A tiny red speck, spotted by the James Webb Space Telescope, has set a date on an important era in the Universe's history.
The speck, known as JADES-GS-z13-1-LA, is one of the earliest galaxies ever seen, observed at just 330 million years after the Big Bang.
But it's not just its age that makes this galaxy special.
According to data published today in Nature, the galaxy heralds the start of the "cosmic dawn", when light from the first stars shone through the early Universe.
It is the first time the process, known as "cosmic reionisation", has been directly detected in a galaxy from this age.
And the discovery took astronomers by surprise.
Until now, it was thought this process began about 500 million years after the Big Bang, according to Joris Witstok, an astronomer at the Cosmic Dawn Center in the University of Copenhagen, who led the international team behind the study.
"All the current evidence points towards reionisation occurring fairly 'late', and quite suddenly," Dr Witstok said.
"Our evidence that it may have already begun 300 million years after the Big Bang means we may have to adjust our models."
How do we know this galaxy marks the 'cosmic dawn'?
Dr Witstok and his team are part of an international effort to survey early galaxies using the James Webb Space Telescope and trying to understand what happened just after the Big Bang.
The age of these very early galaxies is inferred by their "red shift", which is measured using a technique known as photometry.
As the Universe expands, light from distant objects shifts towards the red end of the light spectrum, making very distant and old objects appear red.
JADES-GS-z13-1-LA has a redshift of 13, which correlates to 330 million years after the Big Bang (whereas the other brighter galaxies in the image are much more recent).
Karl Glazebrook, an astrophysicist at Swinburne University of Technology who wasn't involved in the research, said astronomers know of a couple of galaxies with similar or slightly higher red shift than the one the researchers recently discovered.
But JADES-GS-z13-1-LA's the only one to show a particular signature which caught astronomers' attention.
Scientists use a technique called spectroscopy to break light down into different wavelengths to study its properties.
A signature spotted in JADES-GS-z13-1-LA's ultraviolet light — called the Lyman-alpha emission — is a telltale sign of reionisation.
"This line is normally absorbed by neutral hydrogen gas," Professor Glazebrook said.
It means that photons from the galaxy changed the nature of the hydrogen and helium surrounding it, turning it from an impenetrable gas fog to a bubble that light could pass through.
Michele Trenti, an astrophysicist at the University of Melbourne who wasn't involved in the research, said the discovery of the Lyman-alpha emission was unexpected.
"It surprised the researchers, and it surprised myself," Professor Trenti said.
The galaxy itself was also unusual, he added, with "blue light" signals suggesting it either had a lot of young, hot stars, or a supermassive black hole vigorously eating up matter.
But more observations are needed to see if it was an unusual fluke, or a common property of reionising galaxies, Professor Trenti, who has written an accompanying editorial in the journal Nature, said.
"It's very hard to do robust science when you have samples of only one object.
"Did we win a lottery ticket in seeing this line and is it one in 1,000 galaxies that show this behaviour? Is it one in a million? Is it one in a few times?
"I think it will be very important to see what future observations will tell us."
What's next?
Dr Witstok said there were already two plans underway to learn more about cosmic reionisation: looking for Lyman-alpha emission in similar galaxies, and looking more closely at JADES-GS-z13-1-LA to refine their measurements.
While the James Webb Space Telescope spotted the galaxy, Professor Trenti said radio telescopes such as the Murchison Widefield Array and the incoming Square Kilometre Array will be important for understanding the transition from the cosmic dark age to the cosmic dawn.
These telescopes can tune into signals emitted by the primordial gas, allowing astronomers to learn more about reionisation.
"We see these galaxies as tiny, tiny dots in the sky. It's very, very hard to collect light and study them," Professor Trenti said.
"It's really amazing that we can learn so much."
