A cosmographic analysis using DESI-DR2 and strong lensing: I. Time-Delay measurements

TL;DR

This work develops a model-independent cosmographic analysis by combining strong-lensing time-delay distances with the distance sum rule and multiple distance indicators, notably DESI-DR2 BAO and three SNIa samples (PantheonPlus, Union3, DESY5). By expanding distances in a $y$-redshift cosmography up to the fourth order and applying the generalized DSR, the authors jointly constrain $H_0$, $\Omega_{k0}$, and cosmographic parameters $q_0$, $j_0$, $s_0$ via Bayesian MCMC. Without DESI-DR2 the results prefer an open geometry and show broad constraints, while including DESI-DR2 markedly tightens the parameter contours and brings central values closer to ΛCDM predictions, though $s_0$ remains weakly constrained. The study demonstrates the critical role of DESI-DR2 in reducing degeneracies and improving the robustness of cosmographic inferences, and it lays the groundwork for a second companion paper on lensing distance ratios as an additional cross-check of cosmic curvature and expansion history.

Abstract

Strong gravitational lensing time-delay measurements, together with the distance sum rule (DSR), offer a model-independent approach to probe the geometry and expansion of the universe without relying on a fiducial cosmological model. In this work, we perform a cosmographic analysis by combining the latest Type Ia supernova datasets (PantheonPlus, DESY5, and Union3), baryon acoustic oscillation data from DESI-DR2, and updated time-delay distances from strong lensing systems. The analyses using SGL with individual SNIa datasets (SGL+PantheonPlus, SGL+DESY5, and SGL+Union3) indicate a preference for an open universe, though they remain consistent with spatially flat universe at the $95%$ confidence level. When DESI-DR2 data is included in each combination, the constraints tighten and shift slightly toward a closed universe, while flatness remains supported at the $68%$ confidence level. The best-fit values of $q_0$ and $j_0$ agree with $Λ$CDM expectations within $95%$ or $99%$ confidence depending on the dataset, whereas $s_0$ remains weakly constrained in all cases. This work is the first in a series of two companion papers on cosmography with DESI-DR2 and strong lensing.

Figures (4)

• Figure 1: (a) Posterior contours for SGL+PantheonPlus and SGL+PantheonPlus+DESI-DR2 datasets; (b)–(c) corresponding correlation matrices for each dataset combination.
• Figure 2: (a) Posterior contours for SGL+Union3 and SGL+Union3+DESI-DR2 datasets; (b)–(c) corresponding correlation matrices for each dataset combination.
• Figure 3: (a) Posterior contours for SGL+DESY5 and SGL+DESY5+DESI-DR2 datasets; (b)–(c) corresponding correlation matrices for each dataset combination.
• Figure 4: Tension matrix showing the statistical deviation (in units of $\sigma$) of cosmographic parameters --- deceleration parameter ($q_0$), jerk parameter ($j_0$), and snap parameter ($s_0$) --- with respect to the reference $\Lambda$CDM values ($H_0$ from Planck, $\Omega_{k0}=0$, $q_0 = -0.55$, $j_0 = 1$, $s_0 = -0.35$) across seven different combinations of observational datasets.