When the James Webb Space Telescope (JWST) launched on Christmas Day 2021, astronomers hoped it would revolutionise our understanding of the early stages of the Universe. And they were right.
Almost as soon as the telescope was switched on, astronomers saw something surprising: there seemed to be big, bright galaxies everywhere in the early Universe.
They had predictions about how many galaxies they should see and how large those galaxies would be, but there were far more of them than they had imagined.
This led to a flurry of headlines and a whole lot of head scratching in the field, because, at first glance, the galaxies the astronomers were seeing seemed to be impossible.
There simply wasn’t enough time since the Big Bang for that many stars to have formed according to the standard model of cosmology.
So, was there something wrong with our understanding of the Universe? Did the Big Bang not happen when we thought it did? Was cosmology broken?
To understand why this question arose, you need to understand what data from the JWST looks like. The telescope is the most powerful tool ever built for looking at the Universe and is effectively capable of peering back in time.
That’s because it looks at galaxies that are so extremely distant – billions of light-years away, in some cases – that the light from these galaxies takes a huge amount of time to reach us.
When we see these galaxies now using the JWST, we’re seeing them as they were billions of years ago, when the Universe was young.
Looking at such distant objects means you don’t get a nice, clear image of a galaxy.
Most of the images the JWST collects of these distant objects look like fuzzy blobs and scientists have to act as cosmic detectives to figure out as much as they can about these galaxies from limited clues.
Scientists like Katherine Chworowsky of the University of Texas at Austin, in the US, work with the data from the JWST to figure out the mass of these early galaxies by looking at images of the same object taken at different wavelengths of light.
They then compare these to ‘model’ galaxies with different distributions of big and small stars, to see which best fits the observations. “So it’s an estimation,” they explain. “It’s not a direct observation.”
In the earliest JWST observations, the instruments were still being calibrated and refining the calibrations solved part of the problem.
Another batch of the ‘impossible’ galaxies turned out not to be standard galaxies at all, but a new type of object that became known as ‘the little red dots’, which are thought to be huge black holes.
What’s left is a puzzle of strangely heavy, early galaxies, though not ones that necessarily break the Universe as we understand it.
So, did the JWST disprove the Big Bang? No, but it has raised intriguing questions about how galaxies and stars formed when the Universe was young.
Surprisingly prolific
There are two parts to the problem of overly massive galaxies in the early Universe:
Firstly, there’s a group of extremely early galaxies, from the first 400 million years of the Universe, that glow brightly in the ultraviolet – more brightly than they should.
Secondly, there’s the large number of massive galaxies found in the first two billion years of the Universe.
These two unusual findings could be related or they could be different issues entirely. That’s because the relationship between a galaxy’s brightness and its mass isn’t always clear.
“A galaxy can be bright in the ultraviolet because of its mass, but also because new stars are forming,” says Dr Jeyhan Kartaltepe of the Rochester Institute of Technology, in the US, leader of the COSMOS-Web survey using JWST to study galaxy formation.
“Because massive [younger] stars are brighter in the ultraviolet, while older stars that tend to make up most of the bulk of a galaxy emit most of their light in the optical or infrared.”
To work out how bright or how massive a galaxy should be, astronomers work with models based on what we know about star formation and how quickly that can happen in given environments.
But galaxies in the early Universe are quite different from those we see today.
“The biggest difference is in the amount of gas versus the amount of stars,” says Kartaltepe.
“Today, in the Universe, most of the mass is in stars and there’s only a little bit of gas left. But in very early times, it was pretty much all gas and there’s fewer stars. So the properties of those galaxies are going to be different.”
This preponderance of gas leads to more turbulent galaxies that are less likely to have the neat, thin disc structure we see in galaxies today. When early galaxies did form discs, they tended to be thicker and less stable.
“All of these estimates come from cosmological simulations,” says Kartaltepe.
“So the ingredients of these simulations are what our best understanding of the physics of the early Universe is – what the ingredients are, when star formation starts, how it progresses, what the dust is like.
"So there’s a lot of assumptions and a lot of things we don’t know well that go into these simulations.”
These models do a good job of matching to the Universe as we see it at later times, but they don’t match up with what we’re seeing in the earliest part of the Universe.
This means that the kinds of massive galaxies that Chworowsky is studying aren’t outside the bounds of possibility for the early period that they’re found in, but they are surprisingly prolific.
“They’re supposed to be a rarity, but we’re seeing more,” they say. “So what’s going on there?”
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A big wheel
In early 2025, the JWST delivered another surprise. Researchers discovered a huge galaxy with a spiral structure (earning it the name the Big Wheel) – and it existed within the first two billion years of the Universe.
“It being big and having this beautiful spiral structure makes it interesting,” says Dr Themiya Nanayakkara of Swinburne University of Technology, in Australia, one of the researchers who discovered the galaxy.
You would expect such a massive early galaxy to be elliptical, with a smooth and featureless shape. But the Big Wheel has a spiral structure similar to that seen in much later galaxies, including our home galaxy, the Milky Way.
“That means whatever happened for it to grow has either happened in a very neat way, which allowed it to keep on spinning without more violent mergers happening, or most of the mass is formed in situ,” Nanayakkara says.
This points to the two ways that galaxies can grow: either by merging with other galaxies or by creating new stars from material already within the galaxy.
The location of the Big Wheel, in a busy and crowded area of space, suggests that it's likely experienced collisions or mergers with other galaxies in its time.
But these violent events don’t seem to have disrupted the delicate spiral structure. Normally, mergers would disrupt or destroy a galaxy’s elegant spiral arms, smearing them into more of a featureless elliptical.
“The fact that this galaxy didn’t become elliptical means that there might be ways that galaxies can grow that we haven’t encountered in the local [observable] Universe,” Nanayakkara says.
As striking as the Big Wheel is, though, it’s just one galaxy from around two billion years after the Big Bang. So it’s not necessarily representative of the puzzle of all over-massive early galaxies, though it is an intriguing clue to their development.
“There are always outliers,” Nanayakkara says. In the case of this particular galaxy, “Everything worked out in a perfect way for it to exist.”
Potential explanations
Scientists are pursuing several possible theories to explain the overly massive galaxies, including that they might be the result of the conditions in the early Universe, which was smaller and more tightly packed than the Universe is today.
“We know the Universe early on was very dense,” Chworowsky says. “Gas that is dense does form stars faster, because more dense gas can collapse more easily and that’s how star formation happens.”
So it’s perhaps unsurprising that early galaxies would be forming more stars, though exactly how dense gas needs to be to increase star formation is still being investigated.
Another theory relates to the amount of dust that was present in the early Universe. Dust tends to block light, so a very dusty galaxy will appear less bright at certain wavelengths.
The amount of dust in early galaxies could be affecting how bright they appear. If these distant galaxies have less dust than we think they do, that would mean less of their light was being blocked.
This would cause them to shine brighter, leading astronomers to assume they’re more massive than they actually are.
Then there’s a different set of potential explanations that focus more on the nature of star formation than on the conditions of the early Universe.
It could be that star formation in early galaxies doesn’t happen at a stable level, but rather is ‘bursty’ – it happens more dramatically in some periods than in others, and the galaxies that we’re observing happen to be in an intense, ‘bursty’ period.
Alternatively, one area that’s receiving a lot of interest is the concept of initial mass function, or how the different masses of stars are distributed within a galaxy and how that changes over time.
Our current models of what proportion of larger to smaller stars we should expect to find during the evolution of a galaxy are based on observations of galaxies that are close to us, relatively speaking, in the local Universe.
We’re assuming that for every massive star we see, there are a certain number of lower mass stars that we can’t see. But perhaps it was different in the early Universe.
“If we were to have a top-heavy model, meaning far more bright, large stars form initially, then that would make a much brighter galaxy,” Chworowsky explains.
These early galaxies might have a greater proportion of large stars to small stars, so they might not be as massive as their brightness would suggest.
An astronomical holy grail
That leads on to one of the holy grails of astronomy research: the search for the very earliest stars.
Known as Population III stars (populations are numbered backwards, so the stars we commonly see today are Population I, while older stars are Population II and the very first stars are Population III), they have never been observed, but theoretically must have existed when the Universe was at its youngest.
These populations of stars differ due to their lack of heavier elements.
That’s because the heavier elements in the Universe were almost all created within stars as a product of fusion and then spread through the Universe when those stars exploded as supernovae.
The further back in time you go, the less these heavier elements were present, until you reach a time when stars were made virtually entirely of hydrogen and helium – the Population III stars.
Due to their different composition, these earliest stars would have been far more massive than the stars we see today and have lived much shorter lives.
As they’re so very old, though, detecting the signature of such stars for certain is difficult.
Still, it’s possible that the JWST will be able to detect signs of Population III stars, perhaps even among the galaxies that have already been selected for further observations.
“I think there’s hope that we might see them in the galaxies we’ve already chosen for follow-up spectroscopy. There’s hope that maybe we’ll see a Pop III signature,” Chworowsky says.
But there’s still the issue that galaxies are made up of vast numbers of stars and it’s tough to know whether the signatures of these stars would be visible through the light of all the other stars in a given galaxy, even if they are present.
A solvable mystery
Experts are optimistic that, as puzzling as these early massive galaxies are, they’re a puzzle that’s solvable.
The JWST is already working on further observations in the mid-infrared range, rather than the near-infrared red that most of this research has been based on, as well as on spectroscopy observations.
These observations take more time, but they can help reveal the composition and mass of early galaxies more accurately than existing images.
Within the next decade, the astronomers predicted, we’ll have advanced our understanding of early galaxies enough to know if they truly are so massive, and why.
In a field full of big questions and hard unknowns, there’s a sense that this particular oddity is one we can solve and which won’t require rewriting the laws of physics.
“So far, it seems everything can still be explained with our cosmological model and our understanding of when the Big Bang happened. It just needs some fine-tuning,” Kartaltepe says. “It shines a flashlight on where we didn’t understand things.”
That sometimes indirect path toward the truth is part of the reality of science and its joy.
“It’s not always a formula,” Chworowsky says. “There are times when you’re going to see things that are outside of what you thought the path should be. That’ll require you to revise things.
"It doesn’t mean that the things you always said were wrong, but it does mean: Oh. Outliers. That’s interesting. That’s something that now needs to be folded in.”
The JWST has already brought about the revolution it promised: the findings from the telescope are “transformational,” Nanayakkara says.
The ground-breaking observations have already been made in the telescope’s first years of operation, he thinks, and now the challenge is to make sense of what we’ve seen: “It’ll be more fun now, trying to understand the physics as well.”
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