# DESI DR2 Results I: Baryon Acoustic Oscillations from the Lyman Alpha Forest

**Authors:**  DESI Collaboration, M. Abdul Karim, J. Aguilar, S. Ahlen, C. Allende Prieto, O. Alves, A. Anand, U. Andrade, E. Armengaud, A. Aviles, S. Bailey, A. Bault, S. BenZvi, D. Bianchi, C. Blake, A. Brodzeller, D. Brooks, E. Buckley-Geer, E. Burtin, R. Calderon  
**arXiv:** [2503.14739](https://arxiv.org/abs/2503.14739)  
**Date:** 2025-03-18  
**Summary page:** https://www.summarizepaper.com/en/paper/2503.14739v1/

## Key points

- Baryon Acoustic Oscillation (BAO) measurements using Lyman-alpha (LyA) forest data from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) survey
- Analysis includes auto-correlation and cross-correlation of LyA forest absorption with quasar positions
- Sample size is over 820,000 quasar spectra and positions of over 1.2 million quasars
- Achieved statistical precision of 1.1% along and 1.3% transverse to the line of sight for the BAO scale at $z_{eff} = 2.33$, resulting in a combined precision of 0.65% on the isotropic BAO scale
- Significant improvements to analysis include changes in distortion matrix calculations, model metal-line contamination, and introduction of a systematic error term due to nonlinear growth affecting BAO shifts
- Measurements yield ratios $D_H(z_{eff})/r_d = 8.632 \pm 0.098 \pm 0.026$ and $D_M(z_{eff})/r_d = 38.99 \pm 0.52 \pm 0.12$
- Structure of paper includes sections on DESI survey details, spectroscopic analysis methods, redshift measurements, QSO catalog generation, ancillary catalogs for absorption systems identification, and Lyα forest fluctuation extraction techniques
- Future analyses will focus on more precise cosmological parameter measurements through full-shape modeling of galaxy and quasar clustering similar to DR1 studies, as well as full-shape modeling of Lyα forest data with eBOSS datasets
- Supporting papers provide insights into Damped Lyman-α absorbers identification methods in data sets and validation tests using synthetic datasets constructed for analysis verification

## Summary

In this paper, we present the Baryon Acoustic Oscillation (BAO) measurements using Lyman-alpha (LyA) forest data from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) survey. Our analysis includes both the auto-correlation of LyA forest absorption in high-redshift quasar spectra and the cross-correlation of this absorption with quasar positions. With a sample size approximately twice as large as DR1, including over 820,000 quasar spectra and positions of over 1.2 million quasars, we have achieved a statistical precision of 1.1% along and 1.3% transverse to the line of sight for the BAO scale at $z_{eff} = 2.33$, resulting in a combined precision of 0.65% on the isotropic BAO scale. Significant improvements to our analysis are described in this paper, including changes to how we calculate distortion matrices and model metal-line contamination. We also introduce a systematic error term for the first time in LyA BAO analysis due to recent theoretical studies on nonlinear growth affecting BAO shifts. Our measurements yield ratios $D_H(z_{eff})/r_d = 8.632 \pm 0.098 \pm 0.026$ and $D_M(z_{eff})/r_d = 38.99 \pm 0.52 \pm 0.12$, where $D_H$ is the Hubble distance, $D_M$ is the transverse comoving distance, and $r_d$ is the sound horizon at drag epoch. The structure of this paper includes sections on DESI survey details, spectroscopic analysis methods, redshift measurements, QSO catalog generation, ancillary catalogs for absorption systems identification, and Lyα forest fluctuation extraction techniques. Future analyses using DESI DR2 data will focus on more precise cosmological parameter measurements through full-shape modeling of galaxy and quasar clustering similar to DR1 studies, as well as full-shape modeling of Lyα forest data with eBOSS datasets. Supporting papers provide further insights into Damped Lyman-α absorbers identification methods in data sets and validation tests using synthetic datasets constructed for analysis verification. Overall, our results contribute valuable information to ongoing research on dark energy properties and cosmological implications within the context of large-scale surveys like DESI.