Relic Chronometers: First Measurement of the Universe's Expansion Rate Using Compact Galaxies ✛
Context
The expansion rate of the Universe is a cornerstone of modern cosmology, and different measurement methods currently disagree at the 4-sigma level—the so-called Hubble tension. Cosmic chronometers, which rely on the age evolution of passive galaxies, offer an independent check. Related studies in the archive use chronometers to refine the age of the Universe and trace its expansion history; this work continues that line but applies a special class of objects—relic galaxies—for the first time.
Methods
From the E-INSPIRE catalog, 189 relic galaxies were selected in the redshift range 0.07≤z≤0.22. For each, the spectral index Dn4000, sensitive to stellar population age, was measured. Using MILES stellar population models, a calibration between Dn4000 and age was constructed, accounting for metallicity and alpha-element abundance. The Hubble parameter was then derived from the change of Dn4000 with redshift.
Results
The value H(z=0.15)=85±53 km/s/Mpc was obtained. Statistical uncertainty dominates (61% of the slope magnitude), but systematic uncertainty is only 13% (or 8.7% excluding alpha-enrichment)—among the lowest for chronometer-based estimates. Variations in [α/Fe] are shown to contribute significantly to systematics, especially at high metallicities, and should be included in future analyses.
Implications
This work demonstrates that relic galaxies are 'clean' chronometers: their simple star-formation history and lack of mergers reduce astrophysical systematics. Unlike neighboring studies that use ordinary massive passive galaxies, this sample is limited to ultra-compact objects with a high degree of relic status, which provides the systematic advantage. This paves the way for more precise, independent measurements of the Universe's expansion rate.
Future development
With data from Euclid, the Rubin Observatory, DESI, and 4MOST, the number of confirmed relic galaxies will grow by orders of magnitude, drastically reducing statistical uncertainty. Improved stellar population models with broader alpha-element coverage will also be needed to fully exploit the method's potential.
Impact
The result impacts cosmology by offering a new independent way to test the Hubble tension, and galaxy evolution studies by highlighting relic systems as benchmarks of the early Universe.
Next steps
Expand the relic galaxy sample using new spectroscopic surveys and refine stellar population models for high metallicities and alpha-enrichment.
Key open problems
For cosmology, the key unresolved issue is the Hubble tension: the discrepancy between early- and late-time measurements of the expansion rate. This method provides an independent check, free from systematics tied to assumptions about dark energy and dark matter.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study uses relic galaxies as cosmic chronometers: from a sample of 189 objects with measured spectral index Dn4000 and model-based age calibration, the Hubble parameter is estimated as H(z=0.15)=85±53 km/s/Mpc. Particular attention is paid to systematics, including the effect of variations in alpha-element abundances.
Where its strength is
Using relic galaxies with simple star formation histories and no mergers significantly reduces astrophysical systematics compared to ordinary massive passive galaxies, making them promising 'clean' chronometers.
Where it could go next
- It might be interesting to compare the obtained H(z) estimate with results from other independent methods, such as gravitational wave measurements, to check consistency in the context of the Hubble tension.based on: Gravitational waves measure the expansion rate of the universe arXiv:2605.27227
- A related line of work applies cosmic chronometers to test the consistency of cosmological data; expanding the relic galaxy sample to higher redshifts could strengthen such tests.based on: Cosmic Detective: Two Independent Clues Against the Main Cosmological Debate arXiv:2606.03634v1
Worth trying
- Try a joint analysis of relic and ordinary passive galaxies to assess the contribution of systematics related to star formation history.
- Use relic galaxy data from deeper surveys to extend the redshift range and increase statistics.
- Test the sensitivity of the result to the choice of stellar population models (e.g., compare MILES with other libraries).
Why this may matter
Refining the expansion rate of the Universe with an independent method is important for resolving the Hubble tension, and reducing systematics through relic galaxies could make cosmic chronometers competitive with other approaches.
Nearby works in our archive
- The Universe in Meters: How Units of Measurement Set Astronomers at Oddssimilarity 0.65
- Gravitational waves measure the expansion rate of the universesimilarity 0.63
- A Cosmic Ruler Made of Seventeen Lensessimilarity 0.61
- A Cosmic Ruler Without a Rulersimilarity 0.60
- How to Measure the Expansion Rate of the Universe Without Knowing Its Agesimilarity 0.59
- The Cosmic Tuning Fork: Why the Universe Plays Two Different Notessimilarity 0.59
- Cosmic Detective: Two Independent Clues Against the Main Cosmological Debatesimilarity 0.59
The closest work in the archive sits at 0.65 by meaning — the usual level for our corpus.
Original abstract
as written by the authors arXiv:2608.10163v2
Measuring the expansion rate of the Universe is a central challenge in cosmology. Independent and robust methods are essential to validate existing measurements and assess potential systematic effects. We employ the cosmic chronometer (CC) approach, which uses the differential age evolution of quiescent galaxies to measure the Hubble parameter H(z) without assuming an underlying cosmological model. In contrast to previous CC studies, we restrict our analysis to relics -- ultra compact massive galaxies (UCMGs) hosting the oldest stellar populations -- thereby minimising uncertainties related to star formation history and merger history. We select a sample of 189 relic galaxies in the redshift range 0.07< z <0.22 from the E-INSPIRE UCMGs catalogue. Using the D_n4000 spectral index of relic galaxies and its redshift evolution, combined with MILES stellar population synthesis models, we derive the differential age relation required to infer H(z). We account not only for metallicity effects but also for the impact of alpha-element enhancement, which has not been explicitly propagated into the systematic uncertainty budget of the D_n4000 cosmic chronometer method. We obtain an independent measurement of H(z=0.15) = 85\pm53 km/s/Mpc. The total uncertainty is dominated by statistical limitations. The systematic component is 13% (8.7% when alpha-enrichment is not propagated), among the tightest estimates in current CC studies. We find that alpha-enrichment plays a major role in the systematic error budget, particularly in the high metallicity regime, and must be properly accounted for in future analyses. We demonstrate the proof of concept that relic galaxies provide a promising pathway to reduce systematic uncertainties in the CC method. The statistical uncertainties, which dominate the error budget, can be significantly reduced by the increase of data volume expected during the next years.
Relic galaxies are so compact that, despite having a mass comparable to the Milky Way's, their radius is only about 2 kiloparsecs — roughly 10 times smaller. They have survived over 10 billion years without merging with other galaxies or forming new stars.
Key numbers
- H(z=0.15): 85±53 km/s/Mpc
- systematic uncertainty: 13% (including [α/Fe])
- number of galaxies in sample: 189
- redshift range: 0.07–0.22
- median metallicity: Z=0.034