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CORN -- Chronometers of Relic Nature I: The first estimate of the expansion rate of the Universe using compact relic galaxies

This paper presents a model-independent measurement of the Universe's expansion rate at redshift z=0.15, utilizing ultra-compact massive relic galaxies as cosmic chronometers to minimize systematic uncertainties and address the Hubble tension.

Original authors: Krzysztof Lisiecki, Nicola Principi Cavaterra, Agnieszka Pollo, Chiara Spiniello, Laura Hunt, Charlie Rosen, Marek Biesiada, Darko Donevski, Patryk Matera, Giuliano Lorenzon, Katarzyna Małek John Mill

Published 2026-08-12

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CC BY 4.0

Original authors: Krzysztof Lisiecki, Nicola Principi Cavaterra, Agnieszka Pollo, Chiara Spiniello, Laura Hunt, Charlie Rosen, Marek Biesiada, Darko Donevski, Patryk Matera, Giuliano Lorenzon, Katarzyna Małek John Mills

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

Imagine the Universe as a giant, inflating balloon. For decades, scientists have been trying to measure exactly how fast this balloon is stretching. This speed, known as the Hubble constant, is the cosmic speed limit that tells us how the Universe expands. But here's the twist: when scientists measure this speed using two different methods—one looking at the very early Universe and another looking at nearby galaxies—they get two different answers. It's like checking your car's speedometer against a radar gun and getting two different numbers. This disagreement, called the "Hubble tension," is one of the biggest mysteries in modern physics. To solve it, we need a third, independent way to measure the expansion, one that doesn't rely on the same old assumptions. Enter the "Cosmic Chronometer" method. Instead of measuring distance, this method measures time. It treats certain galaxies like ancient clocks, ticking away as the Universe ages. By seeing how fast these clocks tick at different points in history, astronomers can calculate the expansion rate directly.

In this new study, titled "CORN – Chronometers of Relic Nature," a team of astronomers decided to test these cosmic clocks using a very special, rare type of galaxy. They didn't just pick any old galaxy; they hunted for "relics." Think of these relics as the Universe's time capsules. While most galaxies are messy, constantly merging with neighbors and forming new stars like a busy construction site, these relics are the quiet, untouched survivors. They are ultra-compact, massive galaxies that formed all their stars in a violent burst billions of years ago and then just sat there, aging peacefully without any new construction or collisions. Because they are so "pure" and undisturbed, the authors argue they make much better clocks than the messy, chaotic galaxies usually used.

The team gathered a sample of 189 of these relic galaxies, looking at them in the relatively nearby Universe (between redshifts 0.07 and 0.22). They measured a specific feature in the light coming from these galaxies, called the Dn4000 index, which acts like a fingerprint of the stars' age. By seeing how this fingerprint changes as you look further back in time, they calculated the expansion rate of the Universe at a specific moment. Their result? They found the expansion rate to be 85 ± 53 km s⁻¹ Mpc⁻¹ at a redshift of 0.15.

Now, don't panic if that number looks a bit wobbly. The "± 53" part means the statistical uncertainty is still quite large, largely because they only had 189 galaxies to work with. It's like trying to guess the average height of a crowd by measuring just a few people; you're close, but you need more data to be precise. However, the real magic of this paper isn't the final number itself, but how clean their measurement was. The authors discovered that by using these pristine relic galaxies, they drastically reduced the "systematic errors"—the hidden biases that usually mess up these calculations. Specifically, they found that the chemical makeup of the stars, particularly the abundance of "alpha elements" (like oxygen and magnesium), plays a huge role in the error budget. Previous studies often ignored this, but this team showed that if you don't account for it, your clock is off. When they included this factor, their systematic uncertainty dropped to about 13% (or 8.7% if you ignore the alpha elements, which is what others used to do).

The paper explicitly argues against the idea that we can ignore these chemical variations or that standard, messy galaxies are good enough for high-precision measurements. They demonstrate that the "messy" galaxies used in past studies introduce too much noise because they have complex histories of merging and new star formation. The authors suggest that while their current measurement isn't the final, perfect answer yet, it proves the concept works. They are essentially saying, "We found a better clock, and it runs much cleaner than the old ones." The main limitation right now is simply the small number of these rare galaxies they could find. But with upcoming telescopes like Euclid and the Vera C. Rubin Observatory, they expect to find thousands more. This study is a proof-of-concept, showing that if we can find enough of these cosmic time capsules, we might finally be able to solve the Hubble tension and understand exactly how fast our Universe is growing.

Technical Summary: CORN – Chronometers of Relic Nature I

Problem Statement
The measurement of the Hubble constant ( $H_0$ ) and the expansion rate of the Universe ( $H(z)$ ) faces a persistent statistical discrepancy known as the "Hubble tension," where values derived from early-universe probes (CMB) differ from late-universe distance ladder measurements. Independent, model-independent methods are required to validate these measurements and identify potential systematic errors. The Cosmic Chronometer (CC) method, which utilizes the differential age evolution of quiescent galaxies to measure $H(z)$ without assuming a specific cosmological model, is a promising approach. However, traditional CC studies using massive early-type galaxies (ETGs) suffer from significant systematic uncertainties. These arise from degeneracies in stellar population synthesis (SPS) modeling (specifically age-metallicity degeneracy), complex star formation histories (SFHs) involving mergers and rejuvenation, and unaccounted variations in chemical abundance patterns, particularly $\alpha$ -element enhancement ( $[\alpha/\text{Fe}]$ ).

Methodology
This study proposes a novel application of the CC method using "relic" galaxies—ultra-compact massive galaxies (UCMGs) that have avoided significant merger activity and accretion since their formation. The authors utilize a sample of 189 relic galaxies from the E-INSPIRE catalogue, selected within the redshift range $0.07 \leq z \leq 0.22$ . The selection criteria include high compactness ( $\Sigma_{1.5} > 10.5$ ), high stellar mass ( $M_\star > 6 \times 10^{10} M_\odot$ ), and a high "Degree of Relicness" (DoR $\gtrsim 0.41$ ), ensuring the galaxies formed the vast majority of their stellar mass in a rapid, early burst and evolved passively.

The methodology proceeds as follows:

Spectral Analysis: The $D_{n}4000$ spectral index (a proxy for stellar age) is measured from SDSS spectra using the narrow definition by Balogh et al. (1999).
Differential Age Relation: The slope of the $D_{n}4000$ versus redshift ( $z$ ) relation is determined by binning the data and fitting a linear relation using total least squares.
Calibration and Systematics: The relation between $D_{n}4000$ and age is calibrated using MILES SPS models with BaSTI isochrones. Crucially, the authors explicitly propagate the uncertainties associated with stellar metallicity ( $Z$ ) and $\alpha$ -element enhancement ( $[\alpha/\text{Fe}]$ ) into the systematic error budget. This is achieved by mapping the slope parameter $A = d(D_{n}4000)/dt$ as a function of $Z$ and $[\alpha/\text{Fe}]$ using the measured properties of the individual galaxies in the sample.
H(z) Derivation: The Hubble parameter is calculated using the relation $H(z) = -\frac{1}{1+z} \frac{1}{A} \frac{dz}{dD_{n}4000}$ .

Key Contributions

Proof of Concept for Relics: This is the first study to employ relic galaxies as cosmic chronometers. The authors demonstrate that the extreme evolutionary homogeneity of relics (minimal merger contamination and synchronized formation) significantly reduces astrophysical systematics compared to standard passive galaxy samples.
Explicit $\alpha$ -Enrichment Propagation: The paper explicitly incorporates the impact of $[\alpha/\text{Fe}]$ variations into the systematic uncertainty budget of the CC method. Previous studies often assumed fixed or homogeneous $[\alpha/\text{Fe}]$ , potentially underestimating systematics, especially in metal-rich regimes.
Refined Systematic Budget: The study quantifies the specific contributions of metallicity and $\alpha$ -enhancement to the error budget, showing that while metallicity uncertainties are suppressed due to the specific metallicity range of relics, $\alpha$ -enhancement becomes a dominant systematic factor.

Results
The authors derive an independent measurement of the Hubble parameter at the median redshift of the sample ( $z = 0.15$ ):
$H(z = 0.15) = 85.0 \pm 51.5 \text{ [stat]} +10.2_{-8.9} \text{ [sys]} \, \text{km s}^{-1} \text{Mpc}^{-1}$

Precision: The total uncertainty is dominated by statistical limitations (62%), which is expected given the small sample size (189 galaxies). However, the systematic component is notably tight, estimated at 13% (or 8.7% if $[\alpha/\text{Fe}]$ variations are not propagated).
Systematic Reduction: The metallicity-related systematic uncertainty is reduced to $\leq 1.2\%$ , approximately 2.5 times lower than the most restrictive estimates in current literature. This is attributed to the relic sample's metallicity distribution falling near the "turnover point" of the $A(Z)$ relation, where the $D_{n}4000$ -age slope is insensitive to metallicity variations.
Impact of $\alpha$ -Enhancement: When $[\alpha/\text{Fe}]$ variations are explicitly propagated, the total systematic uncertainty increases from $\sim 8.7\%$ to $\sim 13\%$ . The authors find that $[\alpha/\text{Fe}]$ is a major contributor to the error budget in the high-metallicity regime characteristic of relics.
Consistency: The result is consistent within uncertainties with previous CC measurements and the flat $\Lambda$ CDM prediction from Planck.

Significance and Claims
The paper claims to demonstrate the feasibility of using relic galaxies to reduce the dominant astrophysical systematic uncertainties in the CC method. The primary significance lies not in the absolute precision of the current $H(z)$ measurement, which is statistically limited, but in the validation of a "physically cleaner" tracer population.

The authors argue that relic galaxies offer a pathway to competitive precision cosmology because their simple evolutionary histories minimize uncertainties related to SFH, mergers, and rejuvenation. They conclude that the current limitation is purely statistical (sample size) rather than systematic. Consequently, the method is poised to become highly competitive with the influx of data from upcoming surveys (Euclid, Vera C. Rubin Observatory, DESI, 4MOST), which are expected to provide the necessary volume of high-quality relic candidates. Furthermore, the study highlights the necessity for future SPS models to include denser sampling of chemical abundance patterns to fully exploit the potential of relic-based chronometers.

Technical Summary: CORN – Chronometers of Relic Nature I

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