How do we explain the giant black holes appearing in the young universe? Is it possible to directly collapse a black hole, skipping the formation of stars? What does ultraviolet radiation have to do with this? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO GENERATED)

Folks, we've got a problem, and that problem is UHZ one. It's a galaxy discovered in 2023 using a combination of data taken from the James Webb Space Telescope and the Chandra X-ray Observatory. Cool. Side story, they they use gravitational lensing to see much farther than they normally would have been able to, but that's not today's tale. Here's the problem.

UHZ one, which I'm just going to go ahead and call UZ one, isn't just any galaxy. It's a quasar. It's one of the brightest sources of radio emission in the entire universe. Remember, quasars can outshine millions of galaxies at a time. And in order to be a quasar, to power a quasar, to generate those kinds of incredible energies, you need to have a supermassive black hole.

Because it's the material falling into a supermassive black hole, falling into that immense gravitational well that compresses and lights up and becomes a quasar. In the Uzi one's case, we can use the brightness of the quasar to get a rough estimate of the black hole mass, which turns out to be around 40,000,000 solar masses. That's 40,000,000 times the mass of the sun. That's that's big. It's about 10 times bigger than the supermassive black hole at the center of the Milky Way, but it's also much much smaller than the largest known supermassive black holes which can reach a hundred billion solar masses.

So what's the problem? The problem is that this galaxy is living. This quasar is shining. This black hole is blackholing when the universe was only 3% of its present age. This is just about four hundred seventy million years after the big bang itself.

Oozi one lived at the cosmic dawn, the awakening of the very first stars and galaxies to even appear in the universe, and we have no idea how to make that big of a black hole at that young of a cosmic age. The problem is that we can't explain how OUSY one exists, and that's a big problem. Here's why it's a problem. We know of exactly one way to build black holes, and that's through the depths of massive stars. You take a big enough star, something like the eight to 10 times the mass of the sun minimum.

You let it live its life, and then it fashions an iron core. It can't get energy out of the iron core, so the whole thing collapses. It's squeezed down. You have the entire weight of the star crushing in on the core at, like, half the speed of light or something ridiculous, and that squeezes everything down until a chain reaction occurs where the gravity is so strong that it overwhelms every other force, and it creates a black hole. So, yeah, something like Uzi one and its friends.

Don't worry. UZY one is not a one off. It has a lot of friends, and while it's the current record holder for earliest known black hole at the time of this recording, I'm sure that it won't retain its title for long. UZY one and its friends, these giant black holes, could have formed after the appearance of the first stars. Like, you just make us you make you the first stars, you make the first black holes, and then they grow to supermassive status.

But stars don't spit out supermassive black holes. They spit out black holes with masses, you know, ten, twenty solar masses, somewhere around that range. You don't just snap your fingers and make a star go boom and get a giant black hole. That's not how it works. You need to grow these black holes.

And once you've made a black hole, remember, we only have one known way to form black holes through the deaths of massive stars. And once you get a black hole, there are only two ways for it to grow, through mergers and acquisitions. Mergers, a black hole can just collide with another black hole, and then boom, you get a bigger black hole. That's it. And then acquisitions, anything that falls into the black hole, random stars, random blobs of gas, little tiny bits of gas, you know, anything dark matter that happens to pass through the event horizon, anything that crosses the event horizon increases the overall mass of the black hole.

This is the only way we know. Mergers and acquisitions are the only ways we know of how to get a black hole to be bigger. So to make a giant black hole like Oozi one fit into the into the standard and usual picture of black hole growth, you need to have the first stars appear, then you need to have the first stars die, then you need to have those first small black holes merge together, and then you need to have those black holes consume a lot of material, and voila, a 40,000,000 solar mass black hole appearing when the universe is less than half a billion years old. One sticking point. Time.

We know roughly, very roughly, when the first stars appear. We don't have that pinned down at all, but we know the earliest they can appear, and that's around one to two hundred million years after the big bang. Otherwise, the universe is just too warm, too smooth, too dense. It's like a soup that's too hot to eat. You just have to wait for the universe to cool down before you can start forming stars.

And while we don't know exactly how big those first generations of stars were, we can put an upper limit on their size, and you can pick whatever you want. Maybe the first stars were a hundred solar masses. Maybe there were a thousand solar masses if you want to go crazy. But no matter what, the size of the first stars puts a limit on the size of the first black holes because those first black holes have to be born from that first generation of stars. So you can start with the most optimistic rose colored glasses scenario imaginable with the biggest possible seed black holes.

You start with the biggest population of stars appearing as early as you can. Like, we're gonna have thousand solar mass stars appearing when the universe was 100,000,000 years old, like, the the biggest, earliest, and they're gonna produce, black holes that are, like, 500 solar masses, a thousand solar masses. Let's go crazy. The absolute top end of every single estimate you can make, and there's no way that they can merge often enough or create enough material to reach 40,000,000 solar masses when the universe was less than half a billion years old. You just run out of time.

Mergers in the early universe were not nearly common enough. Remember, we need to go from 10 solar masses, a hundred solar masses, maybe a thousand solar masses if you really wanna go crazy, to tens of millions of solar masses. The number of mergers you'll have to encounter to make that happen are too far beyond belief because even if you have a lot of first stars and a lot of black holes, it takes time for those black holes to find each other. It takes time for those black holes to merge and then do it again and again and again, and you just run out the clock. And then when it comes to acquisitions, when it comes to accreting materials, swallowing up material from their environments, again, it's just not fast enough because there is a limit.

There is a limit to how quickly a black hole can consume material, and this is called the Eddington rate or Eddington limit, named after sir Arthur Eddington, handily enough, who figured it out. And there's a certain valve mechanism in place when you have something like a black hole trying to consume material, and that valve is heat. Because if material falls into a black hole, you got a whole bunch of gas around here, and it's feeling the gravitational pull of the black hole, and it starts falling towards the black hole. It's going to compress. It's going to heat up.

It's going to emit light. That radiation is going to push on everything else. It's going to heat up everything else, which prevents it from collapsing down into the black hole. It slows down. It limits how efficiently a black hole can consume material.

Now it is possible. We know of examples, both in nature and in theory and and simulations to break this Eddington limit in certain special scenarios. You can get turbulent flows that can get, material down to the black hole faster than it can be heated up. Like, you can concoct scenarios, but in order to grow, a seed black hole from the first generation of stars up into super massive status in only a few hundred billion years, and I know a few hundred million years sounds like a a long time, but in this case, it's not. You would have to sustain breaking the Eddington limit for over a hundred million years, which just strains credibility, to put it mildly.

So even if you were to put a black hole in the center of an all you can eat buffet, there is a limit to how quickly it can eat. It has to get up from the table every time and refill its plate and then bring it back to the table and then start eating again and then go back up to the buffet. That limits how quickly it can eat even if it's really, really hungry. So it looks like we need something else. We need some other mechanism for creating black holes in the early universe.

To be fair, the young universe was quite different than the modern day cosmos. It was much more smooth. It was much more uniform, had a higher average temperature. The usual galactic nonsense like cosmic rays and giant magnetic fields weren't in play yet because there were no galaxies. There was very little high energy radiation floating around like background ultraviolet light from starlight, from exploding stars.

And most importantly, in the early universe, there were no metals. Metals is this curious word in astronomy. It refers to literally anything but hydrogen and helium. That's right. In astronomy, there are a grand total of three elements in the periodic table.

It's not so periodic anymore. For an astronomer, there is hydrogen, there is helium, and then there is metals. I'm not in charge of naming things. We gotta go with what we got. But metals, heavier elements, anything heavier than helium, come from stars.

It comes from fusion inside of stars. It comes from fusion inside of supernova. It comes from fusion inside of merging neutron stars, all these different pathways for producing and manufacturing heavier elements that we take for granted in the modern day universe simply didn't happen in the early universe because it hasn't had time. You need multiple generations of stars to pollute the atmospheres, to pollute new systems, to pollute new star clusters, and protostellar nebulae, all that. You need multiple generations to build up those heavy elements, and that was simply lacking in the early universe.

What do we have in the universe? Giant clumps of hydrogen and helium. Pristine, clean, unpolluted, unspoiled, just hydrogen and helium, which means we have a little bit of freedom. We know of only one way to form black holes, in the modern universe with modern astrophysics. The early universe was different.

It wasn't all grown up. It was different enough that maybe it could play some games that it's simply too old to play now. Games like seeing who can ride their bike downhill without wearing a helmet and standing on the handlebars. At some point, you just get too old for games like that. Or maybe it had ways of manufacturing black holes that aren't available to it now.

Maybe the universe could make black holes in a different way when it was very young. In this epoch of the first generation of stars coming online, maybe it could make not just regular black holes, but big black holes, and it simply can't. It's too old now, the universe is too cold, and it's too polluted with heavy elements to be able to make big black holes in the same way. Some form of this idea has been around for over a decade now, pretty much as long as we've realized that the early universe is really, really good at producing large black holes. And these ideas have ranged from the exotic, like, maybe dark matter itself interacts and collapses into a black hole, to the really exotic, like, maybe black holes formed just after inflation when things were going kind of nuts and the primordial black holes grow up to be supermassive black holes.

While exotic ideas are fun and all, and I never get tired of talking about them, they always run into several issues. One, exotic ideas are hard to physically motivate because they require so many new ingredients and processes in the universe. Some of these theories really blur the line between theoretical physics and fantasy bedtime stories. And all these ideas, exotic ideas run into other issues like, you know, the evidence for them is really scarce. Or if you concoct some scenario to make giant black holes, you end up breaking some other observation.

Like, this is the whole journey with primordial black holes. I know I've talked about this before. Like, it'd be really awesome if the super infant post inflationary universe made black holes right away. And, yeah, you can tweak and twist things to get them to be super massive by the time you need them to be super massive, like, to explain something like Oozi one. But then you end up breaking, like, everything we know about cosmology, like, basic observations of the cosmic microwave background.

So as even though primordial black holes will always have a soft spot in my heart, they're they're probably not going to work out. And and and if that wasn't enough if it wasn't enough, like, yeah, we all love exotic processes, but they're hard to motivate physically because you need lots of stuff. You need to say, oh, and then there's this process and then this new particle, and then it also does this. And, like, that gets exhausting, and it's kinda easy to break known observations with exotic scenarios. You also need to explain why that fun, interesting, exotic process doesn't happen anymore.

Because we're not directly forming large black holes anymore. We're not creating large black holes in in the snap of our fingers in the modern universe. So if you invoke some exotic process in the early universe, you need to explain why it shut off and that have fun with that. What we like are narrow solutions. Narrow solutions that get in there, solve one problem, and get out.

Where we don't have to invent entire new classes of dark matter particles. We don't need to, like, break some fundamental symmetry of of particle physics. No. When it comes to large black holes in the early universe, it means we try to leverage the unique conditions of the early universe. The early universe was a little bit different than the modern day universe, and maybe there is something special about the conditions that were naturally happening in the young universe that led to the creation of large black holes.

That way, whatever process we concoct simply shuts off as the universe gets older and wiser. And the more narrow we can make our explanation, if we can just get in there and fix this one tiny little problem, it ensures that we don't break any of the other known observations and measurements when it comes to cosmology. So with all that in mind, in the past few years, an idea has emerged that seems like it might be promising. I don't even want to say that it works because we're we're not quite there yet. It's just it's just promising.

It seems to check all the boxes. It's not hugely exotic, takes advantage of the unique conditions of the early universe, it doesn't evoke super duper crazy new physics, and and this is the most important bit, so I should have led with it it's actually capable of producing black holes in the right mass ranges in time. And this idea has the totally boring unoriginal name of direct collapse black holes, which is exactly what it sounds like. It's black holes that form from direct collapse, not through the route of stellar depths. Here's the deal.

You've got lots of clouds of hydrogen and helium floating around in the early universe, and they're big, gigantic. I mean, 10,000 solar masses, maybe more. And all you need to do is compress the entire gas cloud, like, all at once at the same time, which is totally possible, but ironically, only if the cloud doesn't cool off. Because if the gas cloud cools off too quickly, then it will fragment into lots of smaller clouds and just produce a bunch of normal stars, which you don't want because you're trying to make one big black hole. So you want to prevent fragmentation.

You want to prevent the gas cloud from cooling off too quickly. Thankfully, giant clouds of hydrogen and helium aren't so great at cooling themselves off. That's because there are no metals. The heavy elements in the modern day universe actually do a fantastic job at cooling off the cosmos. That's because they can radiate light.

They can emit radiation at a variety of wavelengths, which is able to dump heat out. If you've got a whole big gas cloud and it's sprinkled with heavy elements, they will radiate energy at all sorts of different wavelengths, and it's like a massive heat sink. It's like a cooling fan for a giant gas cloud that can just cool stuff off, and it's super efficient in the modern day universe. It is how we are able to produce lots and lots of stars all the time because of the high metal abundance in every galaxy. But in the early universe, the pristine universe, the unpolluted universe, there are no metals.

And so these clouds of hydrogen and helium, they're hot, and they just stay hot. And they cool off just a tiny bit, and then they shrink. And then they cool off just a tiny bit, and then they shrink just a little. And so if you can manage that collapse, you can get the whole gas cloud to collapse at once instead of cooling off rapidly and fragmenting. There is one way they can cool off, which is a major piece of this puzzle, and that's if the hydrogen bonds together to make molecules, just diatomic hydrogen, you know, hydrogen, hydrogen.

That's it. Simplest possible molecule you can ever imagine. That molecule can emit lots of different kinds of radiation and cool off the gas cloud. We don't like that. We don't wanna cool off too much.

We don't like fragmentation. We wanna keep things nice and hot because as soon as we fragment, we get lots of little stars instead of one big collapsing cloud that might turn into a big black hole. But as long as you have enough, say, ultraviolet radiation floating around to stop the hydrogen from molecularizing, I may or may not have made up that word, but I like it, Then the gas cloud can't cool off rapidly. It stays hot, and it just compresses without fragmenting. And then eventually, you reach a tipping point with this giant gas cloud weighing 10,000, a hundred thousand solar masses collapses, and once again, you reach this, like, runaway point just like you reach it in the heart of a giant star that's dying.

You reach this tipping point. Gravity overwhelms everything. And briefly, very, very briefly, you form a giant star, a real whopper of something like a thousand solar masses. But before it can even get going, boom, instantly, a black hole forms in its heart. Now that black hole ain't all that big, just 10 to a hundred solar masses, but it's surrounded by all this stuff.

And this stuff is still gravitationally collapsing, so the whole Eddington limit breaks down. You don't need to worry about that because it can just hover down as much as it wants. Just just, you know, settling up right to the the buffet, skipping the whole plate business. Just give me one of those big spoons and let me go at it. It swallows it as quickly as it can, and before you know it, you've got a 10,000 solar mass black hole.

Boom. Like that. You snap your fingers, and you convert this giant gas cloud all at once into a 10,000 solar mass black hole. This black hole, at 10,000 solar masses, if it's formed early enough, say, right around the same time that the first stars are forming, then it can merge, then it can continue accreting material within the Eddington limit safe and sound and have plenty of time to make supermassive status within a few hundred million years. There's a direct route from a direct collapse of a gas cloud when the universe is around a hundred million years old to create a 10,000 solar mass black hole that can then grow up to be, say, a 40,000,000 solar Uzi one.

Like I said, it's a pretty reasonable idea. It takes advantage of the conditions of the early universe. It doesn't radically transform the astrophysical or cosmological landscape, and it contributes to Patreon. That's patreon.com/pmsutter, where we're not making black holes. We're just making fun science shows, but, you know, it that's cool too.

Patreon.com/pmstar. I truly do appreciate all of your contributions. Now the big idea of direct collapse black holes, it can do is that it can do the job of producing big enough black holes. It checks. It that that's what it set out to accomplish.

One teensy tiny itty bitty witty super small problem to the story, and that's the UV radiation needed to break apart molecular hydrogen. Because once you get molecular hydrogen, this whole thing falls apart. The molecular hydrogen can cool off the gas cloud, serves as a heat sink, then the gas cloud doesn't collapse. It just fragments, and you get a whole bunch of stars, and this whole story is ruined. So you need some source of energy.

You need something like ultraviolet radiation, which is really, really good at breaking apart molecular hydrogen and turning it back into just normal atomic hydrogen. But this is the time before, you know, stars the time before galaxies these direct collapse black holes formed in the dark ages the deep depths of cosmic history to make lots of ultraviolet radiation you need a bunch of stars but if you have a bunch of stars then you probably have a lot of leftover stellar material, a k a the metals. And if you have metals, you can't directly collapse a gas cloud because they're really efficient at cooling off the gas clouds, and instead you just fragment them, which means you don't get direct collapse. So how do you get enough UV radiation in the early universe without having a bunch of stars? Well, this is where the latest research is.

And like all things when it comes to the latest research, there are some options. For example, maybe there are stars. Maybe the first direct collapse black holes are happening alongside the first generation of stars. Maybe what happens is you get environments where, say, a batch of first stars form. You know, they pop out.

They pop out. But then right next to them is a big giant cloud that hasn't collapsed yet. And those stars heat up the gas cloud, emit lots of UV radiation, and keep the next door neighbor adjacent gas cloud from fragmenting, and then that next door neighbor collapses into a supergiant black hole. Maybe. Now that kinda works.

That also seems like a really odd coincidence, but also there's a lot we don't know about the early universe. Maybe magnetic fields, which I wish had more of a role in this story. May but maybe then they they can do the work of keeping the gas smooth. Maybe they can, prevent fragmentation. Maybe you do form molecular hydrogen, and it wants to fragment, but maybe there are some crazy strong magnetic fields that keep the gas from from tangling up and fragmenting.

Maybe we don't know a lot about the early universe. Maybe something crazy like axion dark matter, which I've talked about recently. It can it has the ability to turn into photons. Maybe it like, you get these pockets of high density axion dark matter that then convert themselves into radiation that then get energized through turbulence, and then those keep the gas clouds warm. It's a little exotic, but, hey.

You know, this is where we are. Maybe it's just the right combination of physics. Maybe it's nothing special. Maybe there are flows of gas that keep the clouds cool or or shock waves that trigger rapid collapse before they have a chance to fragment and cool off. Maybe it's just, you know, the detailed astrophysics.

If we were to go in there with with a scalpel and a microscope and really open things up and see how things play out on a on a decade by decade time scale in the early universe, we'd say, oh, there's nothing exotic happening. This is just what the early universe did. Maybe Well, well, there are a lot of options. Like I said, we don't know if this idea can work. It's promising.

It's promising because it's so narrow. We've replaced a giant problem, which is how to make supermassive black holes like Oozi one in time, with a smaller one, which is how to get the right UV radiation into the mix in the early universe. And that counts as progress, I think. Thanks to James w and Scott m for the questions that led to today's episode. Please keep those questions coming.

That's askaspaceman@Gmail.com, or you can visit the website, ask a space man Com, and plug in your question there, and it goes right to me. Thank you so much for all the questions. Thank you so much for your continued reviews and positive support on your favorite podcasting platform. It really does help the show. And, of course, thank you to all the fantastic Patreon contributors this month.

We've got a lot of them. It's it's such a joy to see so many people supporting the show, wanting to keep it going. I will keep going. Maybe someday we'll we'll find the answer to direct collapse black holes because we'll have lived long enough and done enough episodes that, you know, the successor of the James Webb would have cracked it. I hope so.

That'd be fun. That's patreon.com/pmsudder to contribute, and I'd like to thank my top contributors this month. They're Justin g, Chris l, Alberto m, Duncan m, Corey d, Michael p, Nyla, Sam r, John s, Joshua Scott m, Rob h, Scott m, Louis m, John w, Alexis, Gilbert m, Rob w, Jessica m, Jules r, Jim l, David s, Scott r, Heather, Mike s, Pete h, Steve s, Wat Wat Word, Lisa r, Koozie, Kevin b, Michael b, Eileen g, Tahoe Warrior, Stephen w, and Brian o. That's patreon.com/pmsutter, and I will see you next time for more complete knowledge of time and space.

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