Cosmic chronometers with galaxy clusters: A new avenue for multi-probe cosmology

¹ Dipartimento di Fisica e Astronomia “Augusto Righi” – Università di Bologna, via Piero Gobetti 93/2, I-40129 Bologna, Italy
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Piero Gobetti 93/3, I-40129 Bologna, Italy
³ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, via Saragat 1, 44122 Ferrara, Italy
⁴ Dipartimento di Fisica, Università degli Studi di Milano, via Celoria 16, I-20133 Milano, Italy
⁵ Institute of Cosmology and Gravitation, University of Portsmouth, Burnaby Rd, Portsmouth PO1 3FX, UK
⁶ INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Napoli, Italy
⁷ INAF – IASF Milano, via A. Corti 12, I-20133 Milano, Italy
⁸ Università di Salerno, Dipartimento di Fisica “E.R. Caianiello”, Via Giovanni Paolo II 132, 84084 Fisciano (SA), Italy
⁹ INFN – Gruppo Collegato di Salerno – Sezione di Napoli, Dipartimento di Fisica “E.R. Caianiello”, Università di Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano (SA), Italy
¹⁰ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany

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Received: 25 November 2025
Accepted: 18 January 2026

Abstract

We provide a new measurement of the expansion history of the Universe at z = 0.54 based on the cosmic chronometer (CC) method, exploiting the high-quality spectroscopic VLT/MUSE data for three galaxy clusters in close-by redshift bins: SDSS J2222+2745 (z = 0.49), MACS J1149.5+2223 (z = 0.54), and SDSS J1029+2623 (z = 0.59). The central one, MACS J1149.5+2223, hosts the well-known supernova Refsdal, which enabled H₀ measurements via time delay cosmography. This represents the first step for a self-consistent probe combination, where different methods are applied to the same data sample. After selecting the most passive and massive cluster members (38 CCs), we derived their age and physical parameters via full spectrum fitting. We used the code Bagpipes, which was specifically modified to remove the cosmological prior on ages. On average, the CC sample shows super-solar metallicities (Z/Z_⊙ = 1.3 ± 0.7) and a low dust extinction (A_V = 0.3 ± 0.3 mag) and seems to have formed in short bursts (τ = 0.6 ± 0.2 Gyr). We also observe both an ageing trend in redshift and a mass-downsizing pattern. From the age–redshift trend, implementing the CC method through a bootstrap approach, we derive a new H(z) measurement: H(z = 0.542) = 66⁺⁸¹₋₂₉ (stat) ± 13 (syst) km/s/Mpc. We also simulated the impact of increased statistics and extended redshift coverage, finding that H(z) uncertainties can be reduced by up to a factor of 4 with ∼100 CCs and a slightly broader redshift range (dz ∼ 0.2).