About a billion years ago, two enormous black holes – each about 30 times as massive as our sun – crashed into each other.
Black holes are the leftovers of giant stars that burned out and then exploded, eventually collapsing in on themselves to form something so dense that even light can't escape their gravitational pull.
The collision of the black holes created a burst of gravitational waves; ripples in the fabric of space itself. And 1.3 billion years later – September 2015, to be precise – those waves reached Earth. It was a big deal.
It was the first time gravitational waves had been detected, 100 years after they were first predicted by Albert Einstein. The waves are made whenever objects accelerate or slow down, but they're so tiny that it takes something as dramatic as colliding black holes to make them large enough to detect.
They were spotted using two enormous and incredibly sensitive instruments in the Laser Interferometer Gravitational-Waves Observatory (LIGO) in the US, and the results were published in February.
All of which was very exciting. But now scientists have gone back over the data from the collision, and have started to use it to try to find out when those black holes were first formed, and what the universe was like back then.
Their findings are in a paper published on Wednesday in Nature by researchers at universities in Warsaw, Chicago and New York.
The main findings are that these black holes are probably incredibly old, having lived and died together as stars in a brief blaze of glory. That tells scientists about what the early universe was like.
It's also important because it means gravitational waves are becoming a useful tool for astronomers, not just a novelty.
The study says it's likely that the two black holes came into existence when the universe was still young.
"The system was probably formed 11 billion years ago, in the early universe," Krzysztof Belczynski, an astrophysicist at Warsaw University and one of the authors of the paper, told BuzzFeed News.
That's because for a big black hole to form, you need a really big star, made of simple elements.
When the star dies in an explosion called a "supernova", a lot of its mass is hurled out into space. For complex reasons to do with how radiation passes through the star, that's especially true if the star contains lots of heavy elements, rather than just hydrogen and helium, the main ingredients of stars.
If just 2% of the star consists of anything else – the amount in our sun – then as much as 80% of its mass will be lost in the explosion, meaning it's less likely that there will be enough left over to form a black hole. But if there's much less of those heavy elements, then far less gets lost. "When you have 0.02% heavy elements, only 20% of the mass is blown away," said Belczynski.
And in the early universe, there weren't enough heavy elements to "pollute" the stars.
All the interesting elements that make up the world around you – all the carbon and iron and silicon and everything – was made in supernovas, in the explosive death of giant stars. When the universe was young, there hadn't been very many of them, so there wasn't much of the heavier elements around. "That first generation of stars polluted the later ones with metals," said Belczynski.
And that has the somewhat spooky implication that these stars burned brightly for a few glorious years, then died and spent 10 billion years as dark, silent ghosts before they finally crashed into each other.
"High-mass stars only have a very short life," Graham Woan, an astrophysicist at Glasgow University who was not involved with the study, told BuzzFeed News. "The higher the mass, the shorter the life.
"So these stars only had a few million years as a star, and then became black holes for a large fraction of the life of the universe."
The study also says that they were probably neighbours when they were stars – the two halves of a "binary system".
"Were these black holes formed by a pair of ordinary stars orbiting each other and then evolving into black holes?" Woan said. "Or were they a pair of isolated black holes which started to orbit each other later?"
If it's the latter, then they must have been relatively near to each other, so they were probably in a really dense part of a galaxy called a "globular cluster". "But there are many more stars elsewhere in galaxies than in globular clusters, so I think it’s 10 to 50 times more likely [that they were a binary system]," Belczynski said.
It's interesting to know about these particular black holes. But what's really important, say the scientists, is that it starts to paint a picture of how stars formed, evolved and died in the early universe.
"We don’t fully know how stars evolve," Belczynski said. "We know how they produce energy but a lot of major things, like how they form black holes and neutron stars when they die, we don't know.
"We had various models – that they make lots of black holes, that they make very few, that they make a moderate amount – and each model has very different physical models. And this finding shows it probably isn't the very high or very low models, so we're left with the moderate one."
That's not all. "There's an idea that there was a population of very large stars created in the early universe," Woan said. If these black holes do come from that period, it's evidence that that theory is true. It also supports the theory that early-universe stars didn't have many heavy elements, although that was pretty well confirmed already.
And this is all coming from the very first time gravitational waves were detected. So far there have been two confirmed detections. The really interesting discoveries will come when there have been lots.
To confirm the findings so far – and to start checking other things – LIGO will need to see dozens more. But because they've seen two so quickly, they're hopeful that they will see 10 or more a year, so it won't be too long before the data starts to build up.
"The universe is always more complicated than you think," Woan said. "It’ll be interesting to see what happens when you have a few more. You probably need about 100."