Astrophysics

DESI DR2 Results IV: Alcock-Paczyński Measurements from the Lyman Alpha Forest and Cosmological Constraints

DESI Collaboration, A. G. Adame, J. Aguilar

Featured August 9, 2026

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Simply

By carefully measuring how cosmic patterns appear stretched or squashed in the distant universe, the authors precisely map the universe's expansion, finding a tighter fit for its history than ever before.

In depth

This paper significantly improves cosmological constraints at high redshifts by applying the Alcock–Paczyński (AP) effect to the full shape of Lyman-alpha (Lyα) forest correlation functions from DESI DR2 data. By leveraging the anisotropic distortions imprinted on cosmic structures, the authors achieve a 1% precision on the distance ratio

D_{M} / D_{H}

, which is nearly twice as tight as previous BAO-only measurements, providing a more precise anchor for the universe's expansion history.

Key Takeaways

1
The study achieves a 1% precision on the Alcock–Paczyński (AP) effect from the full shape of Lyα forest correlations, significantly improving high-redshift distance measurements.
2
A novel small-scale marginalization technique is introduced to accurately model distortions from quasar continuum fitting, ensuring robust parameter constraints.
3
The new measurements reduce the tension between DESI and CMB data regarding the matter fraction ( $Ω_{m}$ ) and provide a 2% constraint on the Hubble constant ( $H_{0}$ ) independent of CMB anisotropies.

Conceptual Flow

HIGH LEVEL

Methodology: Measuring Cosmic Distortions

The scientists measure how cosmic patterns look stretched or squashed to figure out how fast the universe is growing.

Light from Quasars

Gas Clouds

Measure Patterns

Stretched Cosmic Map

Results: Sharper View of Expansion

Their new measurements give a much clearer picture of how the universe expanded long ago, helping to solve big cosmic puzzles.

Old Cosmic Map

New Cosmic Map

Compare Details

Clearer Growth Rate