The problems started with six little red dots. Six small fuzzy spots, among thousands of others, were enough to suddenly bring everything into question.
The dots appeared in images captured by the James Webb Space Telescope (JWST) and, whichever way astronomers and astrophysicists looked at them, they just didn't make sense.
Everything about them seemed wrong – their colour, the wavelengths of light they were giving off and the fact they were so numerous. Their existence didn’t fit into our understanding of the Universe.
And yet, there they were. Six little dots, sitting – as plain as the nose on your face – right in the middle of images collected by the most advanced telescope humanity had ever built.
A telescope with the ability to look so far into space that it can effectively see back in time, almost to the dawn of the Universe.
There were only three possible explanations for the existence of these six troubling dots. One, there was a problem with the telescope; two, there was a problem with the experts’ interpretations of the data; or three, there was a problem with our understanding of the cosmos.
Galaxies that seemed too massive to make sense
It was Dr Ivo Labbé, an associate professor of astrophysics at the Swinburne University of Technology in Australia, and his colleagues who first spotted the curious dots and began investigating them. And the closer they looked, the more puzzling the dots became.
The dots appeared to be six massive galaxies – far too massive to make any sense.
The galaxies were more massive than could be explained by the amount of gas available in the Universe at the time they formed. By our understanding, it just wasn’t possible for galaxies so massive to exist at such an early time.
And yet there they were, in images from the JWST, blissfully unaware of the fact they were contradicting everything we thought we knew about the cosmos.
The JWST is able to see the very early periods of the Universe where these ‘cosmology breaking’ little red dots appeared because of the way light travels.
As the Universe expands, the galaxies in it are flying apart to such great distances that it takes time for light to travel between them.
When scientists use an incredibly sensitive tool like the JWST, they’re able to see objects that are billions of light-years away.
The light from these objects has been travelling for billions of years, so when we see it, it’s like we’re looking back in time.
The more distant an object, the earlier we’re seeing it. And the JWST is able to see over such great distances, it’s finding objects that appeared in the first few million years after the Big Bang.
The JWST initially spotted the dots with its near-infrared camera (NIRCam), but a whole new view of them became available when Webb unleashed another of its instruments: not a camera, but a spectrograph.
The instrument, called NIRSpec (near-infrared spectrograph), detects light from distant galaxies and runs it through a prism, effectively splitting that light into the colours of the spectrum. The spectrum will be incomplete, though, because different elements absorb different wavelengths of light.
By looking at which wavelengths are missing, experts can determine which elements are present at the light’s source.
"Spectroscopy is key, because it means we can understand the nature of galaxies. We can understand the elements they’re composed of,” says Prof Andy Bunker, an astrophysicist at the University of Oxford.
This kind of data can tell you far more about a distant galaxy than just an image can, Bunker explains.
“We’re interested in their distances, their redshifts [which are related to the galaxies’ ages], their chemical composition, and the strengths of these emission ions that give you a handle on the star-formation rate.
"It also tells you – perhaps – if there’s a supermassive black hole there. So, there’s a whole bunch of science we can do with the spectra that we couldn’t really do with imaging.”
When researchers finally got to use NIRSpec to peer at these six cosmology-breaking dots, it was immediately clear that the imaging had misled them.
They weren’t galaxies at all. Instead, they looked more like enormous black holes.
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How the mistake had been made
To understand how this confusion arose, it helps to know what data from the JWST looks like. With objects that are so tremendously far away, scientists aren’t getting a clear and detailed picture that they can study.
Instead, they’re getting something that looks more like a fuzzy blob. They can see a colour – in this case, red – and they can see data called spectra, which show the different wavelengths of light plotted on a graph. And that’s it.
The interesting thing about these little red dots is that they had an unusually shaped spectrum. They didn’t look quite like galaxies, but they didn’t look quite like the black holes that we know today either.
“These are not your normal black holes,” Labbé says. “These are not garden-variety black holes.”
The emerging consensus is that the most likely explanation for the odd shape is a small galaxy that’s in the process of creating stars, with a supermassive black hole in the middle.
That means that these objects aren’t breaking the Universe as we know it, but they are something new and strange.
Labbé and his colleagues had correctly identified there was something odd about these objects, but they’d incorrectly assumed that they must be galaxies when they’re something quite different.
“You either have something that’s breaking the laws of cosmology (you have the option that your models [of the early Universe] are just not correct). Or you have the option that you’re looking at a completely new phenomenon,” Labbé says. “Turns out it’s the latter.”
Trying to understand these black holes
Assuming these objects are black holes solves some of the problems they present – it means that cosmology isn’t broken and there aren’t impossibly massive galaxies hanging around in the early Universe. It does, however, create some other problems.
That’s partly because even if you know (or suspect) that an object is a black hole, inferring its features is no simple task.
“There’s one property we can measure of a black hole, which is its mass, and even that’s fairly uncertain,” says Prof Steve Finkelstein, a black hole researcher at the University of Texas at Austin.
The JWST has detected plenty of objects in the early Universe that look like black holes – in fact, it’s detected far more black holes than anyone expected it to.
But like the galaxies they were initially mistaken for, these early black holes seem like they might be too big.
“I think the one thing we’re finding that’s surprising is that, if the masses we’re measuring for the black holes are correct, they’re overmassive compared to their host galaxies,” Finkelstein says.
When we look around the Universe today, practically every galaxy we see has a supermassive black hole at its centre, and the more massive the galaxy, the more massive the black hole. This relationship is generally fairly linear and has been observed throughout the Universe.
The black holes we’re spotting in the early Universe with the JWST don’t seem to operate within that understanding, though.
These black holes are “ten or maybe even a hundred times more massive than you’d expect,” based on the mass of their host galaxies, Finkelstein says.
While that finding is strange, it could also help to answer a longstanding question about galaxy evolution.
If every galaxy has a supermassive black hole at its centre, is that because the black holes form first and then galaxies form around them? Or do galaxies form from clusters of stars and somehow create the conditions for a black hole?
According to Finklestein, the large mass of these early black holes suggests that the former explanation is more likely, as the black holes are so massive they could be forming first. But it’s far from a sure thing.
It’s also entirely possible that the way we measure the mass of black holes is wrong somehow, further complicating matters.
Looking for the tell-tale signs of a black hole
If there’s one thing everyone knows about black holes, it’s that they’re dark. Really dark. Their gravity is so great that they suck in anything that comes too close to them, even light. So you might wonder: how do you spot a black hole?
The answer is that astronomers don’t actually observe the black holes themselves, but rather the gas that surrounds them.
This region, called the accretion disc, attracts gas and dust, which then begins to rotate due to the immense forces of gravity, swirling faster and faster until the matter passes a point called the event horizon and gets pulled into the black hole, never to be seen again.
As the gas swirls around the black hole, it heats up due to friction, becoming so hot that it glows, producing high-energy radiation that’s detectable as X-rays.
These tell-tale X-rays are created by matter that’s so hot it can only be found in a fast-moving accretion disc, so observing these emissions is a clear giveaway of the presence of a black hole.
But while the strange little red dots spotted by the JWST make more sense if we presume they’re black holes, that still leaves us facing a big problem: they don’t give off X-rays.
X-rays are the definitional feature used to identify supermassive black holes, Labbé explains, so not finding them raises questions about whether these objects are black holes at all. But if they aren’t galaxies and they aren’t supermassive black holes, what else could they be?
“We’ve not really seen this before and that makes it quite hard to interpret what’s going on,” says Bunker. “But exciting. These [objects] weren’t predicted in advance.”
Despite the lack of X-rays, researchers think these objects must be black holes because the light they do give off shows a feature called broad emission lines.
That happens when gas is spinning around a central point, so when we’re looking at it, some gas appears to be coming toward us while some is moving away.
The spectra from the gas travelling toward us has a slightly lower redshift, while the gas travelling away from us has a slightly higher redshift (due to the Doppler effect).
So, when you look at the object in its totality, the spectrum of light it’s giving off is stretched over broader wavelengths.
To be detectable in this way, the gas has to be moving at thousands of kilometres per second, and a supermassive black hole is the only thing we know of that could make gas travel that fast. That means something strange is still going on.
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These strange objects can be found across the early Universe
Labbé and his colleagues acknowledge that whatever the six little red dots are, they’re certainly strange objects.
And now that they know what to look for, they’re finding more – possibly hundreds of them. These objects seem to be numerous across the early Universe.
“It seems that we’re looking at a population that’s mostly black holes [but unlike any] black holes that we’ve ever seen,” Labbé says.
A theory that scientists are working on is that these black holes are actually similar to the ones we see today – it’s just that the views we’re getting of them with the JWST show the black holes as ‘newborns’.
They’ve only recently formed and conditions in the early Universe were so different to the way the Universe is now that it makes them look different.
The early Universe was much smaller and denser than the Universe is today. There was a lot more gas floating around that hadn’t yet been pressed together into stars, planets and other objects.
Maybe these clouds of dust could be obscuring the X-rays coming from these objects, stifling their radiation and making them appear differently.
Currently, scientists are using computer models to see whether pockets of dense gas could absorb X-rays and account for this strange X-ray absence, but it’s not yet clear whether that’s a viable theory.
For Labbé, however, that unknown is part of the appeal of this work. “It’s the joy of working at something that’s not understood. It’s, I think, one of the most exciting discoveries that has come out of [JWST] – this whole new population of things that we don’t quite understand.”
Astronomers are still debating exactly what the nature of these little red dots are and some of the questions that their existence brings up – are these really early Universe black holes?
Or are they some different kind of object? Do black holes form first and galaxies form around them? Or do galaxies form first and then somehow generate a supermassive black hole at their centre?
With more JWST data, they’ll be able to begin answering these questions.
“It’s a very special time to be an astronomer,” Labbé says. “Whenever there’s such a big leap in technological advances, that’s a very special time.”
About our experts
Dr Ivo Labbé is an associate professor of astrophysics at the Swinburne University of Technology, in Australia. He is published in various scientific journals including The Astrophysical Journal, Astronomy & Astrophysics and Monthly Notices of the Royal Astronomical Society.
Prof Andy Bunker is an astrophysicist and professor of the same subject at the University of Oxford, in the UK. His work has been published in the likes of The Astrophysical Journal American Astronomical Society, Monthly Notices of the Royal Astronomical Society: Letters Oxford University Press and Astronomy & Astrophysics EDP Sciences.
Prof Steve Finkelstein is a professor of astronomy and black hole researcher at the University of Texas at Austin, in the US. He's published in the likes of The Astrophysical Journal, Publications of the Astronomical Society of Australia and Monthly Notices of the Royal Astronomical Society.
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