Chapter 1: What fictional scenario introduces the concept of primordial black holes?
Picture this. It's the year 2500. The first probe to leave the Milky Way is finally passing our galaxy's outermost star. A historic moment that's being broadcast to every settled planet. The probe's name is Wanderlust 1. Its destination, our closest neighbouring galaxy, Andromeda. But just as onlookers celebrate, something strange happens. Impact.
Worlds watch in shock as the middle of the probe crumples in on itself, causing its metal to distort, buckle, tear, some vanishing entirely. The remains of the probe start spinning in space, its systems dead, and then the culprit becomes clear. Wanderlust 1 was hit by a primordial black hole. Of course, what I just told you is fiction.
But tiny black holes, with masses so small they are comparable to an adult rhino, might not be. They could be out there, circling our galaxy's edges, and we may have just found proof that they actually exist. I'm Alex McColgan and you're watching Astrum. Join with me as we delve into the darkness to solve the mysteries of primordial black holes. What are they? Where do they come from?
And should we be worried? For a long time, tiny black holes, what I'm going to call black hole rhinos, seeing as they could have around the same mass, were thought to be impossible. Until the 1960s, we knew of exactly one way to make a black hole, and that was through stellar collapse. A star 20 times the mass of our sun, or greater, running out of fuel and exploding in a supernova.
All that exploded matter collapses, leaving behind an object so dense, not even light could escape from its crushing gravity. This is a black hole. And because it formed from a star, it's a stellar black hole. But this isn't how all stars end. If their mass is less than 20 times that of the sun, it still goes supernovae, but the collapse isn't quite powerful enough to create a black hole.
Instead, we see a neutron star, a tiny celestial object that's still incredibly dense and made up almost entirely of neutrons. And for stars smaller than eight stellar masses, they don't go supernova at all.
So there is a limit to how big stellar black holes can be at birth, no larger than the largest stars, maybe a few hundred times the mass of our sun, and no smaller than a few times more massive. So why do we think that smaller than possible black holes could actually exist? Strangely, the answer is that we discovered bigger than possible ones.
Today we know that at the heart of almost every large galaxy lies a gargantuan monster, a supermassive black hole. Back in the 60s, we were just beginning to discover these enormous beasts. The first quasar, a highly luminous and redshifted galactic centre, was found in 1963, kickstarting a roughly decade-long golden age of black hole research.
The first predictions of a massive black hole at the centre of a galaxy came in 1971. As the years passed and more evidence of these titans came to light, we realized they were mind-bogglingly massive, hundreds of thousands to billions of times the mass of our sun. But they are something of an enigma, and left scientists with one question in particular. How do they form?
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Chapter 2: What are primordial black holes and how do they differ from stellar black holes?
It inhabits a surprisingly small galaxy. Scientists were able to do a spectral analysis and found that it's incredibly low in heavy metals, elements other than hydrogen and helium. This galaxy had less than 1% of the oxygen that we see in our own, and researchers called it one of the most chemically unevolved systems found in the early universe, which is telling.
Stars usually produce these elements in just their first few generations, so the fact that they are absent in QSO1's galaxy suggests that very little stellar formation has taken place yet, compelling evidence that wherever QSO1 had come from, it had likely not been birthed by a star.
Now, although not smoking guns, these two examples lend weight to the idea that the early universe was capable of producing primordial black holes. In fact, this finding may explain where all supermassive black holes came from. And if that's true, then it's almost certain that super tiny black holes used to exist too. But they're all gone, right? By now they should have dissolved.
Well, according to one theory, perhaps not. Astronomy has a problem on its hands. As we track the amount of gravitational pull in the universe, it is much higher than it should be. Scientists conclude that there is additional matter inside galaxies, or circling them in large halos, something they've dubbed dark matter, because we can't see it.
But you know what else carries serious mass and is quite hard to see? black holes, particularly ones with no accretion disks, because they formed directly out of interstellar matter. Of course, scientists have investigated this idea, and there's no evidence of large black holes surrounding the Milky Way, thankfully.
If there were, stars would go flying like bowling pins every time one fell into our galaxy. We'd also see the light from stars behind them behaving strangely through the power of gravitational lensing, as the intense gravity of these black holes distorts the space around them. And we simply don't see anything that matches up to this.
But tiny black holes, our black hole rhinos, would be very hard to detect through either means. There's no proof they're not there. The only argument is to say that they would have all dissolved by now, but this might not be the case.
In April 2024, a study published in the monthly notices of the Royal Astronomical Society found that at tiny levels, Hawking radiation might slow down considerably, almost stopping entirely. Which means these black holes might shrink and shrink until they eventually stop. Which, if true, means that black hole rhinos might be out there.
Many believe dark matter circles our galaxy in a massive halo. What if, instead of all of that, it was tiny black hole rhinos? There could be millions and millions of them out there. A black hole minefield lying invisible and deadly. A swarm we could someday encounter if we ever attempted to leave the galaxy to explore another. We might be caged here without knowing it.
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Chapter 3: How do black holes form and what limits their growth?
And yet, there they were, and they were numerous. Stellar black holes can grow as time goes on, provided you funnel more mass into them. But how quickly? In 1920, an English astronomer and physicist called Arthur Eddington formulated the idea that there was a limit to how quickly either a star or a black hole could grow. This was because photons carry momentum.
A tiny amount, true, but enough to exert a push. This is what pushes solar sails on certain hypothetical spaceship designs, that tiny amount of momentum imparted by photons. For mass to enter into a star, it has to push against a constant stream of photons that are radiating outward. And at a certain level of brightness, not even gravity is strong enough to pull against the flow.
This is called the Eddington limit. And stars that brush against its boundaries, such as Wolf-Royet stars, bright stars at least 20 times more massive than the Sun, emanating powerful stellar winds, are just the slightest nudge away from blowing themselves apart. For black holes, you might think this would be less of a problem.
Isn't the whole point of black holes, that they don't radiate any light? But their accretion disks are a different story. As we discussed earlier, accretion disks around supermassive black holes can be incredibly bright, particularly around supermassive black holes, sometimes dwarfing the brightness of the stars in the galaxy they reside in.
With brightness comes resistance to gravity, and black holes have to obey the Eddington limit too. So even though, given enough time and mass, stellar black holes could theoretically grow into supermassive black holes, it doesn't seem plausible that this actually explains all the supermassive black holes we see in the early universe.
Simulations have been run, and although it is technically possible to grow a stellar black hole into a supermassive black hole in that timeframe, it would require those black holes to be feeding at near the Eddington limit non-stop since their birth, which just doesn't happen. Black holes in real life often run out of mass nearby and need to wait to run into more, or for more to come to them.
To further complicate the matter, we're not completely sure that supermassive black holes are the grown-up version of stellar black holes in the first place. Although it seems like common sense to assume so, scientists have been confused at the lack of the intermediate stage of black holes observable in our universe. To be frank, they've not cited any, at least none for sure.
Supermassive black holes are common at the centre of galaxies, and there are thought to be 100 million stellar black holes in our Milky Way alone, based on the number we've seen. But intermediate black holes are suspiciously lacking, with only a handful of potential candidates. You would think we'd see a lot more.
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Chapter 4: What role does the James Webb Space Telescope play in black hole research?
Struggling for certainty, scientists began to hypothesise that supermassive black holes were instead born in some other way.
Many cosmologists have been exploring the idea that because everything was much closer together in the early universe, things might have been dense enough that interstellar dust itself could conceivably have collapsed to form a black hole directly, skipping the star step altogether.
If this is true, and there is some evidence to support the theory, then perhaps supermassive black holes were once capable of simply being born that size, or near it, right from the offset, even if such a thing is no longer possible in our more spread out universe today. But this is by no means certain. But then LID 568 came onto the scene, and the pendulum swung the other way again.
LID 568 is a very distant black hole, between 12.1 and 12.3 billion light years away. It's so far away from us that we can't see it at all using visible light. The expansion of the universe has redshifted it all into infrared ranges. But even its infrared emissions were too dim to be picked up by heavy hitters like Hubble alone.
It took the Chandra-Cosmos Legacy Survey's combined telescopes and the incredible resolution of the James Webb Space Telescope to see it at all. Even then, LID 568's whole galaxy is little more than a faint red and compact dot. But the light emissions from this red dot are revealing.
X-rays given off by LID 568's accretion disk reveal that it was actively consuming matter in its galaxy's heart in a way that no one expected. You see, LID 568 crucially breaks the Eddington limit. And not just by a little, it's 40 times over the accretion speed limit. It's well on its way to having its license revoked. How is this possible?
It turns out that breaking the Eddington limit is in fact possible, but only for short bursts or in sneaky ways. For example, jets can help you get around the Eddington limit if all your photons are being blasted off in a single concentrated direction, all the other directions can eat to their heart's content, with no photon feedback getting in the way of a good meal.
there are other possibilities. While Eddington's limit says that once the brightness of the accretion disk becomes too high, all the black hole's food will be blown away, there is a period of time before this happens where a greedy black hole can snatch at the escaping matter and potentially enter Super Eddington territory.
Like an over-eager diner, it might pay for it later, but for a short burst, that level of accretion can occur. If this is true, it might just explain how supermassive black holes in the early universe came to exist. And certainly, LID 568 exists, and is the clearest example to date of a black hole accreting this quickly.
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