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DESI (2024–25): is dark energy weakening?
2024· Discovery / experiment
SpeculativeA serious idea with no experimental proof yet.
A huge 3D galaxy map whose 2024 and 2025 results hint, not yet conclusively, that dark energy is weakening over time.
See it on the Science map →How it works
DESI, the Dark Energy Spectroscopic Instrument, sits on the 4-meter Mayall Telescope at Kitt Peak, Arizona. Its 5,000 robotic fibers each aim at one galaxy per exposure, and in three years it recorded more than 14 million galaxies and quasars. It measures a cosmic ruler, the roughly 500-million-light-year spacing left by early sound waves, at many eras over 11 billion years. Combined with the afterglow and supernovae, the history fits a dark energy that peaked several billion years ago and is fading.
Why it matters
If confirmed, dark energy is not Einstein's constant, ΛCDM needs revising, and the universe's long-term fate becomes an open question again.
How we know
First-year results (April 2024, 2.5–3.9σ) and three-year results (March 2025, 2.8–4.2σ) prefer changing dark energy, the strength depending on which supernova sample is combined. Physics demands 5σ for a discovery.
Still a mystery
Is it real or a calibration quirk? Without supernovae, DESI plus the afterglow prefers change at about 3σ. A related puzzle: assuming constant dark energy, DESI leaves almost no room for neutrino mass, which experiments show exists.
Connections (2)
▸DESI (2024–25): is dark energy weakening? hints at Quintessence: dark energy that changesSpeculative
The relationship
DESI's distances, combined with the afterglow and supernovae, fit best with a dark energy whose pressure is drifting: once more negative than a constant's, now less. A weakening push today is what a quintessence field gently rolling downhill would give. So the data point toward a changing field rather than a fixed property of space, though not yet decisively.
How the link was found
The DESI collaboration, more than 900 researchers led from Lawrence Berkeley National Laboratory, in its April 2024 and March 2025 analyses.
Is it proven?
Not proven. A significance of 2.8–4.2σ is suggestive, not conclusive, and depends on the supernova sample used. Hints at this level have vanished before.
Open questions and mysteries
The best fit's earlier phase, with pressure more negative than a constant's, is something simple quintessence cannot produce. Is it a more exotic field, dark energy trading energy with dark matter, modified gravity, or error?
▸DESI (2024–25): is dark energy weakening? challenges The cosmological constant (Λ)Speculative
The relationship
A true cosmological constant has the same density at every era, so the expansion history it produces is fixed once the amount of matter is known. DESI measures distances in seven slices of cosmic time spanning 11 billion years. If dark energy's density has changed at all, Λ is ruled out as its explanation, and the Λ in ΛCDM must be replaced.
How the link was found
DESI's first-year results (April 2024) first raised the challenge; the three-year results (March 2025) strengthened it.
Is it proven?
Not yet decisive. George Efstathiou argued in 2024 that calibration differences between supernova samples could drive much of the signal. DESI's own distances alone sit closer to a constant.
Open questions and mysteries
Will DESI's full five-year data, plus Euclid and the Rubin Observatory, push the signal to 5σ or erase it? The answer decides between constant and changing dark energy.