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Interesting.
from
https://astrobites.org/2026/03/05/how_old_is_the_universe/

It is best to go to the citation to see the graphics.

Do we actually know how old the Universe is?
by Jayde Willingham | Mar 5, 2026 | Daily Paper Summaries | 0 comments

Title: Improved measurements of the age of JWST galaxies at z=6-10

Authors: M.L||pez-Corredoira and C.M.Guti|-rrez

First AuthorrCOs Institution: Institute of Astrophysics of the Canary Islands

Status: Published in Monthly Notices of the Royal Astronomical Society
[open access]

rCLrO-rO2 Our whole universe was in a hot, dense state. Then nearly fourteen billion years ago, expansion startedrCa wait. rO-rO2rCY

No really rCo wait. Did it?

This might sound like settled science, but researchers are still
actively testing our understanding of when and how the Universe began
its hot, rapid expansion. And new results from the James Webb Space
Telescope (JWST) are putting some pressure on our assumptions.

Now, this doesnrCOt mean the Big Bang didnrCOt happen, but it does mean astronomers are double-checking whether the timeline of the early
Universe fully makes sense.

Why JWST matters
JWST is the most powerful space telescope ever built, designed to see
infrared light with incredible sensitivity. This allows it to observe
some of the very first galaxies ever formed, only a few hundred million
years after the Big Bang.

These early galaxies are cosmic time capsules. If we can figure out how
old they are, we can learn when galaxies first started forming, and how quickly structure grew in the young Universe. In simple terms, galaxies canrCOt be older than the Universe itself. So measuring their ages helps
test our cosmic timeline.

(Think of it like this: if you meet a five-year-old child, you
immediately know their parentsrCO relationship started at least five years ago. The child sets a minimum age for the story.)

How do astronomers tell how old a galaxy is?
Galaxies evolve over billions of years, so we obviously canrCOt watch
galaxies age in real time. Instead, astronomers read the information
encoded in their light. A galaxyrCOs light acts like a fingerprint: it
carries clues about the stars inside it. One major technique for reading
this fingerprint is called Spectral Energy Distribution (SED) fitting.

HererCOs the idea:

Astronomers create computer models of galaxies that include different properties (see Figure 1):
star-formation histories
dust content
chemical composition of gas
stellar ages
Each model predicts what the galaxyrCOs light should look like.
The models are compared to real observations until the best match is found.
If the best-fitting model requires stars that formed, say, 400 million
years ago, that becomes our estimate for the galaxyrCOs age.


Figure 1: The theoretical spectrum of a galaxy, showing how brightness
varies over different wavelengths. Different processes (shown in
different colors) contribute to the overall galaxy spectrum (black).
Adapted from Figure 2 in Iyer et al. 2025 rCo an extensive review on SED fitting.
Astronomers also look for specific features in a galaxyrCOs spectrum.
Young stars are extremely hot and tend to glow blue, while older stellar populations appear redder. But colour alone isnrCOt enough to figure out a galaxyrCOs age, since dust can also make galaxies look red.

Where the tension appeared
The authors studied 31 galaxies at extremely high redshift (between z=6
and z=10). Meaning we see them when the Universe was only about 700
million years old, based on how fast the universe is expanding!

HererCOs the puzzle: Earlier studies analysing many of these same galaxies estimated stellar ages of around 900 million years.

But that creates a problemrCa

If the Universe itself was only ~700 million years old at that time, how
could the stars already be 900 million years old? Either galaxies formed incredibly fast in ways we donrCOt yet understand, or something in our age measurements is misleading us.


Figure 2: These are examples of rCLLittle Red Dots,rCY a population of extremely compact, very red galaxies discovered in early observations
from JWST and thought to exist at high redshift in the early universe.
While these are not the specific sources featured in todayrCOs paper, the dataset used in the study includes 18 such objects. (Image credit: ESA/Webb) What do the new authors do differently?
Instead of assuming the earlier results were correct, the researchers carefully re-examined how galaxy ages are inferred.

Better data rCo They included new infrared observations from JWST, giving
a clearer picture of how brightness changes across wavelengths.
Dust matters rCo a lot. Many of these galaxies are known as rCLLittle Red Dots.rCY Their redness initially suggested old stars. But dust can also
redden light. Tiny particles absorb blue light and re-emit energy in
infrared wavelengths, making young galaxies look artificially old..
Emission from hot gas rCo Young galaxies contain glowing gas that produces strong emission lines. At high redshift, these lines fall directly into
JWST filters and can look a lot like emission from older stars.
More realistic galaxy growth rCo Earlier models often assumed smooth star formation over time, but early galaxies likely formed stars in chaotic
bursts. Allowing bursty histories lets galaxies appear bright and red
without requiring extremely ancient stars.
The result
With these improvements, the authors measured these galaxies to have an average age of about 610 million years. That fits comfortably within the
age of the Universe at that time, so no need to panic. However,
uncertainties remain large, and one galaxy still appears suspiciously
old. The authors emphasise that better data and improved models are
still needed.

SorCa is the Universe still 13.8 billion years old?
Yes. For now, all the strongest evidence still supports that number.But studies like this are exactly how science works: bold observations
challenge our assumptions, researchers refine their methods, and our understanding becomes sharper.

The Universe hasnrCOt changed. Our ability to read its history just got better.

Astrobite edited by Elise Koo and Anavi Uppal

Featured image credit: Jayde Willingham

Author
Jayde Willingham
Jayde Willingham

I am a first year PhD student at Swinburne University of Technology. I
study what is happening in the early universe and how the first galaxies
came to be.

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