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Chapter 1: What mysterious red dots have been spotted at the edge of the universe?
We've just noticed something strange that pops up in almost every image of the early universe. Little red dots. They are scattered across the earliest reaches of the universe, some of the oldest objects we've ever observed. Everywhere we turn, there they are. And yet, no one knows exactly what it is we're looking at. And the more we learn about them, the stranger they become.
These little red dots don't fit any of our existing models of star, galaxy, or black hole formation. Something this ancient should be small, chaotic, slowly building up mass over time. Instead, they're incredibly dense, dazzlingly bright, and far more massive than we'd expect. So, what are they? Some now think these little red dots might be a new cosmic body entirely, part star, part black hole.
But is that even possible? And what would their existence mean for our understanding of how the early universe formed? I'm Alex McColgan and you're watching Astrum.
Join me today as we unpack why these little red dots defy explanation, examine the latest theories about what they are, and discover why they could be the missing piece in one of astrophysics' biggest mysteries, the formation of supermassive black holes.
Chapter 2: Why do these red dots defy our current understanding of cosmic objects?
Since the James Webb Space Telescope launched on the 25th of December 2021, it has completed over 32,000 hours of observation and sent back more than 600 terabytes of data. That's enough to fill the memory of more than 2,000 phones. And within these tens of thousands of images... one type of object keeps cropping up.
These dots were first seen in the Eiger and Fresco surveys, observational studies hunting for galaxies at extreme distances. Their purpose was to examine the very beginnings of our universe, some 500 million years to 1 billion years following the Big Bang.
As James Webb scanned the skies, nearly every photo it sent back of this era revealed the same strange object, little red dots scattered across the early cosmos. By 600 million years after the Big Bang, it seems our universe was filled with them. But 1.5 billion years later, they disappeared completely. This instantly made them one of the biggest mysteries in the universe.
But what do we know about them so far? First up, they are very compact. usually no more than 500 light years across, which is about 200 times smaller than our own galaxy. They burn unusually bright for their size, and are very red, which is why they've evaded detection for so long. Until now, we literally had no way to see them.
Chapter 3: What theories exist about the nature of these little red dots?
Hubble was calibrated for shorter wavelengths than we are seeing from these objects, and other telescopes didn't have the power needed to look that far back in time. James Webb is the only telescope specifically designed to see the mid-infrared wavelengths characteristic of these little red dots.
By combining data from its near-infrared camera and mid-infrared instrument, astronomers caught a glimpse of these ancient objects for the first time, though their properties wouldn't be described and categorised until March 2024 in a breakthrough paper written by Jorrit Matti from the Institute of Science and Technology, Austria.
Now, as many of you will already know, the older the object we are looking at, the redder it appears to us. It's classic redshift in action. Since the universe is expanding, the fabric of space and all the light passing through it get stretched too. The wavelengths of light get physically elongated on their journey, and the longer they travel, the more they get stretched, so the redder they look.
But when astronomers broke the little red dots light apart into a spectrum, they found the redness runs deeper than that. The redshift alone couldn't account for just how red they appeared. The researchers' first instinct was that this may be caused by dust. Dust particles are much better at scattering shorter, bluer wavelengths of light than longer, redder ones, making the object appear red.
The same thing that happens to sunlight hitting Earth's atmosphere at sunset, which is why the sun looks red in the evening. But to be sure, the scientists enlisted the help of another instrument, the near-infrared spectrograph, and what they saw just confused them even more.
The nearest speck aboard the James Webb takes the light from a distant object and splits it into its component wavelengths.
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Chapter 4: How did the James Webb Space Telescope contribute to this discovery?
Different elements absorb light at specific wavelengths, leaving gaps in the spectrum like a fingerprint. So, if there is something, like a dust cloud, between the telescope and the light source, not all the light will make it to the instrument. By looking at which wavelengths are missing from the reading, we can figure out what the dust cloud is made of.
Researchers noticed that these little red dots were emitting a lot of blue and UV light, lots of red and infrared light, but not much in the middle. The result is a highly unusual V-shaped spectral energy distribution. Nothing we knew of in the distant universe was associated with such a spectral signature. But we did get one clue. At 364.6 nanometers, the spectrum shoots up sharply.
For some reason, light with wavelengths shorter than this was being readily absorbed, and longer wavelength light was just passing through unobstructed. This is a well-known signature, and it even has its own name, a Balmer break. And when we see this in a galaxy, there's only one explanation for it. Young, hot stars. Lots of them. Scientists thought they had it.
Little red dots must be early starburst galaxies.
Chapter 5: What evidence suggests these dots could be black hole stars?
It certainly would have been a very elegant solution. But that's rarely how these things go. So, theory number one is starburst galaxies. Now, this starburst galaxy theory had a lot going for it. Hot, young stars are rich in hydrogen. We'll come back to the chemistry later, but this could explain the strong balmer break. A dense, stellar population would explain the brightness.
And if these galaxies were packed with dust, as starburst galaxies often are, that would explain the redness too. It was almost all said and done, except for one annoying detail. The maths just didn't work. Researchers soon realised they were running up against two major problems. The first was how much dust they would need.
Dust in galaxies comes from one main source, stars, and there's a very well-established relationship between how many stars a galaxy contains, and how much dust these stars can produce. In late 2024, Kaitlyn Casey and her team at the University of Texas in Austin calculated their expected stellar population based on the optical light detected from the little red dots.
Chapter 6: What challenges do researchers face in categorizing these cosmic objects?
But once they applied the star to dust ratio, their stomachs dropped. Their calculations only yielded 1% of the dust needed to explain the redness they were seeing. And now, a shortfall of two orders of magnitude could only mean one of two things. Either the well-established dust formation models are wrong, or the main light source of these little red dots wasn't stars.
The second problem was the brightness. To explain the overall brightness of little red dots using just stars, you'd need an impossibly high density of stars in a very compact region of space. They'd only be a few astronomical units apart, but this close together, stars would be colliding and merging all the time. and the gravitational dynamics would be far too unstable.
What's more, these little red dots don't look like normal galaxies. Usually we can see some kind of internal structure, but even though James Webb has the highest resolution of any telescope we've ever built, the little red dots just appear as points of light. Single dots in the night sky, like stars appear to the naked eye.
So either the little red dots are really tiny, or their light is so dominated by a single central source that it drowns out any other surrounding signature that might exist. Which brings us to the second theory. What if little red dots aren't star-filled galaxies at all? What else could explain their properties?
Chapter 7: How might black hole stars explain the formation of supermassive black holes?
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Chapter 8: What implications do the little red dots have for our understanding of the early universe?
At the center of most galaxies, you can find a supermassive black hole, surrounded by an accretion disk of gas and dust. As it spirals inward, the gas is compressed and heated to over 12 million degrees Celsius, and it emits a powerful glow. Active black holes are some of the brightest objects in the universe. Could they be behind little red dots?
Surrounding the accretion disk of a black hole is a donut-shaped ring of gas called a torus. If you're looking at the torus side on, that dust sits between you and the bright black hole, absorbing shorter wavelengths and letting longer red and infrared ones through. Sound familiar? Little red dots as black holes made sense.
It would also explain the brightness, point-source morphology, and reddish color. To top it all off, their emissions show broad Balmer lines, which suggests there is a lot of gas spinning at thousands of kilometres per second around something central. Orbiting gas moves both towards and away from us at extreme speeds.
The light moving away from us is redshifted, while the light moving towards us is blueshifted.
This exaggerates the spectral lines in each direction, broadening their profile. The faster the orbital velocity of the gas, the wider that broadening becomes. And this is exactly what we would expect from a massive black hole feeding on lots of gas. So far, things were looking good for the black hole theory. That was… until researchers noticed something vital was missing.
You see, as far as we know, active black holes always emit X-rays from their accretion disks, but when astronomers measured the little red dots, they found no trace of X-rays at all. This was a devastating blow to a promising theory. As if that weren't enough, there was one more problem. The black hole seemed too big for the galaxies they were in.
In the local universe, black holes are usually around 0.1% of the mass of the galaxy around them. This is such a consistently observed relationship, we think galaxies and their black holes must co-evolve together using some kind of feedback loop mechanism that keeps them in tight check with each other. But when researchers started looking at little red dots, they noticed they...
had much higher black hole to galaxy mass ratios than we're used to, closer to 10%. It was almost as though the black hole had somehow grown to full size before the surrounding galaxy had the chance to catch up. Both the starburst galaxy and active black hole theories had their merits, but neither could fully explain what was going on with these little red dots.
Astrophysicist Fabio Pacucchi at the Harvard-Smithsonian Center for Astrophysics explained the dilemma perfectly. If they, little red dots, contain black holes, those black holes are enormous for such small galaxies. But if they only contain stars, the galaxies are too compact to contain all of them, reaching stellar densities that are unthinkable.
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