Probing the matter-dominated expansion with multi-redshift Lyman-$\hat{I}\pm$ BAO from DESI DR2

Abstract

We present a multi-redshift Baryon Acoustic Oscillations (BAO) analysis of the DESI Data Release 2 (DR2) Lyman-$\hat{I}\pm$ (Ly$\hat{I}\pm$) forest, splitting the forest auto-correlation and its cross-correlation with quasars into three redshift bins. We obtain BAO measurements at effective redshifts $z_{\mathrm{eff}}=2.13$, $2.40$, and $2.81$ with $\sim 2.0$--$2.5\%$ precision per bin in the radial and transverse directions, corresponding to $\sim 1.1$--$1.2\%$ precision for the isotropic BAO measurement. Using the same data products and modeling framework as the DESI DR2 Ly$\hat{I}\pm$ BAO analysis, we validate the pipeline on $400$ synthetic datasets and find unbiased BAO recovery with well-calibrated uncertainties. The measurements show an increase in the isotropic dilation parameter $D_V/r_d$ from $30.26\pm 0.39$ to $32.22\pm 0.47$ and in the Alcock-Paczyński parameter $D_M/D_H$ from $3.96\pm 0.15$ to $5.63_{-0.24}^{+0.22}$. The Hubble distance $D_H/r_d$ decreases from $9.40\pm 0.20$ to $7.22\pm 0.17$, providing a direct measurement of the expansion history consistent with $\hat{I}›$CDM and the expected matter-dominated scaling, with $H(z)\propto (1+z{)}^n$ giving $n=1.34\pm 0.16$. The redshift split also provides a self-consistent measurement of clustering evolution: the Ly$\hat{I}\pm$ forest bias evolves as $(1+z{)}^3$ with ${\hat{I}}_{\hat{I}}^3\pm =3.05\pm 0.16$, the RSD parameter has a redshift evolution described by ${\hat{I}}_{\hat{I}}^2=-0.97\pm 0.26$, and the quasar bias evolves with ${\hat{I}}_Q^3=1.56\pm 0.23$, consistent with independent quasar clustering measurements. Combining these three-bin BAO measurements with DESI DR2 galaxy and quasar BAO measurements yields cosmological constraints consistent with the single-bin Ly$\hat{I}\pm$ BAO analysis in flat $\hat{I}›$CDM and $w_0{w}_a$CDM and improves curvature constraints by $\sim 12\%$ in $\hat{I}›$CDM$+\hat{I}{\text{c◯}}_{\mathrm{K}}$.

35 pages, 15 figures