Cosmic chronometers in the era of large spectroscopic surveys: a cosmographic approach to H(z)
Carlos Alonso Alvarez (SISSA, Trieste)
Tuesday 06/10/2026 @ 14:00, Sala Antonio Sollima (IV piano Battiferro)
Cosmic chronometers offer a direct, cosmology-independent measurement of the expansion rate of the Universe by dating massive passively evolving galaxies across cosmic time. Large spectroscopic surveys such as SDSS and DESI now provide spectra for millions of such galaxies, shifting the challenge from statistics to the control of systematics. Here, I present a cosmographic approach that fits the continuous age-redshift relation, offering a complementary view to the standard finite-difference approximation. Stellar ages are derived from Lick indices measured on high signal-to-noise stacked spectra of early-type galaxies selected with strict spectroscopic criteria. Using ~90,000 SDSS galaxies at z < 0.4, I will show how this method yields a direct estimate of H0 consistent with the Planck 2020 results. I will also present a continuous reconstruction of H(z) over 0.36 < z < 0.80 from more than 500,000 DESI-DR1 luminous red galaxies, based on a pivotal-redshift expansion and released with its full covariance matrix. This is complemented by classical finite-difference measurements that show the level of precision attainable with these large datasets. I will then discuss the systematic error budget arising from our methodological choices, which is comparable to the statistical one. Finally, I will outline the lessons learned from these works and the open threads they leave for future research.