Is Dark Energy Fading? What DESI’s 4.2-Sigma Hint Does and Doesn’t Mean

For a quarter-century, cosmology has treated dark energy as a constant, a flat tax on the expansion of space that never changes. DESI, a spectroscopic survey, has now measured something that might not fit that picture. The key word is might.

• Announced March 19, 2025, DESI’s analysis of nearly 15 million galaxies and quasars found 2.8 to 4.2 sigma evidence for evolving dark energy, depending on which datasets are combined ^[1].
• That is short of the five-sigma threshold considered a discovery ^[1].
• DESI collaboration co-chair Mustapha Ishak-Boushaki said: “With a 4.2-sigma significance, I think we are getting to the point of no return” ^[1].
• BAO, the “standard ruler” DESI uses, reads cosmic expansion history from the clustering of matter ^[2].

What DESI measured

DESI uses baryon acoustic oscillations, ripples in the density of matter left over from the early universe, as a “standard ruler” to trace how the universe has expanded over time ^[2]. Its second data release covered the first three years of observations, drawn from nearly 15 million galaxies and quasars, the largest sample ever used for such a measurement ^[1][2]. Announced on March 19, 2025 at the American Physical Society Global Physics Summit, the analysis reported evidence that dark energy may be evolving, with its influence perhaps weakening over time ^[1].

How strong is the evidence?

Between 2.8 and 4.2 sigma, depending on which supernova and other datasets are combined with DESI and the cosmic microwave background ^[1]. The upper end sounds impressive, but particle physics asks for five sigma before it uses the word discovery ^[1]. The collaboration itself reports systematic uncertainties alongside statistical ones and notes the evidence is robust across methods, especially at low redshift ^[2].

The sentence worth quoting

Working-group co-chair Mustapha Ishak-Boushaki put it this way: “With a 4.2-sigma significance, I think we are getting to the point of no return” ^[1]. That is a scientist saying the idea has earned serious attention, not that it is settled.

Under the hood of the DESI result

The headline rests on three years of data. DESI’s second data release uses its first three years of observations, more than twice the size of the first release and, in the collaboration’s words, the largest ever used to measure baryon acoustic oscillations ^[2]. The instrument sits on the Mayall Telescope at Kitt Peak and uses 5,000 robotic fibers to take spectra of thousands of objects at once ^[2]. The statistical precision is striking: about 0.24 percent for the galaxy and quasar BAO measurements, and 0.65 percent for the Lyman-alpha forest above redshift two ^[2].

The results were fitted with a two-parameter description of dark energy, called w0waCDM, and the evidence for change is strongest at redshifts below 0.3; the constraints combine DESI with cosmic microwave background data, supernova samples such as Union3, and weak-lensing measurements ^[2]. A side result concerns neutrinos: the combined mass is limited to less than 0.0642 electron volts, which sits in 3-sigma tension with the laboratory lower limit of 0.059 electron volts if the standard model is assumed, but relaxes to an upper limit of 0.163 electron volts if dark energy is allowed to evolve ^[2]. That is one of the reasons the claim is being taken seriously, and also one reason to wait for more data before calling it settled.

Why this matters even if it fades

If dark energy is weakening, the long-term fate of the universe changes: a cosmos that expands forever at an accelerating rate becomes one that could slow. If the signal evaporates as more data arrives, cosmology will have learned how much a three-year dataset can fool us. Either way, the next data release is the test. Treat this as the starting gun of a race, not the finish line.

Questions readers ask

Did DESI discover that dark energy is weakening?

Not yet. It found 2.8 to 4.2 sigma evidence for evolving dark energy, below the five-sigma level considered a discovery ^[1].

What is BAO?

Baryon acoustic oscillations are density ripples from the early universe that act as a standard ruler for measuring cosmic expansion ^[2].