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.