DESI First-Year Results (2024): A Hint That Dark Energy Changes

The DESI 2024 results, released in April 2024 by the Dark Energy Spectroscopic Instrument collaboration, gave the first hint in decades that dark energy may not be constant. Measuring a cosmic “standard ruler” in more than 6 million galaxies and quasars, DESI found a 2.5 to 3.9 sigma preference for dark energy that changes over time.

The question before the paper

Since the 1998 discovery of cosmic acceleration, the simplest explanation has been a cosmological constant: an energy of empty space with the same density at all times. In the standard Lambda-CDM model it is described by a single number, with an equation of state w = -1. Every test had been consistent with that. But a constant is an assumption, and testing it requires measuring the expansion history across most of cosmic time with very high precision.

What the paper found

• A precise matter density. DESI’s measurements alone gave Ω_m = 0.295 ± 0.015.
• A Hubble constant in line with Planck. Combined with cosmic microwave background data, H₀ = 67.97 ± 0.38 km/s per megaparsec.
• A hint of evolving dark energy. Allowing dark energy to change, described by two numbers w₀ and w_a, the data combined with the cosmic microwave background and supernovae preferred w₀ > -1 and w_a < 0: dark energy that was stronger in the past and is weakening now. The departure from Lambda-CDM was 2.5, 3.5 or 3.9 sigma depending on the supernova sample.
• Light neutrinos. Within Lambda-CDM, the sum of neutrino masses must be below 0.072 eV (95% confidence).

How they did it

DESI measures baryon acoustic oscillations, a preferred spacing between galaxies left by sound waves in the early universe. Because the true size of that spacing is known, its apparent size at different distances tracks how the universe expanded. This first-year analysis used more than 6 million extragalactic objects at redshifts from 0.1 to 4.2, split into seven redshift bins, using galaxies, quasars and the “Lyman-alpha forest”, the pattern of hydrogen absorption in the light of very distant quasars.

The analysis was blinded: the team hid the true results from themselves until the method was fixed, to avoid steering the outcome toward what they expected.

Why it mattered

The result made the nature of dark energy an open experimental question again, rather than a settled parameter. The strongest preference, 3.9 sigma, came from combining DESI with the Dark Energy Survey’s five-year supernova sample. None of the combinations reached the 5-sigma level physicists use to claim a discovery, so the result was presented as a hint, not a detection.

What we know now

DESI’s second data release in 2025, with more than 14 million objects, strengthened the hint to 2.8 to 4.2 sigma. A July 2026 Lyman-alpha analysis of the same data moved slightly back toward Lambda-CDM. The DESI survey finished its planned five-year map in April 2026, and full-survey dark energy results are expected in 2027. Follow the story in is dark energy weakening?

Who wrote it

The paper is signed by the DESI Collaboration, which today involves more than 900 researchers from over 70 institutions. Individual authors follow the collaboration name, starting with A. G. Adame and J. Aguilar. It is the sixth paper in DESI’s 2024 series, the one with the cosmological conclusions.

Read the original

DESI Collaboration, Adame, A. G., Aguilar, J., et al. (2025). “DESI 2024 VI: cosmological constraints from the measurements of baryon acoustic oscillations.” Journal of Cosmology and Astroparticle Physics. doi:10.1088/1475-7516/2025/02/021. Released April 2024, published February 2025. Free on arXiv: 2404.03002.