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Fossil Galaxies: Ancient Clocks That Measure the Universe's Expansion ✛

Original: "CORN -- Chronometers of Relic Nature I: The first estimate of the expansion rate of the Universe using compact relic galaxies"
· 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
arXiv:2608.10163v2 · 2026-08-10 · CC BY 4.0 · 1 min
Cosmology Galaxies
Galaxies that have remained unchanged for billions of years have helped measure how fast the universe is expanding.

Some galaxies seem frozen in time. They formed shortly after the birth of the universe and have never merged with other galaxies or spawned new stars. These "fossils" are perfect clocks: their age can be precisely determined from the light they emit.

Scientists found 189 such galaxies. For each, they measured the spectral index Dn4000—like a hand on a clock face that tells you how old the stars inside are.

Previously, it was unclear whether these clocks could be trusted. It turns out they can: the systematic error was only 13%—one of the best results among similar methods. However, they had to account for the chemical composition of the stars: if they contain a lot of oxygen and magnesium, they age slightly differently, and the clocks could run slow or fast.

Now that the method is validated, it can be applied to thousands of new galaxies that future telescopes will find. This will help answer the question of why different methods for measuring the expansion rate of the universe give different results.

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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.

H(z) = -\frac{1}{1+z}\frac{dz}{dt}
Expansion rate of the Universe at redshift z
D_n4000 = A(Z,[\alpha/Fe],M)\cdot age + B
Linear dependence of the spectral index on age
H(z) = -\frac{1}{1+z} A \frac{dz}{dD_n4000}
Expansion rate from the observed index change
Scientists
Adam RiessBrian SchmidtEdwin HubbleGeorges LemaîtreMaarten SchmidtSaul Perlmutter
Concepts
cosmic chronometry age of the universe hubble constant cosmic expansion kilometer per second per megaparsec observable universe cosmological parameters stellar population chemical galactic archaeology cosmology stellar archaeology high redshift galaxies Hubble Space Telescope standard ruler Hubble's law galactic evolution Cope's rule hubble constant measurement cosmic expansion uncertainty hubble tension