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

This paper presents a new measurement of the Universe's expansion rate at $z=0.54$ using the cosmic chronometers method applied to massive, passive members of three galaxy clusters observed with VLT/MUSE, establishing a framework for self-consistent multi-probe cosmology that combines time-delay and expansion history data from the same sample.

Original authors: E. Tomasetti, M. Moresco, G. Granata, M. D'Addona, P. Bergamini, C. Grillo, A. Mercurio, P. Rosati, A. Cimatti, L. Tortorelli, S. Schuldt, M. Meneghetti

Published 2026-04-08

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: E. Tomasetti, M. Moresco, G. Granata, M. D'Addona, P. Bergamini, C. Grillo, A. Mercurio, P. Rosati, A. Cimatti, L. Tortorelli, S. Schuldt, M. Meneghetti

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Solving the Universe's "Speeding Ticket"

Imagine the Universe is a giant car driving down a highway. For a long time, astronomers have been arguing about exactly how fast this car is going right now. One group of scientists (using the "distance ladder" method) says it's going 74 mph. Another group (looking at the "baby photos" of the Universe, called the Cosmic Microwave Background) says it's only 67 mph.

This disagreement is a huge problem in physics. It's like two traffic cops giving you different speeding tickets for the same car. If they can't agree, it might mean our map of the road (our laws of physics) is wrong, or maybe the radar guns are broken.

This paper introduces a brand new radar gun to help settle the argument.

The New Tool: "Cosmic Chronometers"

The authors used a method called Cosmic Chronometers (CC).

The Analogy:
Think of the Universe as a giant library. Most books in the library are constantly being rewritten (stars are being born and dying, galaxies are merging). But, there are a few very old, dusty, quiet books that haven't been touched in billions of years. These are passive galaxies.

Because these galaxies stopped making new stars a long time ago, they are like aging wine. As time passes, they get "older" in a predictable way. If you find two of these "wine bottles" that are very similar but are located at slightly different distances from us, you can compare their "ages."

The one closer to us is slightly younger.
The one further away is slightly older.

By measuring the tiny difference in their ages and knowing the difference in their distance (redshift), you can calculate exactly how fast the Universe has been expanding during the time it took for that light to reach us. It's like measuring the speed of a car by looking at how much the scenery changed between two snapshots taken a split second apart.

The Experiment: The "Cluster" Strategy

The team didn't just look at random galaxies. They looked at three massive galaxy clusters (groups of galaxies stuck together by gravity) that are neighbors to each other in the sky.

The Location: They picked three clusters at redshifts of 0.49, 0.54, and 0.59. Think of these as three specific mile markers on the highway.
The Special Guest: The middle cluster (MACS J1149) is famous. It contains a supernova (an exploding star) named Refsdal. This supernova has already been used to measure the Universe's speed using a different method called "Time-Delay Cosmography" (which uses the bending of light like a magnifying glass).
The Goal: By measuring the speed of the Universe in this exact same spot using the "aging galaxy" method, they can combine the two results. It's like checking a car's speed with two different radar guns at the exact same mile marker to see if they agree.

What They Did (The "Recipe")

The Hunt: They used a powerful telescope (VLT/MUSE) to take high-definition "photos" and "spectra" (light fingerprints) of the galaxies in these clusters.
The Filter: They had to be very picky. They only wanted the "oldest, quietest" galaxies. They filtered out any galaxy that was still making new stars (like a teenager who is still growing) because those are too messy to use as clocks. They ended up with 38 perfect "clocks."
The Math: They used a super-computer code (Bagpipes) to analyze the light from these galaxies. They asked the computer: "How old is this galaxy? How heavy is it? How dusty is it?"
- Crucial Detail: They modified the code to not assume a specific speed for the Universe beforehand. This ensures their result is a fresh measurement, not just a confirmation of what they already thought.

The Results

The Galaxies: The galaxies they found are massive, heavy, and very "metal-rich" (they have lots of heavy elements, like gold and iron, which means they are very mature). They formed quickly in short bursts and then stopped making stars.
The Speed: Based on the age difference between the galaxies in the three clusters, they calculated the expansion rate of the Universe at that specific time.
- The Number: They found the speed is 66 km/s/Mpc.
- The Uncertainty: The number has a big "plus or minus" attached to it (±81/−29). This means the measurement is currently a bit fuzzy, like trying to read a speedometer through a foggy windshield.

Why This Matters (and What's Next)

Right now, this single measurement isn't precise enough to solve the "speeding ticket" argument on its own. The "fog" (statistical error) is still too thick.

However, the paper is a proof of concept.

The authors ran simulations to show what happens if we get more data. They found that if they can find about 100 of these "aging galaxies" instead of 38, and look at a slightly wider range of distances, they could reduce the error by 75%.

The Future:
New telescopes (like Euclid and the Vera Rubin Observatory) are about to launch or start scanning the sky. They will find thousands of these galaxy clusters.

The Dream: In the future, we will be able to use the "Aging Galaxy" method and the "Bent Light" method on the same galaxy clusters simultaneously.
The Payoff: Combining these two independent methods on the same data will act like a super-accurate GPS. It will tell us if the Universe is actually speeding up, slowing down, or if our laws of physics need a rewrite.

Summary in One Sentence

This paper successfully tested a new way to measure the Universe's speed by using ancient, quiet galaxies as cosmic clocks, proving that with better data from future telescopes, we can finally solve the mystery of how fast the Universe is really expanding.

1. Problem Statement

Modern cosmology faces a significant tension (4–5 $\sigma$ ) between early-Universe (e.g., CMB) and late-Universe (e.g., Supernovae) determinations of cosmological parameters, particularly the Hubble constant ( $H_0$ ). Resolving whether this discrepancy stems from observational systematics or new physics requires independent, complementary probes.

The Gap: While Cosmic Chronometers (CC) provide direct measurements of the Hubble expansion rate $H(z)$ via differential galaxy ageing, and Time-Delay Cosmography (TDC) measures integrated distances via strong gravitational lensing, these methods have rarely been applied to the same data samples.
The Opportunity: Galaxy clusters hosting strong lenses (like MACS J1149.5+2223, which hosts the multiply-imaged SN 'Refsdal') also contain massive, passive member galaxies suitable for CC analysis. Combining CC and TDC on the same system offers a self-consistent probe to break degeneracies (e.g., $H_0$ – $\Omega_m$ ) and reduce systematic uncertainties.

2. Methodology

Data Selection

The authors analyzed VLT/MUSE spectroscopic data and archival HST photometry for three galaxy clusters in close redshift bins:

SDSS J2222+2745 ( $z = 0.49$ )
MACS J1149.5+2223 ( $z = 0.54$ ) – The primary target hosting SN Refsdal.
SDSS J1029+2623 ( $z = 0.59$ )

Sample Selection (Cosmic Chronometers)

To identify robust CCs, the authors applied strict criteria to isolate massive, passive galaxies with no residual star formation:

Velocity Dispersion: $\sigma_\star > 150$ km/s (a proxy for high stellar mass).
Signal-to-Noise: S/N $> 15$ in the 3700–5250 Å range.
Passivity Diagnostic: The ratio of Ca II H to Ca II K absorption lines ( $H/K < 1.2$ ) to exclude galaxies with young stellar components.
Result: A final sample of 38 massive, passive galaxies (37 after excluding one poor fit).

Spectral Analysis (Full Spectrum Fitting)

The authors employed the Bagpipes code to perform Joint Full Spectrum Fitting (FSF) of spectra and photometry.

Key Modification: Unlike standard FSF practices, the cosmological prior on age was removed. The age parameter was allowed to vary freely between 0 and 15 Gyr, ensuring the resulting $H(z)$ measurement is independent of any specific cosmological model.
Model Components:
- Stellar Population Synthesis (SPS): Bruzual & Charlot (2003) models with a Kroupa (2001) IMF.
- Star Formation History (SFH): Delayed exponentially declining (DED) function.
- Dust: Calzetti et al. (2000) attenuation law.
- Calibration: Second-order Chebyshev polynomial to correct flux mismatches between spectra and photometry.
Validation: The fit quality was verified via reduced chi-square ( $\tilde{\chi}^2 \approx 0.54$ after noise factor inclusion) and visual inspection.

Cosmological Derivation

Redshift Binning: Galaxies were assigned to their parent cluster redshifts ( $z \approx 0.49, 0.54, 0.59$ ).
Mass Binning: To ensure homogeneity, the sample was split into High-Mass (HM) and Low-Mass (LM) subsamples based on $\log(M_\star/M_\odot) = 11.05$ .
Bootstrap Analysis: A bootstrap approach ( $N=1000$ realizations) was used to calculate the differential age ($dt$) between adjacent redshift bins.
Calculation: $H(z)$ was derived using the relation $H(z) = -\frac{1}{1+z} \frac{dz}{dt}$ .

3. Key Contributions

First Self-Consistent CC+TDC Framework: This work establishes the first step toward combining CC and TDC on the same galaxy cluster (MACS J1149), leveraging the unique properties of the cluster hosting SN Refsdal.
Methodological Rigor: The explicit removal of cosmological priors in the age fitting process ensures the $H(z)$ measurement is model-independent.
Environmental Control: The study demonstrates that massive cluster members can serve as reliable CCs, provided strict passivity cuts (like $H/K$ ) are applied to mitigate environmental rejuvenation effects.
Forecasting: The paper provides simulations forecasting the precision gains achievable with future surveys (Euclid, WST, LSST).

4. Results

Physical Properties of the Sample

Stellar Mass: High mean mass of $\log(M_\star/M_\odot) = 11.2 \pm 0.3$ .
Metallicity: Super-solar on average ( $Z/Z_\odot = 1.3 \pm 0.7$ ).
Dust Extinction: Low ( $A_V = 0.3 \pm 0.3$ mag).
Star Formation: Short burst timescales ( $\tau = 0.6 \pm 0.2$ Gyr).
Trends: The sample exhibits a clear mass-downsizing pattern (higher mass galaxies are older) and a consistent ageing trend with redshift (ages decrease as $z$ increases). 95% of the sample is consistent with the age of the Universe in a flat $\Lambda$ CDM model.

Cosmological Measurement

The primary result is a new measurement of the Hubble parameter at $z = 0.542$ :
$H(z=0.542) = 66^{+81}_{-29} \text{ (stat)} \pm 13 \text{ (syst)} \text{ km s}^{-1} \text{ Mpc}^{-1}$

Uncertainty: The error budget is currently dominated by statistical uncertainties due to the small sample size (35 usable galaxies after binning). Systematic errors are estimated at $\sim 20\%$ of the statistical value.

Future Prospects (Simulations)

Simulations indicate that increasing the sample size and redshift coverage significantly improves precision:

Sample Size: Increasing to $\sim 100$ CCs reduces uncertainty by $\sim 20\%$ .
Redshift Coverage: Extending the redshift interval from $dz \approx 0.1$ to $dz \approx 0.2$ reduces uncertainty by $\sim 30\%$ .
Combined Effect: A combination of $\sim 100$ CCs and a broader redshift range could reduce the total uncertainty on $H(z)$ by a factor of 4.

5. Significance

This paper validates galaxy clusters as dual-purpose laboratories for cosmology. By successfully applying the CC method to the same cluster used for TDC (MACS J1149), the authors pave the way for a joint CC+TDC analysis.

Synergy: Combining $H(z)$ (expansion rate) and TDC (distance) measurements from the same object will allow for tighter constraints on cosmological parameters, potentially resolving the $H_0$ tension.
Scalability: With upcoming surveys like Euclid and LSST, which will discover orders of magnitude more strong lenses and provide high-quality spectroscopy for cluster members, this multi-probe approach is poised to become a standard, high-precision tool in cosmology. The current results serve as a proof-of-concept that the statistical limitations can be overcome with future data.