# Age-dating Luminous Red Galaxies observed with the Southern African Large Telescope

## TL;DR

This study measures the cosmic expansion rate at redshift 0.47 using the differential age technique applied to Luminous Red Galaxies observed with SALT, providing a key data point for understanding universe expansion.

## Contribution

It demonstrates the application of the differential age technique to LRG spectra from SALT, yielding a precise estimate of H(z) at z~0.5, advancing cosmic chronometer measurements.

## Key findings

- Measured H(z) = 89 ± 23 (stat) ± 44 (syst) km/s/Mpc at z ≈ 0.47
- Used high-quality SALT spectra of LRGs at z ≈ 0.40 and 0.55
- Achieved one of the best H(z) estimates at this redshift range

## Abstract

We measure a value for the cosmic expansion of $H(z) = 89 \pm 23$(stat) $\pm$ 44(syst) km s$^{-1}$ Mpc$^{-1}$ at a redshift of $z \simeq 0.47$ based on the differential age technique. This technique, also known as cosmic chronometers, uses the age difference between two redshifts for a passively evolving population of galaxies to calculate the expansion rate of the Universe. Our measurement is based on analysis of high quality spectra of Luminous Red Galaxies (LRGs) obtained with the Southern African Large Telescope (SALT) in two narrow redshift ranges of $z \simeq 0.40$ and $z \simeq 0.55$ as part of an initial pilot study. Ages were estimated by fitting single stellar population models to the observed spectra. This measurement presents one of the best estimates of $H(z)$ via this method at $z\sim0.5$ to date.