Measuring the expansion history of the Universe with DESI Cosmic Chronometers

Abstract: Studying large samples of massive, passively evolving galaxies (called cosmic chronometers, CC) provides us with the unique ability to measure the Universe's expansion history without assuming a cosmological model. The Dark Energy Spectroscopic Instrument (DESI) DR1 is currently the largest, publicly available, homogeneous set of galaxies with reliable spectroscopic redshifts, and covers a wide range in redshift. We extracted all massive galaxies (stellar mass $\log M_{\star}/M_{\odot} > 10.75$, and velocity dispersion $\sigma > 280$ km s$^{-1}$), with no emission in [OII] $\lambda$ 3727 $\r{A}$, with reliable redshifts as well as reliable D4000${\rm n}$ measurements from DR1. From this sample of 360 000 massive, passive galaxies, we used D4000${\rm n}$ and the method of cosmic chronometers to get three new direct, independent measurements of $H(z)=$ 88.48 $\pm\ 0.57(\rm stat) \pm 12.32(\rm syst)$, $H(z)=$ 119.45 $\pm\ 6.39(\rm stat) \pm 16.64(\rm syst)$, and $H(z)= 108.28 \pm 10.07(\rm stat) \pm 15.08(\rm syst)$ $\rm km\ s^{-1}\ Mpc^{-1}$ at $z=0.46$, $z=0.67$, and $z=0.83$, respectively. This sample, which covers $0.3 < z < 1.0$, is the largest CC sample to date, and we reach statistical uncertainties of 0.65$\%$, 5.35$\%$, and 9.30$\%$ on our three measurements. Our measurements show no significant tension with the $\textit{Planck}$ $\Lambda$CDM cosmology. In our analysis, we also illustrate that even amongst samples of massive, passive galaxies, the effect of downsizing can clearly be seen.