Cosmography with DESI-DR1 Cosmic Chronometers: Direct H(z) measurements from Luminous Red Galaxy ages · arXivDesk
2608.13178Aug 13, 202614 main text pages (7 figures), 4 appendix pages (6 figures). Supplementary material will be provided in the journal version of the article, currently under correction
Cosmography with DESI-DR1 Cosmic Chronometers: Direct H(z) measurements from Luminous Red Galaxy ages
Carlos A. Álvarez, Marcos M. Cueli, Balakrishna S. Haridasu, Michele Moresco, Martina Torsello, Alessandro Bressan, Lumen Boco, Luigi Danese +1
Providing robust redshift estimates for almost 3 million luminous red galaxies (LRGs), the Dark Energy Spectroscopic Instrument (DESI) offers a unique opportunity to test the expansion rate of the Universe with independent approaches. We apply the cosmic chronometer method to derive new, independent constraints on the Hubble parameter at 0.3<z<1.2 from the differential age evolution of DESI LRGs. We select spectra applying spectroscopic cuts to ensure sample purity and remove contamination by star-forming objects, then build a robust sample of cosmic chronometers (CCs) by stacking to obtain stable, high signal-to-noise (S/N) spectra, which also serves as a democratic binning choice for the t−z plane. Ages are estimated by measuring Lick indices on the stacked spectra and fitting them with a theoretical stellar population model. We obtain t−z relations from which we derive H(z)
Nearby in the stack
constraints via two independent approaches: a fit with a pivotal-redshift cosmography, and a direct estimate from the original CC method. The cosmographic fit yields posteriors for the kinematic parameters
{Hz0,qz0,jz0}
compatible with currently considered cosmologies, giving a precision-level estimate of
H(z)
. We provide the maximum-a-posteriori (MAP)
H(z)
estimate, an array of the median confidence region in the
H−z
plane, and its covariance matrix. We also leverage the redshift distributions of the
t−z
relation for different velocity dispersion groups to obtain two independent local measurements using the discrete approximation
H(z)≈−Δz/[Δt(1+z)]
; the one from the reddest envelope of CCs gives
H(z≈0.61)=88.5−12.6+6.7
(stat.)
±8.1
(syst.) km s
−1
Mpc
−1
. Systematic uncertainties for both the cosmographic and discrete
H(z)
measurements come from a comprehensive analysis of all methodological choices in the data treatment.