Particles of Thought

Hakeem Oluseyi:

Dark matter is found all over our universe, but we don't know what it is. What if dark matter is just black holes? Tiny ones, ancient ones. Stephen Hawking, among other scientists, described this idea decades ago, and now it has really gained steam. My guest, David Kaiser, MIT physicist and science historian, walks us through it.

Hakeem Oluseyi:

All right, so you think that dark matter might be black holes.

David Kaiser:

I do. And I think they're pretty amazing. And they're not the black holes you might have thought about.

Hakeem Oluseyi:

Since we said the phrase black hole—

David Kaiser:

Yeah, yeah.

Hakeem Oluseyi:

Define what a black hole is.

David Kaiser:

Good. Okay, well—

Hakeem Oluseyi:

Before we go further.

David Kaiser:

Yeah, yeah, so a black hole is, the way we think about these stellar black holes, the ones we know are littering our universe all over the place, they really are the remnant of a once thriving star. Okay, they're a star, they're a dense concentration of matter that exhausts its nuclear fuel. And what that means is the nuclear reactions make an outward going pressure that balances the inward crunch of the gravity. It's really massive, it wants to collapse.

Hakeem Oluseyi:

Absolutely.

David Kaiser:

And while that furnace is burning, it has all this outward going pressure keeping it in a kind of equilibrium. Right. And when the fire goes out, when you don't have that balance, Gravity's going to win. Yeah. And it's going to be a runaway collapse. And so what's left over is ultimately, as far as we know, a rupture in spacetime. That's pretty dramatic.

Hakeem Oluseyi:

Right.

David Kaiser:

Which also, as far as we know, is hidden from us by something we call the horizon, the event horizon. The event horizon. Okay. And so if it's, if the black hole forms from the sequence that we've, that I was just talking about, if it's the end state of a star, then the mass of that object has to be at least as big as the mass of our sun. And it could be bigger.

David Kaiser:

So there's a floor below which the mass could never be for these black holes. It formed from a star. If it was below that mass, it wouldn't even lit up, et cetera. It won't form a black hole.

Hakeem Oluseyi:

So do you have an idea of the lowest mass, like solidly confirmed black hole we've observed?

David Kaiser:

Yeah, a few times the mass of our sun.

Hakeem Oluseyi:

So we're talking like two?

David Kaiser:

Yeah, on the order of two, two or three, something like that. That's right, the experts will know better, but it's comparable to, slightly bigger than the mass of our sun.

Hakeem Oluseyi:

Got it.

David Kaiser:

And what's also cool, because it has that runaway contraction, take the mass of our sun, you know, and turn it— if that were to collapse tomorrow into a black hole, it won't, right? But if it were to, if you take that, take the equivalent mass, okay?

Hakeem Oluseyi:

Yeah, it will never actually, right?

David Kaiser:

This one won't because it's below the threshold. Exactly. Of all the things to worry about, that's not it. But if we took that mass and made it so small, so dense that it would become a black hole, it would be the size of like a few city blocks. This huge massive star, take all that mass and squeeze it so densely. So black holes, at least theoretically, if we follow Einstein's equations, black holes are the densest state of matter possible by the laws of physics, at least as we understand them. So some of the earliest ideas about primordial black holes by theorists, you know, hypothetical ideas, were by people like Stephen Hawking, a name you might be familiar to many folks.

Hakeem Oluseyi:

I've heard of him.

David Kaiser:

You've heard of him. Did some pretty good stuff. So there was an earlier anticipation that we now know to go back to and look at by some very very prominent theoretical physicists in Moscow in the late '60s, led by Yakov Zeldovich and Igor Novikov.

Hakeem Oluseyi:

Oh yeah, yeah.

David Kaiser:

Tons of amazing stuff that they had done. They have a paper that was published in Russian, translated into English. It was available to many readers who might not have known Russian at the time. And they wonder about, could there be a second route to make black holes? Would they form the universe? They actually conclude it probably wouldn't have happened. But they raised the idea in the late 1960s. Independently at first, Stephen Hawking comes around to this idea in 1971, more than 50 years ago. And he realizes that there actually could be— if you really take general relativity very carefully, very seriously, Einstein's beautiful theory of gravity— if you had a dense enough lump of stuff in the early universe, that could collapse directly into a black hole. You wouldn't need a star that burns out and collapses. It could be a kind of direct process, goes straight to black hole.

Hakeem Oluseyi:

Yeah, right. So like nowadays, in recent years, there have been these observations, what they call unnovas. Is that the idea? So if a star is so massive, it just goes bloop right to black hole, no supernova explosion?

David Kaiser:

That's right. All kinds of, fill in the blank novas. Kilonovas.

Hakeem Oluseyi:

Kilonovas, right, yeah, yeah.

David Kaiser:

Blame it on the Bossa Nova. I mean, I have to write that paper.

Hakeem Oluseyi:

Chevy Nova.

David Kaiser:

Chevy Nova. That's right, but those would still have been at very large masses. What Hawking pointed out in his very first article, he actually says amazingly these might make up what was called the missing mass. He even says these things maybe are a candidate to be what we would now call—

Hakeem Oluseyi:

And what mass scales was he thinking of?

David Kaiser:

What's amazing is he writes out in his very first article, because these things are not coming from stars, these black holes, if they formed at all, would not have to be anywhere near the mass of a typical star. Not bigger than, could be much, much bigger than, could be much, much, much smaller than the mass of, say, our sun. Much smaller than what we call a solar mass. What they're pinned to, and he works this out beautifully, and one of his then graduate students, Bernard Carr, works out some really foundational work in the '70s since then, and continued, very active now. The idea was that the typical mass of these primordial early universe black holes is not set by a star. Stars wouldn't have formed yet.

Hakeem Oluseyi:

Right.

David Kaiser:

It's set by how much stuff was in the kind of sphere of the observable universe before it got so big that we measure it today. The universe, as we know, has been expanding.

Hakeem Oluseyi:

Right.

David Kaiser:

We now know with great precision the rate at which it's been stretching over time.

Hakeem Oluseyi:

Yeah.

David Kaiser:

It was much, much, much smaller at earlier times.

Hakeem Oluseyi:

So we're talking about really much, much—

David Kaiser:

Typically a fraction of a second after the Big Bang.

Hakeem Oluseyi:

Oh.

David Kaiser:

It'd be maybe a second, you know, but it's really, really early.

Hakeem Oluseyi:

Got it.

David Kaiser:

And so at that time, our observable universe was correspondingly tiny, tiny, tiny on human scales. It had 14 billion years to stretch and grow. What was it doing before that history?

Hakeem Oluseyi:

Right.

David Kaiser:

And so what Hawking points out, and Bernard Carr helps clarify 50-plus years ago, is that the typical mass you'd expect for this direct collapse black hole, these primordial black holes, is set by how much stuff was available to it within our observable universe. It won't gobble up everything. It could gobble up on the order of sort of 10% of what was available to it. So a big puzzle is even the black holes are now studied in great detail. We know they're in our universe. All over the place. They're kind of almost, almost mundane, like boring black holes. Can you imagine when you and I were younger? That was like— I mean, this was— to have these things be boring now is pretty amazing. Or mundane. So they, as you say, they come in kind of typical sizes.

David Kaiser:

Two kind of clusters of sizes. And we can make sense— we try to make sense of the really big ones by saying, well, they formed early and had time to eat. Had time to, what we say, accrete, to get more matter, right? They're huge, dense concentration of matter. Gravity says they should attract other nearby matter. And they could kind of get bigger over time. And getting the details right is tricky. Getting them really so big so early, as the James Webb Space Telescope is now finding more and more evidence for. That's a bit of a puzzle.

Hakeem Oluseyi:

Yeah.

David Kaiser:

But in general, we think, well, they probably formed around the size of, you know, of the mass of our sun, say comparable, a couple of times bigger, and maybe they gobbled up their surroundings. And that, you know, works pretty well for a lot of the black holes that we now know about. The ones that I'm super excited about and many, many folks are getting excited about these days could have formed not from having a star that exhausts its fuel, collapses, leads to a black hole, maybe gobbles, becomes a supermassive one. It's an end route around stellar evolution.

David Kaiser:

You don't even need stars. You don't need atoms yet to have formed.

Hakeem Oluseyi:

Wow.

David Kaiser:

When these things were, the ones that I'm mostly focused on would have formed. So we call them primordial, early universe black holes.

Hakeem Oluseyi:

So not only that, so they, the universe at this time, their primordial early universe, the universe was very different than it is today.

David Kaiser:

Very different, and that matters a lot for our understanding of these things. Still hypothetical objects that we're really curious about. Absolutely. So these would have formed, if they formed at all, our best understanding today, these would have formed before there were atoms, before there were even protons that go inside the nuclei of atoms. This is really a different universe. I mean, it's our universe, but under very different conditions. When the universe was so hot and so dense, really just an eye blink after the Big Bang itself.

funder

Hakeem Oluseyi:

Particles of Thought is supported by Carlisle Companies, a manufacturer of innovative building envelope systems. With buildings responsible for over a third of total energy use, Carlisle's energy-efficient solutions are built to reduce strain on the energy grid. For example, Carlisle's UltraTouch Denim Insulation, made from sustainable recycled cotton fibers, delivers energy efficiency while being safe to handle and easy to install. Made with 80% recycled denim, Ultra Touch diverts nearly 20 million pounds of textile waste from landfills each year. Operating across North America, Carlisle is working towards a more sustainable future. Learn more at carlisle.com.

Hakeem Oluseyi:

Man, man, man, you said a lot, but you left something out.

David Kaiser:

How?

Hakeem Oluseyi:

Yeah, good.

David Kaiser:

So we have a bunch of ideas, but the main idea, which really does go back to people like Stephen Hawking and Bernard Carr from more than 50 years ago. We've been able to learn a lot more about the early universe since then. But the idea that Hawking himself put forward in 1971, really early, was that if, if for some reason— we'll fill that in a second— if for some reason, Hawking said, there was a kind of lumpiness in the distribution of matter at very early times, not just a smooth, you know, kind of vanilla porridge, but if there was a little more stuff here than average than there, then gravity will— can, can do its thing. And if it's dense enough early on, it could collapse directly into a black hole.

Hakeem Oluseyi:

So let me ask you another question, right? So when we look at processes like star formation and, you know, as we got into more detail, we begin to learn like things like turbulence inside of giant molecular clouds are relevant and things like magnetic fields are relevant. So at these early times, right? So turbulence creates these little, you know, for lack of a better phrase, whirlpools, right?

David Kaiser:

Swirlies.

Hakeem Oluseyi:

Yeah.

David Kaiser:

Right, right.

Hakeem Oluseyi:

So have people looked at processes on that fine scale?

David Kaiser:

Yeah, terrific question. So we're beginning to, we're getting better with our computer simulations for things like that. So typically we think the universe was actually, it sounds funny to say, actually pretty simple. It was mostly a nearly uniform kind of process.

Hakeem Oluseyi:

So you wouldn't have thermal gradients that would set up those sorts of—

David Kaiser:

That's right, so typically we would not expect those.

Hakeem Oluseyi:

You wouldn't expect that, right.

David Kaiser:

Maybe some new physics we haven't thought about could have made them, but the typical or the standard cosmological scenario And so we think it was mostly equilibrium, mostly smooth distribution of more roughly same amount of matter here as there. But there could have been, as we've come to understand better and better in more recent years, kind of flukes, these spikes of unusually high density in very short little patches of space, little length scales.

Hakeem Oluseyi:

Due to quantum fluctuations yet again?

David Kaiser:

Of course they are. Of course they are. You knew that.

Hakeem Oluseyi:

I didn't know that. I'm guessing.

David Kaiser:

What else could they be? Due to quantum fluctuations. Exactly, that's at least our leading explanation. Exactly right. And so we've gotten really good since Stephen Hawking's day, certainly since his work in the '70s. We in cosmology and astrophysics, as you know, at trying to understand these very, very early universe quantum fluctuations, which we use to model things like the cosmic microwave black holes.

Hakeem Oluseyi:

Let's define quantum fluctuations. So I'm gonna give you my explanation, but I wanna hear yours.

David Kaiser:

All right, good.

Hakeem Oluseyi:

Because you probably have something better. So I think of it like the surface of a serene lake or pond. It looks like a mirror, it's so smooth. But then if you zero in with a microscope, you see that things are fluctuating and that it's not smooth at all, right?

David Kaiser:

I think that's exactly right. I like that one. You know, analogy I like a lot is from the great science writer Isaac Asimov.

Hakeem Oluseyi:

Oh, yeah.

David Kaiser:

Going back in the day, classics, right? So Asimov made this analogy to Heisenberg's uncertainty principle, which is the heart of this notion of quantum fluctuations. People might have heard about that. It's at the core of our understanding of quantum theory.

Hakeem Oluseyi:

Yeah, yeah.

David Kaiser:

He said if we really take the uncertainty principle seriously, then quantum particles are like young schoolchildren. They just literally can't sit still. Like, they're always jittery, right?

Hakeem Oluseyi:

Right.

David Kaiser:

So on average, they're well-behaved in the classroom. The teacher turns his or her back. They're doing all kinds of wild stuff, just not getting caught. That's the uncertainty principle. On average, they can break the rules, but over kind of short distances or short amounts of time. So otherwise, when we look at that, as you say, that kind of smooth pond, on average it looks kind of pristine. But boy, is there a lot of activity happening on different scales if we zoom in.

Hakeem Oluseyi:

So those fluctuations could have changed the density, even though it's super smoothly, uniformly distributed.

David Kaiser:

That's right.

Hakeem Oluseyi:

Those quantum fluctuations could have created a little more matter here.

David Kaiser:

Yeah, a little more here. And it could be— that's right. And so we already use that kind of framework with real precision. We really make quantitative calculations these days to predict the pattern of these very subtle kind of bumps and wiggles in things like the cosmic microwave background. We use those to feed into our large simulations for things like large-scale structure.

Hakeem Oluseyi:

And so are there like spikes at this particular scale that would give us these primordial black holes?

David Kaiser:

Let me put it this way. We can make models of the very, very, very early universe, that's 3 verys, super early universe, like during cosmic inflation, that will have all kinds of quantum perturbations. And if we have a model that has these features, it should then lead to a spike on those scales.

Hakeem Oluseyi:

Okay, and is there a way to distinguish whether or not that actually occurred.

David Kaiser:

Yeah, good. So it's a great, great topic of research now. So it's becoming more and more clear the kinds of ingredients you need in those models, so on the theory side.

Hakeem Oluseyi:

Right.

David Kaiser:

It doesn't always happen. It's not completely generic. It's less completely rare than people first thought as well. We're finding it, you know, it could happen in families of models and this if the parameters are in a certain regime. So it doesn't look super far-fetched, not that it had to have happened. Now the question is, did it happen?

Hakeem Oluseyi:

Right.

David Kaiser:

One of the best routes to try to find out, which is, is really becoming realistic based on advances in technology, would be to measure a certain kind of gravitational wave. So a kind of very diffuse, kind of large, so-called stochastic background of these gravitational waves. Here's why. Those— if there were these, these very sharp peaks in those quantum fluctuations, those would be with a particularly simple form. But if they're really large enough, they could scatter into each other and make a more complicated form of gravitational wave, like the kind that that we can now measure with increasing accuracy. So if we know the pattern of how much, how much height versus length scale.

David Kaiser:

If we know what we needed to have accounted for these tiny black holes, we can now do those calculations pretty well. Well, if it was really that elevated, a kind of larger departure from the average density, that should have excited these other kinds of gravitational waves too, in a regime that we're now getting better and better to be able to measure soon.

Hakeem Oluseyi:

So in terms of their population density, so again, making note that since the cosmic microwave background radiation was formed, 380,000 years after the Big Bang, the universe has grown by about 1,100 times. But between the very beginning and that, it was, Many, many, many, many, many, many, many, many more times the universe has grown. And so I say that because I'm gonna define a volume. And the point is that these volumes are rapidly changing. So whatever number you're about to give me has to evolve to today's universe. And the number is, I don't know if it's say a cubic meter or a cubic centimeter, but what, how, you know, what is the population density in terms of number of microscopic black holes per cubic meter in the early universe, and then what would that translate to in today's universe?

David Kaiser:

Good. Great question. So the short answer is very, very rare in the very early universe. And so they would form by such a rare process. There would be kind of on average, on average much less than one in any sphere the size of what would grow into our observable universe.

Hakeem Oluseyi:

Wow!

David Kaiser:

Yeah, so it's not like they formed chock-a-block right there.

Hakeem Oluseyi:

How the hell could they be, dark matter today if there was one per observable universe?

David Kaiser:

Less than one, because our observable universe keeps growing. These patches that were neighbors move into our horizon. They become part of our world, right? And so over time, we have a little patch like this. There's a neighboring patch next door. There might be a black hole here and none here. These two both grow and merge, and that happens a gajillion times to make the observable universe we have today. So in any patch of a typical size, of the— of what our observable universe was back then, you'd have many less than one on average of these black holes. They're very rare. But then you have, as you say, the universe has expanded so much, that means these things have come in from the neighborhoods nearby. So within what we now consider our observable universe, these things are a lot. And I can tell you how much, like even today, now they would be— they would potentially be the dark matter density, whether there'd be five times more in any kind of unit of volume than ordinary matter. But you get there because you get lots of these little tiny patches that kind of coalesce and make the big universe we inhabit today.

Hakeem Oluseyi:

So what is the mass of a single one? We say the size is really tiny physically, but what masses are we talking about?

David Kaiser:

So we tend to talk about something called the asteroid mass range, and as the name suggests, the mass is typical of asteroids in our own solar system. And that means smaller than the mass of our moon, much smaller than the mass of the Earth. And so we can put numbers on that. It really means the biggest ones that we can kind of, that could still fit through, you know, kind of thread the needle and not be ruled out by certain kinds of observations on one side or the other. They start— the biggest they could be is roughly 10 billion times smaller in mass than the mass of our sun.

Hakeem Oluseyi:

Oh, wow.

David Kaiser:

And that's— that is the typical size of the asteroids, like in the asteroid belt between Mars and Jupiter.

Hakeem Oluseyi:

So objects that dense— Yeah. —and those huge numbers, it would seem to me that it would play games with light and you get a sort of scintillation effect.

David Kaiser:

Good. However, because they're so massive per object, you need few of them around to make it the average mass density. The astronomers are measuring mass density, how much mass of dark matter per volume.

Hakeem Oluseyi:

Right.

David Kaiser:

And then you ask, well, what's the mass per object?

Hakeem Oluseyi:

Yeah.

David Kaiser:

If it's big, like the mass the equivalent of a big space rock, you need fewer of those objects. So then what we call the number density could be quite low. So not all over the place. To put that in numbers, and we can talk about this more later on, to account for the kind of the amount of dark matter per volume in our really local neighborhood within the solar system, say, we need like 1 or 10 of these things.

Hakeem Oluseyi:

Oh geez.

David Kaiser:

In the inner solar system.

Hakeem Oluseyi:

Wow.

David Kaiser:

They're not in the room, right? They're not on Earth. They're so, sparsely distributed. And then if you say, well, the solar system's pretty tiny compared to our galaxy, they start adding up. But on length scales much beyond even the size of our solar system, typically.

Hakeem Oluseyi:

So you know what you're making me think of now? You know, there was this old idea that stars have black holes in their core. So I would imagine that if these things are moving through galaxies where there are stars, so these concentrations of mass. And so we have this notion that they're moving, it's cold dark matter, so it's moving slow. Are we seeing or imagining microscopic primordial black hole stellar collisions?

David Kaiser:

We are. We are. We're trying to say, if that happened, how would we know?

Hakeem Oluseyi:

Yeah, how would we know?

David Kaiser:

That's right. And so first we wanna say that—

Hakeem Oluseyi:

And even that, excuse me, even then I would imagine that just like supermassive black holes in the cores of galaxies would have halos of stellar-mass black holes. Would they also have, be surrounded by a swarm of these primordials, or is it just too few of them?

David Kaiser:

I think the answer is it's too few of them, at least in our current understanding. That's right. So these things are, they have the mass like a large, huge space rock, but remember, to make it a black hole, the densest thing that matter could be, the mass of a space rock is squeezed into the size of a single atom.

Hakeem Oluseyi:

These have to be much less, right? Much smaller than a— like nuclei or something, right?

David Kaiser:

No, I mean, they could be nuclei. There's a range, what we call the so-called asteroid mass range. The middle of that range is the size of a hydrogen atom.

Hakeem Oluseyi:

It's like 10 to the minus 10 meters.

David Kaiser:

Exactly right. Exactly. It's an angstrom. That's right. And it could be a couple orders of magnitude smaller. It could be a couple bigger. But it's less than a micron, less than a millionth of a meter. And could be quite a bit less than that within this range we think about. So they're tiny in spatial size. But they're packing a lot of heft because it has the mass of an asteroid. Maybe say 10,000 times less mass than our moon. That's a lot of mass still to be in such a little point. So my own group, many people have been studying things like encounter rates or collision rates for these tiny things encountering regions that are full of stars like within a galaxy. And again, the encounter rate's actually not so— it wouldn't happen all the time. These things are traveling. You say they're cold. They would be cold. But that's compared to the speed of light, not compared to my car on the Mass Pike. That, my friend, is slow. Believe me, that's slow. These things would be actually zipping around pretty fast on human terms. In our neighborhood of the solar system, if these things are the dark matter, they'd be moving around at like 200 kilometers a second, not an hour.

David Kaiser:

That's about 10 times faster than these asteroids move in the asteroid belt, 10 times faster than the Earth moves around the sun. So in solar system terms, they'd actually be pretty quick.

Hakeem Oluseyi:

So, man, this is leading everyone listening, I bet, to think the same thought. And this goes back to the Large Hadron Collider, right? Some said, Oh, it's gonna produce these microscopic black holes. And I guess people are thinking now, What? There are these microscopic black holes flying around at 200 kilometers per second? What if they collide with Earth?

David Kaiser:

Yes, yes, yes. That comes up once or twice. Yeah, I've heard that. This is not the first time I've heard that.

Hakeem Oluseyi:

Yeah, that's— yeah.

David Kaiser:

You know, here's what— here's my thought on that. I don't know about you. I stay up at night worrying about a lot of things. I have a list. My list is getting longer. I'm a worrier. I worry.

Hakeem Oluseyi:

Yeah, yeah.

David Kaiser:

I don't worry about this.

David Kaiser:

This is not on my list. First of all, I admire your sleep habits. But, you know, what keeps me up at night ain't this.

Hakeem Oluseyi:

Right.

David Kaiser:

And we can put some numbers in. I was saying a little while ago, the local dark matter density in our solar system, where our Earth is and all that, is such that if the dark matter is all or even mostly these tiny black holes, there's a handful of them distributed across an enormous volume of space. Like between here and planet Jupiter, there might be 1 or 10 sprinkled around in a whole ball that size. You start putting in numbers, what's the odds that one would be striking the Earth, let alone striking you or me?

Hakeem Oluseyi:

Right.

David Kaiser:

We're not as big as the Earth, right?

Hakeem Oluseyi:

I might emit 'em, you know, when I flex.

David Kaiser:

I mean, it's a new research paper. We haven't even detected 'em. But if that's not the case, you know, the odds that one would hit the Earth are about 1 in a quadrillion. Quadrillion's a fun number. It means a million billion.

Hakeem Oluseyi:

A million billion. But let's put that to time. You know how, for example, the probability of a proton pair fusing in the core of the sun is likely to happen once in 10 billion years. So a collision with one of these and a planet in a solar system is likely to occur once in—

David Kaiser:

Approximately once in the age of the universe.

Hakeem Oluseyi:

Okay, the current age of the universe. Current age of the universe. Okay, so we may have lost, we may have had one planet that has a tiny micro— Maybe that's what happened to Jupiter's core. We thought Jupiter's core was gonna be nice and solid, turns out it's not, 'cause of that microscopic black hole in there.

David Kaiser:

I can't say it didn't, but I can't say it did.

Hakeem Oluseyi:

Nor can I.

David Kaiser:

That's right.

Hakeem Oluseyi:

It's just a wild-ass guess.

David Kaiser:

But the idea that, so we're not subject to bombardment by these things.

Hakeem Oluseyi:

So one of the criticisms lately about fundamental physics is the whole idea of, oh, we're gonna dream up a new particle and then build a collider to go look for it. So in terms of how well motivated this idea is, uh, you know, are we playing that same kind of game, or, or do you feel that there's more— How am I gonna say substance? 'Cause it's all substance.

David Kaiser:

Yeah, that's right.

Hakeem Oluseyi:

However, I guess you would imagine what I'm trying to ask without having the words.

David Kaiser:

I think I know what you mean. So I think these are, again, it's kind of funny to say it, but the idea of tiny black holes formed at the Big Bang, to my mind, is one of the most conservative, least out-there explanations we have for dark matter these days. Doesn't mean it's right.

Hakeem Oluseyi:

'Cause we know black holes exist, let's start there.

David Kaiser:

Correct, and it'd only require the normal matter– of the standard model particles.

Hakeem Oluseyi:

No new stuff has to be invented and created.

David Kaiser:

They could be, but they're not required anymore. So that's what's kind of— so it requires the least new ingredients. Yeah, yeah. Took us 50 years to get there, or 40 years. And so that doesn't mean it's right. But as a kind of thing to kind of bet on with your pencil, right? Say, is that worth another few months of digging in? I think it is. Another thing that's awesome is that the way in which we can try to learn more about where they're not, right, to put that in— make that box— where it has to fit within. Often that comes from piggybacking on measurements being done for other reasons anyway.

Hakeem Oluseyi:

Right.

David Kaiser:

So to date, we have not been building our own massive infrastructure saying, oh, you're measuring this thing with great precision. If the black holes were there and had these properties, you should see this thing lighting up and you don't.

Hakeem Oluseyi:

That's right. Yeah.

David Kaiser:

So for example, the microlensing surveys, we're using beautiful existing telescopes. You need some telescope time and some very smart people to stay up all night, which we have luckily. But you're not— it's not the kind of capital outlay, right?

Hakeem Oluseyi:

Yeah.

Hakeem Oluseyi:

So that helps a lot too. So, okay, so this idea of primordial black holes, Hawking introduced it, and they were just like, this is a process that could have happened, right? And so now we have this dark matter problem, and they might conveniently fit into that box. Are there other, uh, cosmological, astrophysical problems that these primordial black holes could resolved.

David Kaiser:

They could. They're a gift that keeps on giving. So one would be maybe— let's go back to these supermassive black holes you mentioned, the ones that now with the latest data, like from the James Webb Space Telescope and other, other inputs, are finding many, many more enormously massive black holes very early.

Hakeem Oluseyi:

Yeah.

David Kaiser:

And that's hard to square with our typical understanding.

Hakeem Oluseyi:

Right.

David Kaiser:

If the only route to make black holes is from stellar collapse, that fixes the typical size and it's much, much smaller mass to start. Than these enormous supermassive black holes people are finding. And then you say, well, if they're that early, they didn't have time to gobble up. They couldn't accrete, at least in our best understanding of stellar processes. And so one idea would be, what if the seeds for supermassive black holes came from this alternate route? What if primordial black holes formed at around a second or a few seconds, not a fraction of a second? Then their typical mass at formation would be maybe 1,000 times the mass of our sun to start. They start already with a head start, right? Then the more typical accretion rate that we otherwise think we understand from astrophysics, the rate at which the star would grow or the black hole would grow by gobbling its surroundings.

David Kaiser:

That's now much, much more straightforward to say why could this thing be 1 billion times the mass of our sun at such an early time. So a lot of work that's motivating primordial black hole research these days would be not even about dark matter, about the structure of galaxies and these enormous anchors, these supermassive black holes that are being found at much larger numbers much earlier than expected. If you give them a head start, they have less work to do to get that big.

Hakeem Oluseyi:

You know, that's a beautiful thing about how nature works is that you look at a situation today and you define a normal and that sets up an expectation. But then you think deeply about it and you realize, like one example is life, right? So right now, you know, life depends on oxygen, right? And, and, you know, but early life it was poisonous to oxygen. So if you think, oh, we need this, an environment like us to create life like us, but you don't start with life like us, nor do you start with an environment like us.

David Kaiser:

Right.

Hakeem Oluseyi:

Like the current environment. So it could be that when you think about the fact that this environment of the early universe was so different—

David Kaiser:

Yeah, that's right.

Hakeem Oluseyi:

And the black holes that formed, formed differently. They weren't star first.

David Kaiser:

Exactly right. Exactly right. And honestly, just stepping back and saying, one of the things I love about this topic, many things I love about it, is that it forces me, encourages, allows, forces me and my gang, my students, my great collaborators, to really think about a large range of topics across physics because it could matter about how do quarks behave, 'cause quarks were it in the early universe before there were protons.

Hakeem Oluseyi:

That's good.

David Kaiser:

That's not something I'd focused on before. I have amazing colleagues who can coach me and be very patient with me. It forces me on the other end to think about how do telescopes measure microlensing? I didn't know that. You know, what about gravitational waves? So for me, it's been just a feast of new ideas, new to me ideas. And then we can say, oh, let's put them together. Can we really figure something out? So that it feels, I mean, a little like Indiana Jones. I don't get to feel like that very often in my line of work. But here's some weird clues from like all over to the map and what can we do to kind of put that jigsaw puzzle together?

David Kaiser:

It's really fun. With these primordial black holes, I'll just say the interest is growing. There are certainly skeptics. That looks less natural to me, but natural is, you know, that's based on what we're used to.

Hakeem Oluseyi:

When you're right, there are skeptics.

Hakeem Oluseyi:

Fair enough.

Hakeem Oluseyi:

When you're proven right to high precision, there are skeptics

David Kaiser:

But the kind of dismissing it as not even a good question, I think that really is, I don't hear that anymore.

Hakeem Oluseyi:

So let's talk culturally here, because what I've observed over the course of my career is there is a bandwagon phenomenon. In science, and sometimes there are strong personalities, and so people move around, right? They go from WIMPs to axions and this sort of thing. So if you look at where the energy of, or modified gravity, right? If you look at where the energy of the upcoming graduate students and postdocs are, how do you say the health of this black hole approach, is it starting to get its own bandwagon, or?

David Kaiser:

Not a bandwagon, certainly its own enthusiastic community.

Hakeem Oluseyi:

Yeah, its own enthusiastic community.

David Kaiser:

Maybe to outsiders it looks like a bandwagon. It is, it is. And so the number of young, amazing, super skilled young people, I have undergraduates at MIT working with me on some of these projects, amazing. Lots of grad students, postdocs, and not just MIT. So we have annual meetings and the number of attendees grows every year and the number of people who want to attend, people propose to come, grows even faster. This year because of calendar, we have two specialist meetings happening on just the same topic the same week in different cities by accident. But the point is there's that much interest.

Hakeem Oluseyi:

That's enthusiasm. Yeah, that's enthusiasm.

David Kaiser:

And it's not to say we're right, but saying this is a real question, we have better and better ideas for the next questions.

Hakeem Oluseyi:

I love it, love it, love it.

outro

Okay, so this isn't just a hypothesis on a whiteboard. Scientists are actually trying to find these things right now. But how? Join us on the next episode when David Kaiser shows us the way.