Astronomers have used the Very Large Telescope (VLT) in Chile to study a fundamental change in the cosmos more than 13 billion years ago.
The scientists studied a series of galaxies through time to determine the pace at which the neutral hydrogen that pervaded the Universe back then was turned into a diffuse, highly charged cloud of gas.
The research pushed what is one of the world's premier telescopes to its limits.
And it revealed that the change in the cosmic environment progressed relatively rapidly.
Understanding this "Epoch of Re-ionisation" is now one of the key quests in astrophysics, because it helps to explain why the Universe looks the way it does to us today.
"We see the end of this re-ionisation with the VLT," said lead researcher Laura Pentericci of INAF Rome Astronomical Observatory.
"The whole process took probably several hundred million years," she told BBC News.
Studying this epoch is not only a compelling exercise; it is also an extremely difficult one.
It involves careful observations of galaxies that are so distant as to be barely detectable.
In a galaxy far away...
Dr Pentericci and colleagues looked for a very characteristic light signal in these far-off galaxies.
They are so far away that the VLT sees them just 750 million to a billion years after the Big Bang.
The characteristic Lyman-alpha line, as it is called, is a tell-tale for the ultraviolet emission from these ancient galaxies' stars.
It would have been this UV output that "fried" the neutral hydrogen, ripping electrons from atoms to produce the ionised gas.
It would have been something like trying to clear a fog, however.
Much of the UV energy would have been absorbed in the process. Only in the end phase could the radiation spread unhindered across the cosmos.
The VLT sees these events unfold in the changing nature of the Lyman-alpha line in galaxies at varying distances.
"We see a dramatic difference in the amount of ultraviolet light that was blocked between the earliest and latest galaxies in our sample," said Dr Pentericci.
"When the Universe was only 780 million years old, this neutral hydrogen was quite abundant, filling from 10 to 50% of the Universe's volume.
"But only 200 million years later, it had dropped to a very low level, similar to what we see today.
"It seems that re-ionisation must have happened quicker than astronomers previously thought."
End of the beginning
The VLT is only studying the end of the Epoch of Re-ionisation.
Despite the telescope's immense power, its 8m-wide mirror system and associated instrumentation cannot resolve the information required to study the epoch's beginnings.
Better instrumentation is coming that should push the VLT slightly deeper in time, but the final knowledge will only come with the next generation of astronomical observatories.
One of these is the 40m-wide Extremely Large Telescope, which will be built about 20km from the VLT in Chile's Atacama Desert.
Another is the James Webb Space Telescope, the "successor to Hubble", due for launch later this decade.
Both of these facilities will be tuned to see the light from the very first stars to shine in the Universe. Those behemoths, perhaps a hundred times more massive than our Sun, would have burnt brilliant but brief lives, producing the very first heavy elements.
"We call them the Population III stars," said Dr Pentericci. "These stars may have contributed a lot to the re-ionisation process."
The research, which took three years to conduct, will appear in the Astrophysical Journal.