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The largest three-dimensional map of the universe ever assembled is forcing physicists to reconsider a foundational assumption about the cosmos: that dark energy, the mysterious force accelerating expansion, behaves as a constant.The Dark Energy Spectroscopic Instrument collaboration, working from a mountaintop telescope in Arizona, has now catalogued 18.7 million celestial objects stretching back 11 billion years. Their data, released in stages since April 2024 and culminating in a landmark March 2025 analysis, points to something Albert Einstein's equations did not predict. Dark energy appears to be changing over time.
For a quarter century, the Lambda-CDM model has served as cosmology's standard framework. It treats dark energy as a cosmological constant, a fixed value Einstein introduced to his field equations in 1917 and later called his "greatest blunder." The model has withstood every major observational test, from the cosmic microwave background measured by the Planck satellite to galaxy surveys conducted by the Sloan Digital Sky Survey over more than a decade.
DESI was built to stress-test that framework with unprecedented precision. Mounted on the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, the instrument deploys 5,000 robotic fiber positioners that reconfigure every 20 minutes, capturing spectra from roughly 100,000 galaxies per night. Construction cost $75 million, finishing $1.9 million under budget.
The collaboration's Year-3 results, drawn from 15 million galaxies and quasars, achieved 0.5 percent precision in measuring the expansion history of the universe. For the period between eight and 11 billion years ago, precision exceeded one percent, a threshold no prior survey had reached.
When combined with data from the Planck satellite and Type Ia supernovae observations, the measurements favor a model where dark energy strengthens and then weakens over cosmic time. The statistical significance ranges between 2.8 and 4.2 sigma depending on the dataset combination, short of the five-sigma threshold physicists require to claim discovery but strong enough to demand attention.
"We're seeing some potentially interesting differences that could indicate that dark energy is evolving with time," said Michael Levi, DESI director at Lawrence Berkeley National Laboratory.
The collaboration spans more than 900 researchers across 70 institutions worldwide, with Lawrence Berkeley National Laboratory managing the project and the National Energy Research Scientific Computing Center processing the 270 terabytes of data contained in the first public release alone.
If dark energy is indeed weakening, the consequences reach far beyond academic journals. The standard prediction for the universe's fate, an eternal cold expansion known as heat death, would need revision. A weakening dark energy could allow gravity to eventually dominate again, potentially leading to a contraction.
Carlos Frenk, a professor at Durham University and veteran cosmologist, described the findings as revealing "chinks in the armor" of the standard model, suggesting the field may be approaching a fundamental rethinking of cosmic physics.
For full coverage, visit https://www.linos.ai/science/desi-cosmic-map-dark-energy-challenge/
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