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Measuring the expansion history of the Universe with DESI Cosmic Chronometers

S. I. Loubser

TL;DR

This work demonstrates a model-independent approach to tracing the Universe's expansion by employing cosmic chronometers (CC) with DESI DR1 data. By selecting a vast sample of massive, passive galaxies and using the D4000_n spectral index, the authors derive three direct measurements of the Hubble parameter, $H(z)$, at $z=0.46$, $0.67$, and $0.83$ with high statistical precision and a careful treatment of systematics, including metallicity and SPS-model dependencies. The analysis reveals a clear downsizing signal and provides a robust calibration of the $D4000_{\rm n}-z$ relation, enabling a cosmological interpretation of the CC measurements. Deprojecting to $H_0$ within a flat $\Lambda$CDM framework yields $H_0=72.82\pm6.03$ km s$^{-1}$ Mpc$^{-1}$, illustrating the CC method's potential to complement early- and late-Universe probes in constraining cosmology.

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 $σ> 280$ km s$^{-1}$), with no emission in [OII] $λ$ 3727 $Å$, 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 $\textit{Planck}$ $Λ$CDM cosmology. In our analysis, we also illustrate that even amongst samples of massive, passive galaxies, the effect of downsizing can clearly be seen.

Measuring the expansion history of the Universe with DESI Cosmic Chronometers

TL;DR

This work demonstrates a model-independent approach to tracing the Universe's expansion by employing cosmic chronometers (CC) with DESI DR1 data. By selecting a vast sample of massive, passive galaxies and using the D4000_n spectral index, the authors derive three direct measurements of the Hubble parameter, , at , , and with high statistical precision and a careful treatment of systematics, including metallicity and SPS-model dependencies. The analysis reveals a clear downsizing signal and provides a robust calibration of the relation, enabling a cosmological interpretation of the CC measurements. Deprojecting to within a flat CDM framework yields km s Mpc, illustrating the CC method's potential to complement early- and late-Universe probes in constraining cosmology.

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 , and velocity dispersion km s), with no emission in [OII] 3727 , with reliable redshifts as well as reliable D4000 measurements from DR1. From this sample of 360 000 massive, passive galaxies, we used D4000 and the method of cosmic chronometers to get three new direct, independent measurements of 88.48 , 119.45 , and at , , and , respectively. This sample, which covers , 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 CDM cosmology. In our analysis, we also illustrate that even amongst samples of massive, passive galaxies, the effect of downsizing can clearly be seen.

Paper Structure

This paper contains 18 sections, 8 equations, 11 figures, 1 table.

Table of Contents

  1. Introduction
  2. Data
  3. The Dark Energy Spectroscopic Instrument (DESI)
  4. Sample selection and composition
  5. Sample properties
  6. D4000$_{\rm n}$ measurements
  7. Results
  8. Downsizing and the D4000$_{\rm n} - z$ relation
  9. Calibration of the D4000$_{\rm n} - z$ relation
  10. Systematic and total errors
  11. Our measurements in a cosmological context
  12. The evolution of $H(z)$
  13. Deprojection and $H_{0}$ probes
  14. Summary
  15. Nominal velocity dispersion values
  16. ...and 3 more sections

Figures (11)

  • Figure 1: The sample selection, illustrated in the D4000$_{\rm n}$--redshift plane, as detailed in Section \ref{['sample']}. The number of galaxies retrieved with each selection cut is given in the legend.
  • Figure 2: Top panel: The selected sample divided into velocity dispersion ($\sigma$) bins (with objects with the default velocity dispersion of 250 km s$^{-1}$ removed as shown in Appendix \ref{['velocitydisp']}). Lower panel: We bin redshift (from $z=$ 0.2 -- 1.1 in steps of 0.02), and we only use galaxies with $\sigma >$ 200 km s$^{-1}$. The plots illustrate why we fit the D4000$_{\rm n}$--$z$ relation (Section \ref{['downsizing']}) in the range $0.3 < z <1.0$.
  • Figure 3: Distributions of sSFR, light-weighted SSP-equivalent Age, $g-r$ colours, and stellar mass ($\log M_{\star}/M_{\sun}$), for the massive galaxies ($\sigma$ > 250 km s$^{-1}$). The distributions for all three velocity dispersion bins are very similar.
  • Figure 4: We divide the galaxies into velocity dispersion bins, and we bin the galaxies in redshift bins between $0.2 < z < 1.1$ where each bin is $\Delta z=0.02$ wide -- the density of galaxies in these bins are shown in grey-scale in the background. The straight lines fitted to the medians of the bins between $0.3 < z < 0.9$ (the range where there is a sufficient number of galaxies in all five bins) is plotted together in the bottom right panel, and clearly illustrates the effect of downsizing. The 68% confidence levels on the fitted lines are shown in shaded bands.
  • Figure 5: The D4000$_{\rm n} - z$ relation for galaxies with $\sigma >$ 280 km s$^{-1}$, with a redshift binning of $\Delta z = 0.01$. We fit a piece-wise linear function (PWLF) between $0.3 < z < 1.0$ to the median (and standard error on the median) values, using iterative sigma-slipping, with the breakpoints at $z=0.55$ and $z=0.78$.
  • ...and 6 more figures