What kind of crazy physics made primordial black holes? What does that have to do with the LIGO observations of merging black holes? How can we possibly detect them? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
In 2015, LIGO, the laser interferometer let me see if I get this right. Right. Gravitational wave observatory. I guess the w is silent. Otherwise, it'd be LIGO, and, anyway, that's a different episode.
LIGO detected its very first gravitational wave, and it was the merger of two black holes. And this detection was a huge milestone in physics. I've talked about gravitational waves before, but I really will dedicate a whole episode someday to gravitational wave detection and LIGO and all this. The short version is that it took twenty five years of development to get LIGO to actually see something. I mean, this signal gravitational wave signal is super tiny.
Even Einstein didn't think it'd ever be technically feasible to see this. It was it was a big deal, which is kind of why they wanna know about price. And amongst all the hoopla of that first detection, which was a lot of well deserved hoopla for sure, but tucked in there was a very strange surprise, and it was almost lost in that noise and hoopla. The surprise was the masses of the black holes involved in the merger event. The masses were weird.
And you know me and weird things in the universe. To me, weird means interesting. The two black holes that collided to generate this gravitational wave signature that LIGO detected had a mass one of them had a mass of 30 solar masses. Solar mass means, in case you've never encountered this term before, the mass of the sun. And, yes, I know that's very solar centric and biased, and we're imprinting our own units on the cosmos, but whatever.
Solar masses. One of them had 30 solar masses, which means it weighed 30 times the mass of the sun, and the other one had a mass of 35 solar masses. And these numbers, thirty and thirty five, when it comes to the size of black holes, are very awkward. To explain why they're awkward, we have to look at how black holes form. Black holes form from the depths of massive stars.
You have a big giant star, at least eight times the mass of the sun, burning lots of elements deep in its core. Eventually, it fuses iron, and then it can't fuse iron and get energy out of that reaction anymore, so there's a catastrophic collapse. That catastrophic collapse squishes down the core, and if it squishes down the core enough, out pops a black hole. Also, the star explodes in an awesome supernova explosion. Okay.
That is how the universe manufactures black holes. It does so through star formation. First, you need stars, then you get the black holes. But to get a black hole the size of 30 solar masses, you either need a big giant star to just pop out something with 30 solar masses. Like, imagine how big of a star that has to be in order to make a core, just the core, weigh 30 times the mass of the sun.
We have to talk about stars upwards of one or 200 solar masses. A real beast of a star, and those stars just don't really exist, at least nowadays. We don't see stars like that. So you can't get a black hole that big from just a single star. But you could get a black hole from mergers, or you could get a black hole that big from mergers.
Right? I mean, these two black holes merged, and they ended up with something like 60 solar masses. Some of the mass was converted into energy, and so it just left, but you got a big black hole out of it. So maybe you can smash together a few black holes, normal ones, like five or 10 solar masses. And after enough collisions, you can you can build these up.
But black hole mergers are relatively rare. But black hole mergers, if they're rare, then you can never you don't have enough time in the age of the universe to build up 30 to 35 solar masses out of five to 10 solar mass parts. If you only if you only get a collision once every few billion years, you just don't have enough time to do this. And if they're common, if black hole mergers are super duper common, then very quickly, you're gonna go way past 30 to 35 solar masses. Just blow by, and then you'll be in the hundreds and the thousands and the millions and the billions of solar masses.
It's just really, really awkward. Like I said, it's not impossible to get a 30 solar mass black hole out there in the universe. Obviously, not because they exist. And it's not impossible to explain how they get there, but it's just it's really awkward, folks. It's a really awkward size for black holes.
We prefer them smaller, and we prefer them bigger, but we don't prefer them right in the middle. So it seems tough to make these black holes the normal way, which is through the depths of massive stars and then mergers and collisions, but that doesn't rule out the abnormal way. Maybe these black holes weren't created inside the hearts of stars burning in our universe. Maybe they're a relic of an exotic age. Maybe these black holes that Lego saw were from the very first moments of the big bang.
And we're talking about what we call primordial black holes, which our P B H for short. These are black holes that formed in the early universe. And I should caution you every time I read PBH, I see PHB, not primordial black hole. I see pointy haired boss and my mind refuses to go any other way. So if I ever say PHB instead of PBH, you'll all know what I'm talking about.
As a side note, your boss might not be a relic from the Big Bang, and we can talk about it privately if you like. Anyway, primordial black hole, PBH, a black hole that was formed in the early universe. How the heck does a black hole form in the early universe? Well, the short version is the early universe is crazy, and anything is possible. Alright?
It's new physics. It's exotic physics. It's high energy physics. It's high density physics. You you're I mean, you're cooking the fundamental elements of the cosmos.
Maybe it's not so crazy that you just pop out a few black holes without even realizing it. How would the universe do it? Well, to make a black hole in general, you need to cram a lot of material into not a lot of space. So much so, you need to get those densities so high that the escape velocity, the velocity you need to leave the surface of your thing, is greater than the speed of light. Once you've done that, you have achieved the creation of a black hole.
Stars do this through the crushing gravity of their own weight, just squeezing and squeezing and squeezing and squeezing. And in the early universe, it can just, you know, happen. Seriously, it's crazy enough times back then that just randomly, you could have a bunch of gas just hanging out, minding its own business, and then, whoop, it gets squeezed together like this, like and you create a black hole. In order to get the crazy amount of physics that's required to manufacture black holes, we have to talk about the really early universe. I mean, early universe because the later universe is just too cold, too spread out, and too boring for these kinds of crazy random high density patches to simply happen.
How early we're talking about, like like less than a minute old. If you wanna make black holes in the early universe, by the time the universe is a minute old, possibly even a second old, it is too old. I told you. It's early universe stuff here, folks. Now here's the thing that we're going to explore when it comes to PBHs.
They've been a topic of interest and curiosity for decades. I remember Stephen Hawking and Hawking radiation, this whole idea that black holes aren't entirely black, that they do give off a little bit of radiation through this very exotic, very hard to understand, somewhat mystical process involving quantum mechanics. Stephen Hawking got interested in the topic of black holes evaporating because he was really working on primordial black holes, and if they might survive to the present day. That's what got him motivated. That was back in the seventies.
So at least fifty years ago, people were thinking about primordial black holes. So as you can imagine, through all these decades of theorists working on the problem, there are approximately 4,300,000 different theories of how primordial black holes can form. I may be exaggerating, but not by much. And depending on the model and the age of the universe, when the black holes pop up in the model, what the conditions are, the physics behind it, and on and on and on, all the details of all these different theories of primordial black holes, you can come up with just about any population of black holes you want. You want a whole lot of little tiny black holes no bigger than your thumb?
We've got it. We've got a theory for that. You want just a few super dry ones? We can make that happen too. You want a little bit of both a little metal column and a little bit of column b?
Sure thing, boss. We got dozen theories that can cook up black holes that look like that. My point is that models of primordial black holes are flexible. So that's why I'm not going to go into detail about how they form because it basically doesn't matter. There are a variety of scenarios for making BBHs.
Some of them are more well grounded than others. Seriously, I'm not I mean, just to give you a little hint of this, there are theories involving inflation. Like, at the end of inflation, there were some instabilities and then a bunch of black holes pop out. Maybe there's something to do with string theory. Maybe there's something to do with super strings.
Maybe there's something to do with the universes colliding with ours. There there's a lot of different ways that the universe could potentially create black holes way back in the day. And all these different theories produce different populations of black holes flooding the universe, maybe just a few big ones, maybe a bunch of smaller ones, maybe different sizes, etcetera, etcetera, etcetera. What we care about, since it's so rich and so flexible, we want to know if any PBHs were made and more importantly, how they can be detected today. Right?
We've got all these theories that might or not might not be right about the creation of primordial black holes, but can we see any? Before I get to that, I wanna explain why people care at all about PBH's primordial black holes. What what are they good for? Why have there been decades of interest in this topic of the universe making black holes in the early days? Well, it's been used over the decades to explain all sorts of unexplainable astronomy things.
Like, you make some observation you don't really understand. There's a theorist somewhere in the world that is using primordial black holes to explain it. I swear. Right? Like, oh, oh, did you know that there's this flood of gamma rays in the universe that just generally dissolve into a background fuzz and aren't associated with any particular source?
Maybe it's primordial black holes. Do you know that cosmic rays have a bunch of antimatter in them, and we don't really fully understand why some cosmic rays are made of antimatter? Maybe it's primordial black holes. Did you know that there's some extra radiation in X rays and gamma rays coming from the center of the Milky Way? Maybe there's a bunch of black holes in there.
We really don't understand gamma ray bursts. Maybe it's black holes. We kinda sorta don't understand how the giant supermassive black holes form. Maybe they were seeded by primordial black holes. Some stars appear a little bit hotter than others.
Some stars are moving around a lot more than we would expect. Maybe it's black holes. The large scale structure of the universe, there are some details of it that we don't fully understand. Maybe it's black holes. There's the whole dark matter thing, you know, this major component of the universe that is not emitting or absorbing light.
Maybe it's black holes. Hey, LIGO saw two black holes merge, but we have a hard time understanding their masses. Maybe they're primordial black holes. It's easy, man. You got a mystery in astronomy?
Just blame primordial black holes. That sounds like a lot. Right? If they're so super awesome and wonderful and capable of explaining every single mystery in all of astronomy, then we should be able to detect them. Right?
Right? I mean, if they're causing an effect on our universe, we should be able to find them. Well, maybe. There are two ways to go hunting for primordial black holes, because it's not like we can go back to the early days of the universe again. One way to find them is through Hawking radiation, the radiation that they emit.
Primordial black holes can be any size they want depending on your model of the day. They can be as small as a few kilograms to millions of times the mass of the sun. But if Hawking radiation is correct, then all black holes evaporate. But the bigger the black hole, the more slowly it will evaporate. A black hole, the mass of the sun, emits, like, one photon a year, which is not a lot, and it will take around ten to the sixty years to fully evaporate.
Keep in mind, our universe is barely ten to the ten years old. Ten to the sixty years to evaporate a black hole the size of the sun. So we're not gonna see that. What else do we have? Well, if you're smaller in black in Hawking's theory, if you're a smaller black hole, you actually emit more radiation, which reduces your mass.
It shrinks you a little bit, which now because you're smaller, you evaporate even more, which makes you smaller, more, smaller, more, smaller, more, smaller. You you start to get this runaway effect, and you start to emit a lot of high energy radiation for the the smaller the black hole. So if the early universe manufactured black holes the size of the sun, we can't see them through Hawking radiation. But if they made black holes, say the mass of the earth or a few hundred kilograms, they would be evaporating maybe in the early universe if they're small enough, and maybe they'd be evaporating right now, like, right in front of us. This was Hawking's motivation.
He said, ah, if black holes are manufactured in the early universe, I wonder what their effect might be. Skipping through a bunch of math, he figured out Hawking radiation said, oh, they might be glowing right now. Like, right now when our universe is 13,800,000,000 years old. But there, it could have affected so many other things in the universe. Like this whole big bang nucleosynthesis when our universe was just, like ten to twenty minutes old, and it was manufacturing all the hydrogen and helium in the universe and a few other elements like lithium.
If you have a bunch of tiny black holes in that soup, and they're starting to emit high energy radiation, that's going to affect your nuclear reactions. Right? Imagine dropping a bunch of tiny black holes into a nuclear reactor. It it's gonna go interesting. Or maybe for some bigger black holes, they just hung out minding their own business during Big Bang Nucleosynthesis, and they didn't really start to mess things up until a few hundred thousand years later when we get to the epoch of the cosmic microwave background.
And there, again, if you have a bunch of black holes hanging out, their gravity is important. This is gonna shift around how the matter behaves. They might start popping off from Hawking radiation. This is gonna heat up some patches. It's gonna change the cosmic microwave background.
Black holes evaporating today, once they reach that critical state where it's runaway collapse and runaway evaporation, they're gonna give off some cosmic rays, some gamma rays, some real high energy stuff, so we might just see them, like, pop pop pop pop, like little twinkling stars, except they're dying black holes. It might have affected when the first stars appeared. It might be, like I mentioned, the gamma ray burst, the sources of high intensity gamma ray radiation that largely we don't understand. Maybe it's black holes evaporating. There's also I saw this really interesting paper.
You know, have you ever heard of the Higgs instability? I'll do another show on that for sure, but this is basically saying our universe may not be entirely stable. Our universe has gone through a few phase transitions in its life. It's been stable for around thirteen point eight billion years, but it may not be done. A black hole evaporating might or might not trigger the transformation of the universe and all and the eradication of all physics as we know it.
So sleep tight, folks. But here's the thing. We look at big bang nucleosynthesis. We look at the cosmic microwave background. We study cosmic rays.
We study the gamma ray background. We study gamma ray bursts. We study the formation of the first stars. We study the Higgs instability. So far, we have no evidence whatsoever of any primordial black holes evaporating in our universe.
Period. We see no evidence of evaporating black holes. But that doesn't mean they're dead. Oh, no. No.
No. No. No. No. Because there's another way to detect them.
Right? Maybe Hawking's wrong. Maybe black holes don't evaporate. It's theoretical and it's on the boundary of general relativity and quantum mechanics. So, you know, kind of tough.
Maybe they just don't evaporate. Maybe he was wrong. And so maybe we're not seeing the evaporation of the primordial black holes because they don't evaporate. Or maybe in the early universe, black holes do evaporate, but the early universe just didn't make the smaller ones that have are evaporating in the past or evaporating today. Maybe it only made big ones.
Maybe primordial black holes are only the big ones. And so we're we're just never gonna see them evaporate because we're not gonna hang on to this for ten to the sixty years. Trust me. So are there other ways to see bright primordial black holes without looking at their evaporation, their radiation? Well, you need to find small, populous, dark things like finding a pest.
You know, if you if you have cockroaches or a mouse or something, you're not it's hard to see the mouse or the cockroach, but you can see the damage they do. You can see the little holes in your food. You can you can see the the poop in the corner. It's all disgusting. You can't see the past, but you can see the damage they do.
Maybe we can't see the black holes themselves, but if there's a bunch of black holes just floating around the universe, you bet you we're gonna spot them. Right? Because they're gonna cause some havoc. Maybe maybe we're we'll see them through microlensing. Like, if we stare at a star for a really long time, and then a random tiny black hole just temporarily blocks our point of view to that star, we'll see a little blip because of the gravitational bending of light around that black hole.
This is something called microlensing. We might be able to see a black hole that way because it's messing up, the light coming from a distant star. Maybe I swear these are real papers, real scientists actually writing them. Maybe black holes every once in a while collide with the earth. You know what?
But if they're not big if they're not big, you know, if they're just, say, a few hundred kilograms, they wouldn't have evaporated yet. So we can't see them with Hawking radiation, but they would just slip through our atmosphere. It wouldn't be it wouldn't be like a meteor or something. It just slips right through the atmosphere and then hits the ground and then does something. But I don't know what it would do later, but in the meantime, it would cause an earthquake.
So maybe some earthquakes on Earth are caused by collisions with tiny black holes. Maybe some of the neutron star or white dwarf explosions that we see are triggered by black hole. Like, you're hanging out. You're a white dwarf. You're all stable.
You're gonna just gonna chill out for eleven billion years. No big deal. And then boom, you get hit by a black hole, and that causes a runaway catastrophe instability, and you end up blowing up. Maybe if you're two binary stars and you're circling around each other in this beautiful, graceful, gravitational dance. You've been doing it for billions of years.
You got this and then swings by another black hole, and it's gonna disrupt you. And we get to watch you be disrupted, but we can't tell what's disrupting you. Maybe it's a black hole. Maybe a small dwarf galaxy that's only weakly bound together with gravity, maybe gets injected with a bunch of black holes like a black hole swarm into it. And they're all tiny, and so we can't see the black holes themselves.
But through their gravity, they end up disrupting and and ripping apart the dwarf galaxy. Maybe they contribute to Patreon by going to patreon.com/pmsutter. We can't see them directly, but we can see the evidence of their existence, and they do that by making this show possible just like you. Maybe these black holes mess up the cosmic web. You know, this delicate structure of filaments of galaxies, and it's all gorgeous.
And then, you know, there's a bunch of black holes that fuzz it out here and there. Maybe and again, this is a real paper with a real hypothesis. You know the whole planet nine thing that I talked about recently? Maybe it's not a planet at all. Maybe it's a black hole the size of a tennis ball.
I'm not joking. These ideas are wonderful, aren't they? These are creative. These are smart. These are trying to find ways to find something that is small and invisible.
Maybe black holes are colliding with the earth. Maybe black holes are messing up with the orbits of objects in the outer solar system. Maybe black holes are making some white dwarfs explode. Maybe not. Because as far as we can tell, black holes aren't doing any of this.
We can't find any evidence of primordial black holes. We've looked at microlensing. Nothing matches up. We get microlensing for sure, but nothing matches up with black holes, small black holes. Neutron star and white dwarf explosions, the signatures that you would expect from the collision with a black hole, we don't see it.
We don't see binaries getting disrupted. We don't see dwarf galaxies getting ripped apart. We don't see people contributing to Patreon. I'm just kidding about that. All of you are wonderful contributors.
The cosmic web is about exactly what we expect. Planet nine, well, let's not get into Planet nine. These are all brilliant creative ideas cooked up over the decades, and we don't have any evidence for primordial black holes. But they're not dead yet. The end result of all this wondering and thinking and planning and plotting over PBHs is that the vast majority of them have been ruled out.
You can't have small ones flooding the universe because we don't see them evaporating, assuming Hawking radiation is correct. We can't have the big ones because we don't see them crashing into anything else. Most of the mass range of possible black hole models generated in the early days of the big bang simply don't match up with observations, which is the power of science. You come up with a hypothesis. You come up with ways to test it.
You go out in the universe, and you look for it. And if you don't see it, you have nothing to support your hypothesis. But there is one particular window of mass ranges that aren't ruled out quite yet. The masses that are big enough that they haven't evaporated yet, but small enough that they don't mess up anything else in the universe. And that range is around 10 to a hundred times the mass of the sun.
So you could see why people were so interested in that first LIGO result. Here it is, a black hole that's hard to make through astrophysical means. It's hard to make through the death as a massive star. But primordial black holes, it might might be a relic of a bygone era. Where LIGO's black holes back in 2015, Were they formed in the big bang?
Maybe maybe not. It's honestly hard to tell. LIGO only saw one event. I mean, since then, it seemed like a dozen more. But at that time, we only saw one event.
But that itself was able to tell us about the abundance of these kinds of black holes. Just seeing one event, a merger between a 30 and a 35 solar mass black hole, tells us that black holes of that size are common. Why? Oh, imagine you've never seen a car before. Imagine in your life, you've heard about cars.
You've heard descriptions of cars, but you've never ever, ever, ever seen a car before in your life. And you build over years and decades a car detecting device. And finally, you go out onto the street and you open up your car detecting device and voila, you see a car and it's a Honda Civic. That was your first car. That was the first car you've ever seen in your whole entire life.
The fact that you just randomly went out looking and the first car you happen to see was a Honda Civic tells you that Honda Civics are relatively common kind of car. Because if they were rare, it's very unlikely that you would see them on your first try. And since 2015, we've seen a lot more black holes in a very similar size range, so we know they're common. This tells us something important. The universe really likes black holes in this size range.
What does this mean for primordial black holes? Well, it says, okay. Okay. Okay. If you want if the black hole if or sorry.
If the universe really likes black holes of this size range, which apparently it does because we're seeing them, and if you want these black holes to be made in the early days of the Big Bang, you can work backwards and figure out what other effects they might have. Like, okay. Okay. If the universe in the first second of its existence is going to manufacture black holes around 30 to 35 solar masses at a common enough level that we are able to see them in our very first gravitational wave detection, what other things are they gonna do? What are some other effects they're gonna have on the universe?
Well, some viewers say everything clicks, and it's all kosher. This fits with our picture of the universe, and others say it's nonsense. Really, there's a divide. Some some people say that, yeah, the universe can make black holes this small in abundance in the earliest days of the Big Bang. And others say, no.
No. No. No. No. It doesn't fit.
It's hard. It's a hard problem. And today, in 2020, in honesty, it seems unlikely that primordial black holes are a thing. Because even the models and the theorists that say, yes. It works.
They have to do a lot of tweaks. They have to invoke a lot of extra physics to make it work. And once you start invoking a lot of extra physics, you're like, just I it's hard to get excited about it. It is possible that the black holes that LIGO saw could be explained by massive stars, somewhere around one or 200 solar masses that formed in the young universe. Not primordial young, just regular young, like when the universe was a billion or two years old.
It still might require some mergers. We're not exactly sure how often these black holes can merge to get up to that size of thirty thirty five. It's a little bit messy on the astrophysical post big bang side, but it's not quite as messy as it is on the primordial black hole side. But they're not dead yet. After fifty, sixty years of trying to get the Big Bang primordially to manufacture black holes, they're on really thin ice right now in 2020.
It's just hard to get a universe filled with black holes of any size that agrees with what we see. But to me, this is a great example of how science is actually done in reality. It was not just one idea, then one observation or test, and then boom, we move on. No. This is an idea that's hung around for decades, that we've come up with all sorts of clever ways to test, that we've come up with all sorts of clever ways to manufacture theoretically.
It hasn't worked in almost all those cases. There is still a tiny little possibility that primordial black holes could be a thing. It's unlikely. We'll probably find out for sure in the next ten years, but we've been saying that for, like, fifty years. As for LIGO, it's still trucking.
I will do another episode on it, and we'll talk more about these black holes that are merging in galaxies far, far away. And in the meantime, you can rest assured that it seems likely that almost all the black holes in our universe were made through the deaths of massive stars. I don't know how comforting that thought is, but that's reality. Thanks to Robert k on email, Peter n on email, and Raul p on email for the questions that led today's episode. Thank you to my top Patreon contributors.
What a downer of an ending, but, hey, I can't do it any other way. My top Patreon contributors this month, Matthew k, Justin z, Justin g, Kevin o, Duncan m, Corey d, Barbara k, Neutrude, Chris c, Robert m, Nate h, Andrew f, Chris l, Cameron L, Nalia, Aaron s, Kirk t. Thank you to all the amazing space cadets that signed on for the promotion of the book. You are getting a free autograph copy of my book. If you didn't snag an autograph copy, you can get a regular unadulterated version at a bookstore near you or go or go to pmsudder.com/book.
Or if you just wanna contribute for the sake of contributing, go to patreon.com/pmsudder. Keep those questions coming. Hashtag ask a spaceman, ask a spaceman@gmail.com. Find me on social media. I'm at paul matt sutter, and I will see you next time for more complete knowledge of time and space.