NOIRLab. The Blanco is home to the 570-megapixel Department of Energy-fabricated Dark Energy Camera (DECam) conducting DES.

Credit: CTIO/NOIRLab/NSF/AURA/P. Horálek (Institute of Physics in Opava)

Using DESI’s first three years of observations, which contain the most accurate distance measurements ever achieved for nearly 15 million galaxies and quasars, the DESI collaboration calculated the BAO scale across 11 billion years of cosmic history. DESI’s precision with this approach is the best in the world.

DESI researchers found that when DESI BAO data are combined with other cosmological data — such as observations of the cosmic microwave background (CMB) and weak gravitational lensing, as well as supernova data from DES — it points to inconsistencies with the standard model. This combined analysis suggests dark energy’s impact may be weakening over time. While the statistical significance of the results to date is not yet at the ‘5-sigma’ level — the gold standard in physics that represents the commonly accepted threshold for a discovery — the data are remarkably consistent in suggesting, tantalizingly, that dark energy may be evolving over time. These results are presented in multiple papers that will be posted on the online repository arXiv.

“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.”

Two key measurements from DES — Type Ia supernovae and the BAO scale [2] — provide complementary insights. Type Ia supernovae serve as ‘standard candles’, enabling precise distance calculations. In early 2024 DES published the most extensive and detailed supernova dataset to date, followed by measurements of the BAO scale in the early Universe. The new analysis, which is presented in a paper appearing on arXiv, combines these earlier results with additional measurements to confirm the anomalies seen in the 2024 data and suggest that dark energy might evolve over time.

We are witnessing what may be the first signs of a new paradigm in cosmology.

Together, the robust sample of supernovae across a wide range of distances provided by DES, plus the most precise measurements ever of the BAO scale achieved by DESI, are revealing unprecedented insights into the nature of dark energy and the expansion history of the Universe.

The results from DESI and DES were presented at the American Physical Society’s Global Physics Summit — the largest annual gathering of physicists — in separate sessions on 19 March in Anaheim, California.

This slice of the DESI data maps celestial objects from Earth (center) to billions of light-years away. The large-scale structure of the Universe is visible in the inset image, which shows the densest survey region and represents less than 0.1% of the DESI survey’s total volume.

Credit: DESI Collaboration/C. Lamman/DOE/KPNO/NOIRLab/NSF/AURA

“This research represents a major step toward understanding dark energy,” said Harriet Kung, Acting Director of the Department of Energy’s Office of Science. “The precision and depth of DESI’s and DES’s findings are unprecedented, and we are excited about the potential implications for fundamental physics. This is energy-research at its finest.”

If dark energy is indeed evolving, it could signify the existence of a previously unknown field or necessitate modifications to Einstein’s general relativity. Such a discovery would profoundly reshape fundamental physics and our understanding of the cosmos.

“We are witnessing what may be the first signs of a new paradigm in cosmology,” said NOIRLab Director Patrick McCarthy. “As we push the boundaries of our knowledge, surveys like DESI and DES continue to challenge our understanding of the Universe, opening doors to exciting new possibilities.”

NSF–DOE Vera C. Rubin Observatory on the Cerro Pachón ridge in north-central Chile.

Credit: RubinObs/NOIRLab/SLAC/NSF/DOE/AURA/B. Quint

Future large-scale cosmological surveys, such as NSF–DOE Vera C. Rubin Observatory’s upcoming Legacy Survey of Space and Time (LSST), which will begin in late 2025, will provide further understanding of the nature of dark energy. Additionally, an extension of the DESI mission, named DESI-II, is in planning. DES, DESI, LSST, and DESI-II, represent a planned sequence of experiments, each of successive complexity, aimed at understanding dark energy and the physics of the early Universe.

While definitive confirmation of an evolving dark energy requires more data, the evidence continues to mount. Scientists worldwide eagerly anticipate the next generation of cosmological experiments, as the Universe may be revealing a deeper, more complex story than we ever imagined.

Notes

[1] DES is a Stage III (previous-generation, intermediate-scale) ground-based dark energy experiment, whereas DESI is the first Stage IV (longer-term, larger-scale) experiment to come online, and features improved precision and sensitivity.

[2] The DES and DESI BAO measurements use different approaches; DESI measures very accurate distances to galaxies based on their spectra to then map the full 3D BAO structure, whereas DES uses approximate estimates of galaxy distances based on their brightnesses through different color filters.