# Addressing the Hubble tension with cosmic chronometers

**Authors:** Michele Moresco  
**arXiv:** [2307.09501](https://arxiv.org/abs/2307.09501)  
**Date:** 2023-07-18  
**Summary page:** https://www.summarizepaper.com/en/paper/2307.09501v1/

## Key points


## Summary

Twenty years after the discovery that the expansion of the Universe is accelerating, recent studies have revealed a new challenge to our understanding of the cosmos. The Hubble constant, which measures the speed of expansion today, has been found to provide conflicting values when measured from different sources. Measurements from the Cosmic Microwave Background in the primordial Universe and from Cepheids and Supernovae Type Ia in the local Universe show significant tension in their results. To address this puzzle, there is a need for new independent cosmological probes that can provide additional evidence. One such emerging probe is cosmic chronometers. Cosmic chronometers offer cosmology-independent estimates of the Universe's expansion history. This method derives the expansion rate by measuring how much the Universe has changed in age between two different redshifts, essentially estimating the slope of the age-redshift relation. This chapter introduces cosmic chronometers and discusses its main ingredients and equations involved in estimating necessary quantities from observables. It also explores how to select a reliable sample of tracers to map out coherently the age evolution of the Universe. Additionally, various methods for robustly measuring differential ages of populations are reviewed as they are fundamental to this method. Furthermore, this chapter presents key measurements obtained using cosmic chronometers and provides forecasts for future surveys. It also discusses how these data can offer valuable feedback to address the Hubble tension and potentially resolve discrepancies in our current understanding of our expanding Universe. Overall, cosmic chronometers present an exciting opportunity to gain further insights into our expanding Universe.