NOVA

All right.

So you think that dark matter might be black holes?

I do.

What if its ordinary matter?

The stuff we know about really well from the standard model that's locked up in a black hole, not a black hole.

The sort that we now understand really well.

What if it's locked up into tiny black holes?

That formed right after the Big Bang and are the size of single atoms.

What?

Not not your not your grandma's black hole.

We are going to talk about some amazing stuff today, man.

I'm so happy to have you here.

We're going to cover dark matter.

We're going to cover black holes.

But, you know, before I lean too deep into your theoretical physicist brain, we're going to go into your historian braid.

Because dark matter has this amazing history.

And, you know, I know what dark matter is, okay?

I know what we mean when we say dark matter.

Nobody knows that dark matter is.

Maybe you do.

Maybe you'll break the story today.

I have ideas you have.

I got favorites.

So thank so.

Thank you.

It's a great, great topic.

A long history to the topic.

Yes, almost 100 years by now, actually.

So one of the things I find most compelling about dark matter is that over that nearly 100 years, astronomers and physicists, the community as a whole, has found lots of different types of evidence that we interpret in terms of dark matter.

Not just one thing, not just two things.

It really goes back to the 1930s often is how we start thinking about this.

Yeah, there were astronomers, including Fritz Zwicky, who was a Swiss American astronomer working in California at the times, in the largest telescopes on the planet.

Some huge, great tools.

And Zwicky, like others, was measuring things that we now call clusters of galaxies.

Not one galaxy, but they were so far away that each galaxy looked like it was as if it were an individual star.

Little pinpricks of light and gajillions of them, right?

All all seem to be stuck together.

Bound together like the Coma cluster is one that Zwicky was famously looking at, and he and his assistants were trying to measure the speeds of typical galaxies within that cluster, that bundle of galaxies.

And it kept finding in the mid 1930s, the typical speeds were larger than he would expect, given the amount of stuff he could see or infer.

There would be some balance between the mo-- the energy of motion.

These things having a fast speed.

Why don't they just zoom away from each other?

And the and the the mass that he would have infer from all those galaxies acting through gravity to keep them stuck together.

So the thing was gravitationally bound.

He assumed these things weren't free to fly far apart, and the individual objects had higher speeds than he would have expected.

Okay.

So often I've heard people say that they were observed moving faster than the escape velocity from the cluster.

Yeah, exactly.

Right.

That's right.

And perfect.

And that's why he.

So how could they be stuck by gravity if they're moving faster than it would take to keep them bound?

Right?

They should have been freed of to wander through the cosmos alone, and they appeared not to be.

So he says, as a kind of an offhand remark, is maybe there's some other matter.

And he actually uses the term dark matter that we don't see.

It's not lighting up, but it's acting through gravity.

So maybe each of those fast moving, you know, bits of light is actually feeling more gravity than we otherwise would have thought.

If, hypothetically, there's more stuff in the surrounding area of space that's adding more gravitational tug than he had otherwise assumed, he says maybe he's not sure.

It's interesting.

It sounds like the discovery of, which one was it?

Neptune.

And very similar.

That's right.

Yeah.

So we infer gravitational effects from things you can track through light.

It looks very much like that now on the scale of a huge cluster of galaxies, not just in our solar system.

Same idea, the next big set of data points that we tend to look at.

Again, it's kind of, in hindsight at least, came about 30 plus years after Zwicky.

And now we're looking at the late 1960s, early 1970s.

And here one of the main researchers we tend to, to, to think about is Vera Rubin, amazing American astronomer with her long time colleague Kent Ford.

And again, many people around the world doing similar kinds of things.

Rubin was, I think, especially, dogged in this on this question.

To her great credit.

So unlike Zwicky, who is studying a collection of lots of galaxies, Rubin and Ford were zeroing in on a single galaxy like Andromeda, one of our closest neighbors.

Right.

Pretty closeby Galaxy.

And they were studying individual stars within that galaxy.

It's a it's a spiral galaxy, much like the Milky Way.

Objects in the kind of outer arms are whipping around like a kind of carousel.

And again, they assumed that it would be like planets in our solar system, that the further away an individual star was from the center of that galaxy, the slower its speed would be.

And so.

So Vera Rubin and Kent Ford figured the same would be true of stars in an ideal galaxy, as if they were, you know, if they're further away from where they assumed most of the mass was concentrated, they should have corresponding these slower speeds.

What they found over and over again with really beautiful, precise measurements was what they came to call a flat rotation curve.

So the speed was not falling off like it would have expected.

And they checked their instruments, they checked different galaxies, they were very thorough.

It took many, many years and eventually began to convince the community because they were so, so careful in these measurements more than once, that that would be consistent with the idea.

Still, a hypothesis, right, that maybe there's more stuff that's acting to gravity that's not lighting up.

What if it were dark matter?

So if there's a larger clump, we now call it a halo of stuff that's not lighting up in the form of stars, but is acting through gravity.

If it's more extended through space, a bigger blob.

A bigger blob, so its wrapped around the galaxy.

Exactly.

And then you would then you would expect the speeds to level off and not fall off the way people expect.

Okay, okay, now I want to pause there again.

That didn't alone convince the whole community either.

But now you see something where someone like Fritz Zwicky is finding these on a scale of, you know, hundreds of millions of light years across.

And now someone like Vera Rubin and Kent Ford is finding this on much less than a single light year that starts to make physicists pay attention.

We see the similar phenomenon across such a wide range of scales.

Length scales.

Yeah, right.

Huge change and yet looked kind of consistent.

Interesting.

And then, one of my favorite examples comes from much, much more recent times.

Why we really, really think this is, the stuff filling our world right now.

And that comes from the, the, remarkably subtle patterns in the cosmic microwave background radiation.

Man, I thought you were going for gravitational lensing.

We'll get there.

You went way complicated, now.

You know, it's, you know, it's not comp-- It's beautiful.

It's beautiful.

Its beautiful.

You know, we're remember intro, intro level.

This is the earliest light in the cosmos.

All right, let's pause and enjoy that for a second.

It's amazing.

You just.

You know.

Yeah.

And so astronomers again really with with great skill since the 1990s.

Now it's a kind of past 30 years, not 50, not 100.

Pretty recent more recent, have been able to measure with increasing precision.

As you know, these very subtle bumps and wiggles in the pattern of the CMB, the cosmic microwave background radiation, the, the light that we receive is almost entirely uniform.

The same energy from all directions sky but with little ripples, about one part in 100,000.

And you can actually do a kind of careful analysis of the pattern of those ripples, the height.

So you can make a kind of, what we call the spectrum and the, the pattern of those tiny bumps and wiggles should have been sensitive to the stuff filling the universe when that light was emitted.

Right.

So it tells us, like the kind of ingredients list.

It's like a cookbook.

Right?

Right, right.

And so those bumps and wiggles are consistent with a particular ratio of dark matter to ordinary matter.

The stuff that makes up you and me and everything we measure.

And it's the same ratio as you infer from the rotation curves of individual galaxies from, from the galactic clusters and all the rest.

Okay, so you mentioned scale.

So you have, individual galaxies on a scale of much less than a, light year, you have the galaxy clusters on scales of hundreds of billions of light years.

Now you have the cosmic microwave background radiation, which is another scale in time.

Yeah, another scale in, in size.

Yeah, yeah, yeah.

So how do you characterize the, the CMB is short for that light.

Yeah.

Cosmic microwave background.

So how do you characterize that scale?

Yeah, it fills the entire sky.

So it's the entire observable horizon, even bigger than any individual cluster of galaxies.

So another factor of 10 or 100 or even bigger.

So tens of billions of light years, that we can scan.

And that's again, that's when people started saying, okay, hang on.

There's really if it's consistent across this huge range of types of physical systems of length, scales of moments in cosmic history, more recently, when our friends with really powerful computers, right, can start simulating what we call large scale structure, the distribution of stuff right throughout the entire cosmos.

Some of it's very densely concentrated, lots of matter and activity, energy flow and huge voids.

Let me let me insert here.

So earlier you mentioned the cosmic microwave background radiation was so super uniform.

So that suggests to us at 380,000 years after the universe began, right.

Matter was uniform.

Yeah.

Was that structure built at these super large scales of hundreds of millions of light years that we call the cosmic web.

Exactly.

Yeah.

And so, you know, I often say gravity is the most aristocratic of forces, right?

The rich really get richer.

Yeah.

Right.

So what this what the microwave background tells us is there were little, little lumps, little lumpiness in the distribution of where the stuff was a little bit more, a slightly more mass here than, than a neighboring spot.

But in parts of, you know, much less than a percent, a fraction of a percent.

And just to say, over time, the parts that happen to have a little bit more through gravity start attracting more and more, right?

So you get even more dense concentrations over time.

And likewise, the parts that start out a little bit less than average get more and more, kind of emptied out, more evacuated.

And you see this can create and that works with pencil on paper.

Works now really impressively with these computer simulations to build universes that look like ours today.

And this is having this huge range of scales, a hierarchy of structure, so massive concentrations here and sort of huge empty voids there, the cosmic web.

Okay.

What the what the folks who do these very sophisticated simulations keep finding is unless they put in the amount of dark matter that these other observations are suggesting, then they won't get a universe that looks anything like ours.

The structure will will either not form or you have different statistics of it.

Yeah, not enough galaxies would form if the universe was stretching as we know it was in an expanding universe.

It could have stretched, more quickly than certain clumps of matter could have gotten kind of gravitationally bound together.

You'd have fewer galaxies.

Our own Milky Way might not even exist without dark matter.

And without that, we wouldn't have this, you know, beautiful desk and all the rest of stuff.

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So, so lots and lots of examples.

From over, spanning almost a century of careful observations and theoretical modeling, they all point, not just that there seems to be this stuff that acts through gravity, but doesn't light up, doesn't shine on its own and the same amount of stuff, which is really that's, I think, again, pretty impressive.

The the amount of extra stuff required to satisfy each observation is consistent across all these scales and all these environments.

Exactly.

And at it certainly didn't have to be.

Sounds like quite the coincidence.

Or maybe it's just telling us, right.

And so.

So now we say with great confidence, oh, the universe is filled with dark matter.

We know how much you know, per volume compared.

We can say that now, but that was not so obvious.

Well, okay, let's, let's, let's take it back to how I framed the question.

So what we're actually observing is motions.

And these motions are typically observed through the Doppler shifts.

Right.

And what we infer from that is there is a lot of extra gravity happening here.

Right.

Right.

That's right.

So in most cases extra gravity means extra stuff.

Right.

So we don't actually see stuff.

No.

Perfect.

That's right.

Right.

Exactly.

Right.

Now what we see, what we infer as you said in the beginning, what we what we the way we make sense of these range of observations, different length scales, different moments in cosmic history.

We make sense of that by inferring, by concluding there's more stuff out there that we don't see directly.

We, we see the, the what we assume are the effects of that.

Yeah, tugging and nudging the motion of stuff.

So when we when the community, I should say the community came together saying there is this challenge which got the name dark matter, which is really just covering up what we don't know as you say, it's something that that needs study.

Right?

That really came together as even a question only 50 years ago.

And I say only because some of this evidence was coming in long before that.

That's younger than me, man.

So that oh, and I'm young as hell.

So yeah.

Well that's a separate episode, well cover by the way Hakeem.

Young at heart.

I feel the same.

So in our lifetimes.

Yeah.

Which is actually kind of amazing because absolutely, this is energizing so much the community.

And so that prompts a series of what could it be?

Can we explain this?

Now, there really is a question they all agree.

That is a question.

Go.

What could it be?

One perfectly legitimate idea, is that maybe we've been making the inference based on the observed motions, because we're assuming gravity like we think we know it.

Right.

If we assume we really know the laws of gravity, as Einstein wrote down beautifully just over 100 years ago in his general theory of relativity, encompasses Newton's gravity.

I mean, it just it matches all kinds of tests and, and, and theoretical checks.

Let me just let me just put up a point in that what you basically just said is if we assume gravity as we know it.

Yeah.

And gravity as we know it is Einsteins general relativity.

And we have tested that sucker throughout a lot of scales.

Exactly right.

And and it works every time.

We have reason to be confident.

Yeah, but what people are rightly saying is could dark matter be the first, you know, exception to that?

It could.

Maybe it is.

Right.

Because again, just as you said, we're using our assumption about gravity to fit why this motion looks weird to us.

Maybe it would look normal if we had different laws of gravity.

Right?

That's the, that's the sort of set up.

Yeah.

Often called modified gravitational, you know, models.

And you know, that's again, that certainly could be that's logically a great thing to start from.

And as you also said, so far over the many decades that many people have had, very smart people have worked very hard at this and still do.

Yeah, sometimes they'll come up with a very cool model adjusted, you know, set of laws of gravity and it'll it can make the one kind of physical system make a bit more sense.

Maybe we don't need, you know, dark matter, but then it tends to break when we apply to these other things.

This is why I'm so impressed.

As a as I was saying a moment ago, by the different lines of evidence for dark matter, which are remarkably consistent with each other, but they come across an enormous range of length scales, time scales.

And so, so far it doesn't mean that this won't ever work out.

But so far, at least, my understanding is that these very clever ideas to try to modify gravity.

If they when they work at all, they work at kind of one typical system, one length scale, and they tend to work pretty poorly or sometimes accentuate the mismatch at other length scales.

We want, we want we got to shoot the moon, right, right, right.

So that work goes on and it should go on.

But that's that's one option.

That's still a live option.

But this is why we're not done.

Right okay.

All right.

So let's look at these dark matter is stuff.

Yeah.

Options.

Right.

And, you know, I, was teaching observational astronomy, back at the turn of the century.

Yeah.

And, you know, I used to talk about supersymmetric particles.

Yes you did, and, you know, we'd talk about WIMPs and MACHOs and these sort of things.

Yeah, yeah, yeah.

Now, that was a long time ago.

That was a quarter century ago.

And so a lot of these models we've come to understand are not viable, or at least they're, they're certainly much more constrained than we used to think.

Maybe they're right, but they certainly don't look like we used to think they did.

Okay.

So now what are you know what have we considered and discarded, what remains?

And, you know, are there experiments that can find.

Oh, and I must mention, yeah, that my very first physics experiment ever research ever.

Yeah.

Was in the basement of LeConte Hall at Berkeley.

Yeah.

Bernard Sadoulet at the beginning of what became CDMS.

Yep, yep.

That's why I first did physics.

Yeah, yeah, yeah.

No.

So that's right.

So so as you say, it's a, fairly recent investigation.

I'm going to give you credit for that.

It was only a few months ago when you were a graduate student.

Yeah.

We've been doing this for a long time.

1991.

Who's counting.

What is time?

What does this mean?

It's you know.

Yeah, I right.

And so if we were to do a spot survey today of most physicists and astronomers and cosmologists, what do you think the answer is to this puzzle of dark matter?

Most will still say it must be some new type of particle, or maybe a cluster.

A whole sector of new particles.

Yeah, and that's still the most popular answer, though not doesn't mean it's right.

But that's and just as you say, the candidates that most people will find kind of vote for, right are shifting.

Right.

So when the dark matter question first came together in the mid 1970s, a lot of particle physicists were very confident for the reasons you were saying we have, they would say, they were saying we have all kinds of candidates, right?

For other reasons, for other theoretical modeling ideas.

The universe, they thought could be chock full of all kinds of particles beyond those what we now call the so-called Standard Model.

So the Standard Model is, a remarkable achievement.

I would say it's the most impressive, boringly titled, you know, theory in human history.

Like if you get a committee to write the name... The standard model... it should be like the outrageous, unbelievable, super cool.

I mean, it's it's amazing.

Yeah.

It's amazing.

And it's been now tested, you know, up and down and withstood every test.

Mostly these large accelerators, mostly huge particles to smash stuff together and see this stuff comes flying out.

Exactly.

With increasing, you know, remarkable precision things like the Large Hadron Collider at CERN, similar machines, throughout the United States.

And so one of the biggest, kind of most exciting gets of all those, of those decades of searches came now more than a dozen years ago with the announcement that physicists had really found the Higgs boson.

It had been hypothesized 50 years earlier, all kinds of reasons why people thought it had to be there and was very stubbornly not showing up.

Well, on July 4th, 2012, I remember the date.

Same These two enormous groups based at CERN, international collaborations, announced they had really found conclusively found the Higgs boson was the last missing piece of this beautiful Standard Model.

And people were saying, this machine works so well, we're just going to find all the other stuff, right?

Right.

To find these particles beyond the Standard Model, which had similar kind of motivations, theorists saying, you know, it has to be there.

It should look like this should be you'll find it here.

And people were really excited.

The machine was working great.

Still working great.

And what's happened in the years since 2012 is a lot of kind of hurry up and wait.

Wow.

And so the machines are working great.

The teams are dedicated in doing extremely precise science.

What they haven't yet found is any evidence at all of anything beyond the Standard Model.

And these are particle.

Some of these particles that are beyond the Standard Model might be the dark matter that people are looking for.

It was expected for decades that there would be more particles beyond the Standard Model.

Some would have exactly the right properties people would often call the miracle.

These particles were assumed to exist for other reasons and then could also play the role.

Kind of like, you know, straight out of central casting, would be perfect for dark matter, right?

They're not there.

Or at least they haven't been found yet.

They're not found where everyone, all the theorists at least, were pretty convinced they should be.

Right.

So since 2012, more and more data, beautiful experiments, lots of precision on the standard model.

We know more about the Higgs boson now than before.

It's a great good thing.

But what there's no evidence for at all, is a single particle as yet, beyond the Standard Model.

So that means that we've given up.

We're out of particle.

We're out of options.

We haven't given up.

There's a lot more to explore.

And some extremely dedicated colleagues continue to explore.

Yeah, but what it's done is it pushed the kind of obvious answer.

It seems a lot less obvious right now.

Right.

So we so now we've gone through, all these potential microscopic particles.

But there was also an idea that, oh, it could be like Jupiter's out there, MACHOs And they searched for gravitational or microlensing.

Yeah, right.

That's right.

Signals.

And how did that turn out?

I haven't paid attention to since I was in South Africa.

Yeah.

15 years.

Yeah.

Yeah.

So so the short answer is you I mean, I don't say you didn't miss much.

People still do the work very carefully, but in terms of the like the big answer still still no big answer.

So the idea was, as you say, there's often called MACHOs, massive compact halo objects.

Yeah, they could be like like Jupiters that have a lot of mass, but not so much that they actually became a star that never had quite enough mass to get hot enough in their core to start the nuclear reactions that power the stars.

So they'd be dark, so they'd be dark.

They'd be massive.

Like Jupiter has a lot of mass, right?

But not lighting up.

Right.

That's a cool idea, you know?

And so people were doing dedicated searches, observational searches, starting really in the 1990s with real focus.

And it's very cool.

No pun intended.

I mean, a little pun intended.

And so you take some of these dedicated telescopes, some in Australia were being used at the time.

Right, right.

And stare at again like a galaxy like, say, Andromeda or some nearby galaxy, often doing it for the Large Magellanic Clouds.

Take someplace thats near, near us in cosmic terms, some galaxy nearby.

And what's really amazing, I mean, there's so many things amazing about Einsteins general relativity as you know, one of them, a core idea, is that mass will warp its surrounding spacetime.

Yeah, fantastic.

Beautiful, beautiful.

And that followed on, as Einstein himself, predicted, should then bend the path of starlight because it's tracing out this warped, curved kind of environment.

Right.

So what if you have some large object like Jupiter, right.

Some mass?

Yeah.

Sort of sitting in this, you know, warping its surrounding spacetime, and you have some other object that really is lighting up some bright stars in some nearby galaxy.

Then you should see temporarily when that Jupiter, when that massive object passes in our line of sight between us and the bright thing behind it.

Yeah, right.

Then that should act like a lens temporarily.

And focus light beams that would have passed us, would never have hit our telescope.

But we'll bend them back towards our telescope, so we'll see a temporarily brighter image.

Yeah.

So microlensing should make an object that we can see in the night sky.

Yeah.

Temporarily get brighter.

What is the timescale of that?

Is it like seconds?

It depends.

It depends on on how big the object is and how far away.

So it's the geometry of it, right.

Just like playing with lenses you know would be for... Yeah okay.

So look so so a Jupiter is what kind of smallish dark chunk could be.

You have these dead cores of stars right on a scale.

You have your white dwarf.

Yes.

That's right.

You probably, you know, if it's super old, it was big, right.

So, yeah, you could have a neutron star.

Exactly.

That's that's naked and not giving off any light.

Yep.

Or the ultimate tiny object, which is a black hole.

And then you notice I said these stellar leftover cores.

You did.

Yeah.

The stars that died and left behind their core in some small, compact form.

Yes.

So the question becomes compared to a Jupiter.

Yeah, it would be smaller and it would have a much higher mass.

Right.

So how would that change the microlensing signal.

But yeah, so so good.

So people were able to say they didn't have to assume they knew the mass of the, of the lensing object.

It could have been Jupiter, it could have been... And what they would try to do is infer from the signature, you find the lensing first and then you get out.

Yeah.

But I guess the question begins.

You know, the question is, if I have a particular lensing event, then I'll use that.

No, I'm not going to use the phrase, I'm not going to say degeneracies, but, you know, sometimes it could be like, okay, right.

It's not just one size and one mass.

That's right.

It's a you know, I could match them up in such a way that they all give me the same signal.

That's right.

Exactly.

So perfect.

And so what they want to do is not get one bright spot in the night sky, okay?

You do this over and over.

And they did it for the better part of a decade, okay?

With many, many bright objects to do a high statistical survey.

So you really need lots and lots of examples, just like, you know, if we do, study the human population, not everyone's your height, not everyone's my height, you know, find a range.

Right.

And we can so you can map out the kind of bell curve or where it might be the kind of distribution.

It's always a bell curve.

Yeah.

You know, and so even taking that into account, this group, what's amazing is that they first said this group in the 90s said, oh, we found a couple of these momentary brightnesses.

It looks like they had found objects that were less massive than our sun.

So sub solar mass, that were consistent with, you know, the temporary, lensing.

Then they did what they should have done.

They took more data for many more years.

And guess what?

Yeah, the signal kind of went away.

So those have been a couple flukes and far too few of them for those to be all the dark matter, because we know how much dark matter there should be.

It has to be a lot, much more than the luminous matter we see.

It's much more luminous.

And so if, if, if dark matter is mostly these kind of, you know, chunky bits that never lit up like Jupiter, like, you know, MACHOs right?

Then they should have seen many, many, many more of these microlensing events.

And they clocked in after the better part of a decade of looking.

So that puts constraints.

Maybe those are 1% of dark matter or less, but they can't be the whole story.

Okay.

So it's almost like, okay, the the particle models haven't been completely eliminated, but, you know, pushed in the corner, right.

They're pushed into a corner.

Our direct detection experiments haven't detected anything at all this time.

That's right.

When it comes to the, you know, leftover remnants, they're not necessarily statistically working out.

That's right.

So where does that leave us?

So there's a third possibility that gets people very excited these days.

What if its ordinary matter, the stuff we know about really well from the standard model that's locked up in a black hole, not a black hole, the sort that we now understand really well, what if it's locked up into tiny black holes that formed right after the Big Bang and are the size of single atoms?

What?

Not, not your, not your grandma's black hole.

What?

Yeah, yeah.

So they're far beneath that mass limit.

Way below.

Exponentially below.

Oh wow.

That's right.

And so these are called primordial black holes.

They would have formed in the very early primordial universe.

And they could have bypassed the route by which all the black holes that we really know about by which those had formed.

So a second route, possibly by which black holes could form.

All right.

So you think that dark matter might be black holes?

I do, and I think they're pretty amazing.

And they're not the black holes you might have thought about since we say the phrase black hole.

Yeah, yeah.

Define what a black hole is.

Good.

Okay, well, before we go further.

Yeah, yeah.

So, a black hole is, the way we think about these, these stellar black holes, the ones that 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 that balances the inward crunch of the gravity.

Its really massive.

It wants to collapse.

And while it's 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, it's gonna be a runaway collapse.

And so what's left over is ultimately, as far as we know, a rupture in space time.

That's pretty dramatic, right.

Which also, as far as we know, is hidden from us by something we call the 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.

Right.

So there's a 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, etc.

for a black hole.

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

Yeah, a few times the mass of our sun.

So we're talking like two?

Yeah.

On the order of two, two or three, something like that.

That's right.

The experts will know better.

But it's, it's comparable to slightly bigger than.

Yeah.

The mass of our sun.

Got it.

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

Right.

But if it were to take that ma-- the equivalent mass okay.

Yeah.

It will never actually.

This one wont 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, right?

It would be the size of like a few city blocks.

Yeah.

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.

Yeah.

Were by people like Stephen Hawking, a name you might be familiar to many folks.

I've heard of him.

You've heard of him?

Did some pretty good stuff.

Yeah, yeah.

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.

Oh, yeah.

Yeah, 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?

Could they have formed the universe?

They actually concluded it probably wouldn't have happened, but they raised the idea in the late 1960s independently at first.

Stephen Hawking comes around to that 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, Einsteins 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 dies, collapses, it could be a kind of direct process.

Go straight to black hole.

Yeah.

Right.

So like nowadays, in recent years, there have been these observations, what they call unnovas?

Is that the idea?

So a star is so massive just goes bloop right to black hole.

No supernova explosion.

That's right.

All kinds of fill in the blank novas.

Kilonovas, kilonova.

That's right.

Yeah yeah I so blame it on the Bossa nova.

I mean, I have to write that paper.

Chevy Nova, you know, that's right.

But these would be 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, even says these things may be our candidate to be what we know.

What mass scales was he thinking of?

What's amazing is he writes out in the in this 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.

Yeah, much smaller than what we call a solar mass.

What they're pinned to.

And he works us out beautifully in 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 wouldnt have formed yet, right?

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, right?

We now know with great precision the rate at which its been stretching over time.

Yeah, it was much, much, much smaller at earlier times.

So so we're talking about really much, much earlier.

Typically a fraction of a second after the Big Bang.

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

Got it.

And so at that time the, 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, that history?

Right, right.

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 is 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 that are now studied in great detail, we know they're in our universe.

Yeah, 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 thing be boring now is pretty amazing or mundane.

So they, as you say, they come in kind of typical sizes, two kind of clusters, of sizes.

And we can make sense.

We try to make sense of the really big ones.

We're saying, well, they they formed early and had time to eat.

Right.

Had time to what we say accrete to get more matter.

Right.

They're huge dense concentration matter.

Gravity says they should attract other nearby matter.

Right.

And they could kind of get bigger over time.

And getting the details right is tricky.

Getting them really so big so early as as the James Webb Space Telescope is now finding more and more evidence for.

That's a bit of a puzzle.

But in general, we think, well, they probably formed around the size of, you know, of, of the mass of our sun, say, comparable a couple 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.

You don't even need stars.

You don't need atoms yet to have formed.

When these things were.

The ones that I'm mostly focused on would have formed.

So we call them primordial early universe black holes.

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

Very different.

And that matters a lot for our understanding of these things.

It's still hypothetical objects that were 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, a different universe.

I mean, 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.

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

How?

Yeah.

Great.

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, for 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-- So let me let me ask you another question.

Right.

So so when we look at the 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, whirlpool.

Yes.

Swirls.

Yeah.

Yeah.

Right.

Right.

So are-- have people looked at processes on that fine scale?

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 is actually, It sounds funny to say actually pretty simple right.

It was mostly a nearly uniform kind of.

So you wouldn't have thermal gradients that would set up those sorts, of uh-- That's right.

So typically we would not expect those.

You wouldn't expect maybe some new physics we haven't thought about what could have made them.

But the typical or the standard cosmological scenario.

Yeah.

And so we think it was mostly equilibrium, mostly smooth distribution of 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, it kind of flukes, these spikes of unusually high density in very short little patches of space, a little length scale due to quantum fluctuations, yet again?

Of course they are, of course they are.

You knew that.

I didnt know that!

What else could they be?

Due to quantum fluctuation.

That's at least our leading explanation.

Exactly.

Right.

And so we've gotten really good since Stephen Hawking's day, since certainly since his work in the 70s, we in cosmology, in astrophysics, as you know, and trying to understand these very, very early universe quantum fluctuations, which we use to model things like the cosmic microwave background.

Let's define quantum fluctuations.

So I'm going to give you my explanation, but I want to hear your all right.

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 it with a microscope, you see that things are fluctuating and that its not smooth at all.

That's right.

I think that's exactly right.

I like that one.

You know, an analogy I like a lot is from the great science writer Isaac Asimov.

Oh, yeah.

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 mightve heard about that, because were at the core of our understanding of quantum theory.

Yeah, yeah, he said, if we really take the uncertainty principle seriously, then quantum particles are like like young schoolchildren.

They just literally, they can't sit still like they're always jittery.

Right?

Right.

So on average, theyre well-behaved in the classroom, the teacher turns his or her back.

Theyre doing all kinds of wild stuff just not getting caught.

Right?

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 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 on different scales.

Right.

If we, if we zoom in.

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

That's right.

Those quantum fluctuations couldve create a little more matter here.

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 radiation.

We use those as-- to feed into our large simulations for things like large scale structure.

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

Let me it this way.

There, we can we can make models of the very, very, very early universe, thats three 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.

Okay.

And is there a way to distinguish whether or not that actually occurred?

Good.

So is a great, great topic research now.

So it's becoming more and more clear the kinds of ingredients you need in those models.

So on the theory side.

Right.

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 and 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?

Right.

One of the best routes to try to find out which 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, a 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 we can now measure with increasing accuracy.

So if we know the pattern, how much, how much height.

Yeah.

On a versus length scale.

Right.

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 to in a regime that we're now getting better and better to be able to measure soon.

So in terms of their population density.

So, again, making note that you know, since the cosmic microwave background radiation was formed, 380,000 years after, the Big Bang, the universe has grown by about 1100 times by between the very beginning and that.

Yep.

It was many, many, many, many, many, many, many, many more times.

Thats right.

The universe has grown.

Yeah.

And so I say that because I'm going to define a volume.

Good.

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.

That's right.

And the number is, I don't know if, say, a cubic meter or a cubic centimeter.

Yeah, 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?

A great question.

So the short answer is very, very rare in the very early universe.

Okay.

And so they would form it by such a rare process.

Yeah.

They would be kind of on average, on average, much less than one in any sphere.

Yeah.

The size of what would grow into our observable universe.

Wow.

Yeah.

So it's not like they form chock-a-block, right?

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

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 by, next door.

There might be a black hole here.

None here.

These two both grow and merge, and that happens a gajillion times to make the observable universe we have today.

Right.

So in any patch of a typical size of the... what our observable universe was back then.

Right.

You'd have many less than one on average of these black holes.

They're very rare.

Right.

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 that they would potentially be the dark matter density where they would be five times more in any kind of unit of volume than ordinary matter.

Right, right.

But you get there because you get lots of these little tiny patches, they kind of coalesce and make the big universe we inhabit.

So what is the, mass of a single one?

We say the size is really tiny physically, but what masses are we talking?

So we tend to talk about something called the Asteroid Mass Range.

And there's a name it suggests that the the mass is typical of asteroids in our own solar system.

Okay.

And that means smaller than the mass of our moon, much smaller than the mass of the Earth.

Right.

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.

Yeah.

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.

Oh, wow.

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

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.

Good.

However, because theyre so massive per object, you need fewer of them around to make it the average mass density.

The astronomers are measuring mass density.

How much mass of dark matter per volume.

Right.

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

Yeah.

If it's big, like the mass equivalent of a big space rock, you need fewer of those objects.

Yeah.

So then what we call the number density could be quite low.

Okay.

So not all over the place to put that in numbers.

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.

Right.

We need like 1 or 10 of these things.

Oh, jeez.

In the inner solar system.

Wow.

They're not in the room, right?

They're not on Earth.

They're so, sparsely distributed.

Right.

And then if you say, well, the solar system is pretty tiny compared to our galaxy, they start adding up.

Yeah, but on length scales much beyond even the size of our solar system typically.

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 galaxy, where there are stars.

So these concentrations of mass.

Right.

And so we have this notion that they're moving, you know, it's cold dark matter.

So it's moving slow.

Are we seeing or imagining microscopic primordial black hole stellar collisions?

We are, we are.

It's a... we're trying to see if that happened, how would we know?

Yeah.

How would we know?

That's right.

And so so first we want to say-- Even that.

Excuse me.

Yeah.

Even then, I would imagine that just like supermassive black holes in the course of galaxies would have halos of stellar mass black holes where they also have, be surrounded by a swarm of these primordials?

Or is it just, you know, too few of them?

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.

Remember, to make it a black hole.

The densest thing that the matter could be, the mass of a space rock is squeezed into the size of a single atom.

These are tiny.

It has to be much less, right?

Much smaller than, like nuclei or something, right?

I mean, they could be.

There's a range is what we call the so-called asteroid mass range.

The middle of that range is the size of a hydrogen atom.

It's like ten to the -ten meters.

Exactly.

Right.

Exactly.

It's an extra.

That's right.

And it could be a couple orders magnitude smaller.

It could be a couple bigger, but it's less than a micron.

Less than a millionth of a meter.

Yeah.

And could be quite a bit less than that.

Yeah.

Within this range we think about.

So they're tiny in spatial size.

Yeah.

But they're packing a lot of heft because it has the mass of an asteroid.

Right.

You know, maybe say 10,000 times less mass than our moon.

That's a lot of mass, still.

To be in, such as a little point.

So we, in my own group, many people have been studying things like encounter rates or collision rates for these tiny things, encountering, you know, regions that are full of stars, like within a galaxy.

And again, the encounter rates actually not.

So, it wouldn't happen all the time.

These things are traveling.

You say they're cold, they would be cold.

Right?

But that's compared to the speed of light, not compared to my car on the Mass Pike.

Right.

That, my friend, is slow.

Believe me, thats slow.

These things would be actually zipping around pretty fast by, on human terms.

Right.

In our neighborhood of the solar system, if these things are the dark matter, they'd be moving around at like 200km a second, not an hour.

Right?

That's about ten times faster than these asteroids move in the asteroid belt, ten times faster than the Earth moves around the sun.

So in solar system terms, they'd actually be pretty quick.

So, man, this is leading everyone listening.

I bet to think the same thought.

And this goes back to the Large Hadron Collider.

Right.

So I'm saying, Hey this is going to produce these microscopic black holes. Yeah.

But I guess people are thinking now, What?

There are these microscopic black holes flying around at 200km per second.

What if they collide with Earth? Yes, yes, yes, that comes up once or twice.

Yeah.

Okay.

I've heard that this is not the first time I've heard that.

Well, that's.

Yeah.

You know, here's what here's my thought of 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.

Yeah.

Yeah, I don't worry about this.

Yeah.

This is not on my list.

First of all, I admire your sleep habits.

But you know what keeps me up at night aint this.

Right, right.

And we can put some numbers here.

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 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.

Yeah, you start putting in numbers.

What's the odds that one would be striking the Earth, let alone striking you or me?

Right.

We're not.

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

I might emit them, you know, when I flex.

I mean, I it's a new research paper.

Yeah.

We haven't attacked it.

But if that's not the case, you know, the odds that we would, that one would hit the Earth are about 1 in 1,000,000,000,000,000.

Quadrillion is a fun number.

It means a million, billion.

A million, billion.

So let's let's put that to time.

You know how, for example, the the probability of a proton pair fuzing in the core of the sun is likely to happen once in 10 billion years.

Yeah.

So a collision with one of these and a planet in a solar system is likely to occur once in... Approximately, once in the age of the universe.

Okay.

The current age of the universe.

The current age of the universe.

Okay.

So we may have lost we may have one planet that has a... Maybe that's what happened to Jupiter's core.

You know, everybody, we thought Jupiter's core was going to be nice and solid.

Turns out it's not because of that microscopic black hole in there.

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

Nor can I. It's just it's just a wild ass guess.

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

So one of the criticisms lately about fundamental physics is the whole idea of, oh, we're going to 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, you know, are we playing that same kind of game or do you feel that there's more... Im not going to say substance because its all substantive.

Oh yeah.

That's right.

Yeah.

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

I, you know, I think I think I know what you mean.

So I think these are it's again, it's kind of funny to say it, but the idea of tiny black holes formed the Big Bang.

Yeah.

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.

Because we know black holes exist, lets start there.

Correct, and we and it only require the standard, the normal matter, the standard model particles.

No new stuff has to be invented.

They could be, but they're not required anymore.

Yeah.

Right.

Right.

So that's what's kind of.

So it's it requires the least new ingredients.

Yeah.

Yeah.

Took us 50 years to get there.

Or 40 years.

And so that doesnt mean its right.

Right.

But as a kind of thing to kind of bet on with your pencil.

Right.

So what is that worth.

Another few months of digging in?

I think it is.

Yeah.

Another thing thats awesome is, is that the way in which we can try to learn more about where they're not, right, to put that, make that box, where it has to fit within.

Often that comes from piggybacking on measurements being done for other reasons anyway.

Right.

So to date, we have not been building our own massive, you know, infrastructure.

We're 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.

That's right.

Yeah.

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.

Yeah.

We have luckily.

But you know it's not the kind of capital outlay.

Yeah.

Right.

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.

Yeah.

Right.

And so now we have this dark matter problem and they might conveniently, fit into that box.

Are there other, cosmological astrophysical problems that these primordial black holes could resolve?

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, and 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.

Yeah.

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

Right.

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, theyre that early they didn't have time to gobble up.

They couldn't accrete, at least in our best understanding of of stellar processes.

And, and so, 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 massive formation would be maybe a thousand 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 for astrophysics, the rate at which the star would grow, or the black hole would grow by gobbling its surroundings, that's now much, much more straightforward to say.

Why could this thing be a billion times the mass of our sun at such an early time?

So a lot of work, this motivating primordial black hole research these days would be not even about dark matter.

Yeah.

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.

Right.

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.

Yeah.

Right.

Yeah.

Yeah.

And you know, but early life it was poisonous to oxygen.

So if you think, oh wait, we need this 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.

Right.

So 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, and the black holes that formed formed differently.

They weren't star first.

Exactly.

Yeah.

Yeah.

Exactly.

Right.

And honestly, just stepping back is one of the things I love about this topic.

Many things I love about it, 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 the quarks behave because quarks were it in the early universe before they were protons.

That's not something I 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, you know, what about gravity?

So for me, it's been just a feast of new ideas.

New to me ideas.

Yeah.

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.

Yeah.

But here's some weird clues from all over the map.

Yeah.

And what can we do to to kind of put that jigsaw puzzle together?

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-- When you're right, there are skeptics, right?

Fair enough.

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

Yeah.

But the kind of dismissing it as not even a good question?

I think that really is, is, I don't hear that anymore.

So let's talk culturally here because what I've observed over the course of my career is there is a bandwagon phenomenon.

Yeah.

In, in science.

And sometimes, you know, there are strong personalities, you know, and so people move around, right?

They go from WHIPs to axions.

So if you look at where the energy, you know, or modified gravity.

Yeah.

Yeah.

That's right.

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?

Its, not a bandwagon, certainly it's an enthusiastic community.

Yeah.

All enthusiastic, which maybe to an outsider looks it like a bandwagon might be both.

Right, right.

It is, it is.

And so the number of young, amazing, super skilled young people, I have undergraduates, you know, at MIT work with me on some these projects amazing.

Lots of grad students, postdocs.

They're not just MIT.

Yeah.

So, you know, we have annual meetings and the number of attendees grows every year.

And there are people who want to attend or who propose to come.

Yeah.

Grows even faster.

This year because of, you know, calendar.

We have two specialist meetings happening on just the same topic, the same week in different cities, by accident.

But the point is, theres that much interest.

Thats enthusiasm.

Yeah.

As not to say we're right, but saying this is this is a real question.

We have better and better ideas for the next questions.

I love it, love it, love it.

All right.

So dark matter might have been black holes all along.

Could be could be.

So if that's the case then you know we know a lot about black holes.

Much more than we know about those weird particles that were thought of.

That's right.

So that means that we have a chance of finding them.

So how can we go about doing this?

It's super fun.

And, it's it's a lot of it's an area I focus on now with my own research group.

Many people think about this now with, with increasing, attention.

So there are kind of two clusters of people tend to think about two, two kinds of signatures to kind of look for, signals.

One is through their gravity.

Yeah, right.

We know we infer dark matter in the first way because it affects the motions of things, invisible things we can track.

So what kind of gravitational signals should we be looking for on the one hand.

Yeah.

And the other hand actually has to do with particles again.

So these black holes are so massive and so dense that light can't escape.

That's why we call them black holes.

Yeah.

It cant escape directly.

But we know again from people like Stephen Hawking and many folks since then, that these black holes should also be very weak emitters of their own radiation.

Not something came out from from, from one side of the event horizon of the other, but very subtle quantum mechanical effects just near the edge.

The boundary of black hole.

Yeah.

Should leave the effect of as if the black hole itself were in effect radiating.

We call that Hawking radiation.

Right, right.

And that leads to again, kind of like, particles of light and other kind of, cosmic ray particles to go look for.

So what we're trying to do typically is these kinds of things for the current universe today.

How do we catch these black holes in the act if they're all around us as dark matter?

Looking for certain kind of gravitational wobbles, and even best of all will be combined with certain kinds of particles detected with some other machines.

That's our goal.

Okay.

So as I understand it, what we observe now.

So a black hole is a very small localized thing.

But the effects that we call dark matter are typically on these larger scales.

Like it shows up and, and at a certain size scale it doesn't really affect us at this small scale.

Yeah.

So where do you search, at the small scale or at the big scale where we already see signatures of dark matter?

All of the above.

So so one can do all kinds of careful studies with things that are already measured very well, like gravitational waves, like the cosmic microwave background radiation.

In fact, one of the projects I got going with some amazing younger colleagues.

Is it lensing of the CMB?

Not lensing, but people can use that too.

Okay.

But actually it's about spectral distortions, which is very fancy.

In what spectrum?

The black body spectrum in the CMB?

Exactly right.

Could it be little tiny departures in the spectrum of the of the cosmic microwave background that could have occurred because, the distribution of stuff was not quite as smooth as people would have thought there'd be slightly more energetic areas of the sky here than there.

You would need that for black holes to have formed.

The length scale, where that would have happened is tied to the mass of the black holes it formed.

Right.

So we can use the lack of these measured distortions, say, well, the black holes, if they formed, couldn't have been this size or that size.

So we can do that kind of thing, put them in again in a box.

For the really tiny ones, the ones that I think, that are so exciting, these asteroid mass ones, we have other kinds of things to look for.

One thing that I got very excited about with, again, some terrific coauthors was to say, Well, what about how many of these things should be around our solar system? We know there should be a bunch in our galaxy if they're the dark matter.

Yeah, but the distribution throughout the galaxy is not uniform.

They'd mostly be near the center of the big massive concentration, very dense concentration of whatever the dark matter is near the center of a galaxy.

And we don't live in the center.

We're off in the suburbs of our of our Milky Way galaxy.

Our solar system is way off on one of the arms in, frankly, kind of nowheresville.

Right.

So the density of dark matter, whatever it is, should drop off as we get closer to our solar system in a way that astronomers can measure.

They don't know what it is.

They know how much and where to to pretty good accuracy.

So we can put in their best numbers for how much stuff per volume is a dark matter really locally, near our own sun, near our planets, and it turns out to be equivalent of something like roughly the mass of one hydrogen atom.

Per cubic centimeter.

But not a lot, right?

So like, that's not a lot of, you know.

Yeah.

And so if the dark matter is some other tiny elementary particle that has some small mass, youd say, okay, there could be a lot of them buzzing around.

Yeah.

That's the mass per object.

If the dark matter is all or most these very much more massive black holes, you need many fewer of them, right?

To add up to the same amount of mass per volume.

Right.

So the number per volume falls, as the mass gets big.

So put in some numbers.

Yeah.

If we think these are in the so-called asteroid mass range, these tiny primordial black holes, then the numbers you get are something between like 1 in 10 kind of around all the time in a sphere.

The size of, say, the planet.

Jupiter's orbit.

Its very sparse.

1 in 10.

Roughly on that order, and they're zipping around it to 200, 250km a second, ten times faster than typical solar system speeds.

And that means they should be kind of passing through the solar system once every, let's say, 1 to 3 to 10 years, right.

Not so rare.

Not like if it had waited a century.

Not like it happens every microsecond.

It's a kind of delightfully human scale to imagine.

Okay, we could look for that.

Could it be right?

Yeah.

So one thing we did a project with at the time, an undergraduate named Tung Tran, he's now a PhD student in physics.

He was an undergraduate MIT the time, Sarah Geller, who's a very dear friend.

She was a PhD student.

Now she's a postdoc, Ben Lehmann, postdoc.

The four of us said, well, what would dark matter detection look like?

Right?

If the things we care about are these tiny black holes and not some new exotic particle, that's the first thing we did was say, how many should there be in our local solar system neighborhood?

1 to 10 ish, right?

Right.

Give or take.

And then what would they do?

Well, they're mostly not going to be in the same plane as all the planets orbit.

They're going to be all over the place coming from any direction in the sky.

Typically, or not nearly so similar in their direction, which is mostly they're going to go on these joyrides, they're going to cut through the solar system, not, you know, go straight up.

They're going to be like, Oumuamua.

They could be like Oumuamua, that's on our minds.

That's right.

Yeah, exactly.

Right.

On their own path, on their own path at high speeds, even faster than Oumuamua, it turns out.

So what would happen?

They don't have to hit anything.

So these things are going to go on a joyride, kind of cutting through the solar system at some angle, right?

Typically at some high speed.

Yeah, with a whole lot of mass.

But very little spatial size.

They won't hit anything.

Yeah.

They're going to pass through.

Right.

But you know, even from Newton's physics, let alone from Einsteins fancier physics, you have a lot of mass moving at a high speed.

It's going to perturb is going to affect the motion of things nearby.

Yeah.

So we began saying what are things that we can see in the, in the sky in our solar system that we track really, really well?

Can we see them off, off the path they should be on?

The one we fastened on at the end was the planet Mars.

Okay.

Our red, our red neighbor.

Because of its, humongous orbital eccentricity?

It is not humongous.

I know its not humongous.

As you know everything is damn near circular, right?

No.

That's right.

No.

Even taking that into account, let's let it have this beautiful, you know, orbit with on its ellipse, taking all the fancy corrections from relativity from other, corrections.

People can track the Earth, Mars distance with outrageous accuracy, partly or largely because we've been sending stuff to Mars for decades.

And they have radio transmitters.

We do telemetry back and forth.

So between the orbiters and the rovers, the landers, we have 20 plus years.

We as a community, I don't do it, but our colleagues do.

Humans can now track the Earth-Mars distance with an error margin of something like tens of centimeters.

Mars is not so close by.

No, it is not.

And we know the distance with an error, our uncertainty of much less than one meter.

I find that astounding.

So then what if.

Now let's play that scenario.

Yeah.

What if some little black hole.

Right.

Is far away, even from Mars?

Maybe it's as far from Mars as Jupiter is, you know?

So zips by at some angle with some large mass and high speed.

It's going to set Mars rocking a little bit.

It's not a disaster movie.

It's not Mars falling out of the sky.

It's going to make Mars wobble in its orbit in a predictable pattern.

I see.

That will exceed the uncertainty with which we measure the Earth-Mars distance.

It could wobble by half a meter within a few weeks, within a few months, and so on.

So now you start saying, well, we track the Earth-Mars distance so carefully with such great precision.

Does Mars ever get kind of off where it should be?

Oh wait, one particular one observation is not going to tell you.

Of course not.

Of course not, no that's right.

So that comes into other what would that be combined with.

So I guess the statistics.

How do you, how much time do you have to do this for a century?

No, not necessarily, no.

If they're really a transits every 1 to 3 years.

Yeah.

You get a couple of them, you know.

Right.

I mean, I feel still sounds like too low of a number to me, just to convince anyone.

No.

You're right.

So that alone should should not be convincing.

They'd be right to be skeptical.

Right.

But that'll be awfully intriguing and a reason to dig in further.

So one wobble.

What, what else we can do.

From that wobble, there's a very specific pattern of the wobble to Mars.

If there were a transit such that even this fantastic undergraduate with a laptop, nothing super fancy could reconstruct the path of that perturber.

Say, okay, because of the pattern that Mars is wobbling and we now know the thing, whatever set it in motion should be there now, right somewhere else in the sky.

Let's come back to things like Oumuamua.

Yeah.

Our friends with optical telescopes are very good at finding space rocks.

Yeah.

If it has the mass that we because we we can we can figure out the mass and the path right within some, you know, error bars based on the, on the observed pattern of something like Mars, because Mars is tracked so well, you go through and say, okay, do you see some some object that reflects some light, like an actual rock, like a meteor or an asteroid?

It should be there, right?

The Oumuamua, it turns out, is much, much smaller in mass, than these perturbers would be if they're the dark matter, if the black holes are the dark matter.

And yet the astronomers could track even something as relatively small as Oumuamua.

And dark.

It's that's why it's not lighting up, right.

But it's reflecting, say the sun's light, it has what we call an albedo.

Right.

The black hole would not.

Right.

So another check is if you look there, do you really, really see nothing?

That's still not proof it's a black hole.

Right.

But it's getting further okay.

Yeah.

Now you start combining with other stuff that we care about.

Right.

These black holes I began to say a little while ago, should also be emitting in a very particular way, their own particles.

There's something called Hawking radiation.

It's a quantum mechanical effect.

Again, in the 50 plus years since Hawking figured it out.

As I recall.

Yeah.

A black hole has a temperature associated with it that is relative to its mass.

And the smaller the mass, the hotter the black hole.

Exactly.

Right.

So does that make it detectable?

It really could.

That's great.

And so that's something to go looking for.

So let me back up and say this Hawking radiation, absolutely fantastically brilliant theoretical prediction.

Never observed to date.

We're trying.

Yeah.

Any black hole should have an associated temperature, at least according to Stephen Hawking's and others, you know, calculations since then and as you rightly say, the temperature of that black hole varies like one over the mass, right?

Big mass, low temperature, and the opposite.

Why does that matter?

It's a lot easier to find stuff coming off a very hot, bright source than a cold, dim one.

We know that just from light bulbs and all these things.

So the Hawking temperature of a black hole that has the same mass as our sun or a little bigger would be so cold, we will literally never be able to measure the Hawking radiation.

It's cold, exponentially colder than even the background photons of the CMB.

So Hawking radiation is awesome.

Prediction.

It's beautiful.

People gone over it and clarified over the intervening time to really see it.

You'll never see it from stellar collapse black holes.

You'll never see it from supermassive black holes or even colder.

And for this effect, the only hope to ever see it would be a smaller mass black hole in this asteroid mass range.

Those would be above the background temperature so that they'd be smaller mass hotter, emitting more things more efficiently.

And we can calculate that now really pretty carefully.

So I work with an amazing PhD student, Alexandra Klipfel.

What she and I work on all the time now, to make predictions, as do other people, of course.

So for black holes in the mass range, we care about where they could really be all the dark matter as far as we know today.

What would their Hawking emission be like?

So most of them will be emitting very dimly some photons.

So the only thing that could come out will be photons, and theyd be pretty low energy and very rare.

Right.

That's tricky because to collect them you need a, you need a, you know, a large collecting area to have a chance to see them.

We know the pattern.

We could say if-- Hows the temperature compared to the CMB is it is it hotter?

Much, much hotter.

These would be much, much hotter.

Yeah.

So for the very tiny mass ones we care about, the temperature would be way above the background, which helps a lot.

In fact, then you can do it.

This is something Alexandra's a real expert at.

Take various real experiments, real telescopes in space, on satellites on the ground, this whole range.

Different frequencies which are sensitive.

What are their own background counts, not just the CMB, other sources, you know, sky glow, all kinds of things.

It gets really complicated for the astronomers.

Yeah.

And you can say well above that, could you expect to see some some signal above background with what confidence and there are scenarios you really could.

Yeah.

That's the kind of thing we're working on.

So what you'd love to see in general would be both.

Right.

Yeah.

You'd love to see Mars wobble in this particular way.

Right.

And could you catch some of these Hawking emission, these kind of cosmic rays.

Yeah.

Then you have a really, really tight case.

You don't need to do that a thousand times.

That would not be explained by some space rock.

It wouldnt be explained by some other sort.

You get those two effects together.

What about the rate at which these, so the calculation of how many there should be.

Yeah.

I imagine you start off with some population.

That's right.

That population decreases over time because they're coalescing.

They're falling into things.

Yep.

Yeah yeah.

And so now we're 14 billion years past, and you know so in the vicinity of a supermassive black hole, you know if they're like raining down on it or something like that, you know, are there signals that can just be created by those, the, the sweeping up of them?

Yeah.

Yeah, there could be.

So one thing again, we've worked on-- Would it be gravity waves?

It would exactly.

That's exactly what one thing we're looking at, exactly right.

So this would be different than the gravity waves from the fact that these things formed.

That's one thing we will get hopefully with Lisa and other probes.

Yeah.

But differently would be let's say we have these things just dancing around the universe.

Yeah.

Interacting with some other large mass.

In fact we showed, even interacting with, with just our own sun or a kind of mundane star, let alone more fancy things like pulsars or neutron stars by different rates to encounter them.

Right?

Yeah.

Stars like our sun are more typical.

There's more of them.

Right.

And so you want to figure out a bit bigger.

But but but you know, things like for the mass of a neutron star could be a larger mass but more rare.

Right.

So you develop.

So these things could indeed capture a very tiny mass primordial black hole that could capture them over cosmic history.

Right.

And that would lead to a particular ringing, a pattern we can calculate of gravitational waves depending on when it happened, how far away from us would tell us the frequency today we should go look for.

And that spans a wider range of detectors.

Some would be higher frequency than we're sensitive to, some would be lower.

Some are in the sweet spot for next generation.

We have to wait, you know.

Right.

So so that would be a process that should have been happening naturally throughout a long span of history.

That's right.

So have we actually started running any of these tests to detect them?

Has anyone ever tried?

We, were getting closer.

Well, I think what we're in now is we're using existing data sets that were collected for other reasons with which we can, place constraints.

Yeah.

Well, if there were this many black holes of that mass, you'd see this.

You don't see it.

That's super helpful.

Things like we were talking about a little before the Voyager probe.

Right.

One thing that's really good at is measuring electrons and positrons.

If those were emitted as Hawking radiation from a population of black hole of tiny black holes that are the dark matter, then outside the sun's magnetic field, outside our solar system, that should see a predictable counter rate of, let's say, charged particles.

It sees a much lower count rate.

So it's okay.

So that means they could be black holes.

But if they were, if the black holes were a little larger in mass.

Yeah, slightly lower Hawking temperature, they would be emitting fewer and with less efficiency.

Right.

That's what help bound you know what the box within which these things have to fit.

Right.

Right.

So if you're going to get a dedicated experiment, right, you know, we talk about, oh, we're gonna need hey, ultimately at some point you send in a proposal and somebody is going to fund it, either, you know, from the government level or who knows who knows who, right?

Yeah.

That's right.

So what threshold do we need to cross before you think someone is going to be willing to invest in a mission?

Yeah.

To discover these.

Like where do we need to get.

Yeah, hopefully.

Hopefully were at or near the spot, where were thinking about these proposals right now.

Again, this amazing colleague I mentioned, Alexandra Klipfel, another dear friend at MIT, Peter Fisher, were actually writing these kinds of proposals now.

We're not the only ones.

Right.

So pie in the sky but you know, but we think there's enough motivation for the idea.

Right.

And we can do it with existing technologies.

That's something that again, Alexandra and I show.

Yeah.

So the Voyager is sensitive to these things and that's 1970s technology.

There is an amazing experiment measuring cosmic rays on the space station called the AMS, the Alpha Magnetic Spectrometer.

AMS.

Yeah.

Amazing instrument.

Alexandra and Peter, I showed that technology, which itself is now a couple decades old, decade and a half old for how they track particles.

That would be capable of measuring a fly by if if it were not so close to the Earth's own magnetic field.

So, you know what its making it sound like man?

It's making it sound like there's going to be a relatively cheap experiment.

That's the hope.

It's not like the Webb telescope or it's not that or the Large Hadron Collider.

Right.

That's exactly in fact, what we're looking at now.

So what we've been doing so far are we collectively, not just my own little group, but really these folks who are inside of this, we've been kind of piggybacking on data sets collected by other folks for other reasons.

And you can do a lot of really creative stuff with that.

That's great.

So so first of all, I should say the Rubin telescope is astonishing.

Absolutely.

They're finding something like 1000 new asteroids per week, maybe per night.

I've lost track.

Yeah, crazy.

I mean, that is so, so it can find it can study deep in the past, faraway things.

It can find things literally in our neighborhood with precision that we couldn't have imagined, not long ago.

We can use that in other things.

It wasn't built for us, for this.

But but again, if there's black holes zipping by every 1 to 3 years, yeah, it should make asteroids wobble.

If you're tracking asteroids to unprecedented precision.

True.

Close, you know, like what you do with Mars.

So then we can use those publicly available data as well.

So we're staying tuned to those things.

So are there any alerts in that data that might give you because because in order for an alert to pop up, someone has to do the calculations to say this is something needs to be followed.

Exactly.

Right.

That's so that we don't have that pipeline set up yet.

Okay.

Inspired by our friends with Ligo and other folks they have they literally call it, you know, triggers alerts to go look.

Yeah.

And so I understand a bit better from my nearest neighbors at MIT and other colleagues what that infrastructure is like.

Now for this challenge, it's pretty tricky because you have to compute the motions of well known objects to really, really high precision.

Yeah.

And that means more than just, you know, imagine the sun and Mars, means taking into account the perturbations.

What else is tugging or pushing on Mars from perfectly mundane reasons?

How about even the fact that Mars is not a point?

And so it has some radiation pressure.

It has some surface area.

There are groups in the world who do that really, really well.

A few of them.

And so we've been in contact with one of these groups.

The Paris observatory, and we've had very fun and friendly preliminary discussions.

Maybe we could work, team up with you, work with their expertise on that level of modeling the fine motion of objects like Mars.

So you can say, okay, it wasn't this.

Wasn't it, is it?

What could it be?

So that would take, again, a bigger team a bigger group, right.

One thing we'd like to do more close to home with people like Alexandra and Peter is say, what if we then design our own optimized instruments?

Yeah.

Space based.

Right.

But I think they could be maybe not literally a CubeSat, but a small satellite, not nearly as big as James Webb or Hubble or these, you know, amazing instruments.

That makes it cheaper in general, cheaper to make, cheaper to launch in relative terms.

And what we'd need would be basically really fancy clocks, right?

Like like atomic like atomic clocks.

Yep.

And those are becoming more accurate, smaller, cheaper, less weight.

Right.

So were learning more about that, right.

You need fancy clocks to be able to do things like ranging like where are what's your location?

How can I tell your location?

That really comes down to timing as much as anything else.

Super careful clocks and those exist.

We could do we think we could do off literally off the shelf clocks.

You might need things like the laser ranging, but people know how to do that really well.

Wed learn, wed team up.

And then you need some way to detect, stray light and or stray charged particles.

These cosmic rays.

And so and we know a lot of how to do those.

So and we'd ideally have more than one of these things if they're cheap enough per unit.

Right.

Let's launch maybe not a fleet, but yeah.

Oh or 3 or 5 would be nice.

Maybe ten because then you could do correlations, right?

Right.

If one thing wobbles.

But if two things wobble-- Like having two gravitational waves.

It's exactly like that.

That's right.

So you could do the gravitational detection with more confidence and you could figure out the signature of these particles.

Well, so if you do find one there's still a gap between we find that these things exist and these things are sufficiently populous to solve the dark matter problem.

We would.

And so that would come back to something you were asking about before, the statistics.

Right.

So if we really have, reliable means of detecting and we are able to do this for 10 or 20 years, which is not unusual duration of these things, right?

Then you start saying, well, did you see enough of these events?

Seeing one is awesome.

First of all, let's just come on.

I'd love to see one.

Oh seeing one is great.

Right.

Because then because you know what you get by seeing one?

A Nobel Prize.

You know what you get intellectually for seeing one?

I haven't booked the Airbnb yet, but what we get intellectually, conceptually is there is a second route to make black holes that nature has used.

Third or fourth!

There are multiple paths to make black holes.

I'll agree with you.

And Hawking missions real.

I would love to know that.

I would love it would be.

That's one of the only examples we have that we really think we understand so far.

Yeah.

Where quantum theory and gravity could start to play together.

Man, it sounds like two, two Nobel Prizes, you know, if not three.

So what if it happens?

If we don't detect any?

What if we built very careful experiments and it turns out there are no microscopic primordial black holes that we have detected.

I think if you run those experiments for a few decades and we see nothing, I think it's a win win.

First of all these things, again, I think these experiments would be considerably less expensive than other experiments.

Yeah, but I'm not saying we shouldnt invest in the others.

I'm saying that hopefully this is something we as a group, as a society can hopefully afford.

Yeah.

Number two, we learn either way.

Either we found these amazing objects.

We've only been speculating about for a long time.

Right.

Or we don't.

Then we've also learned something about the conditions of the early universe.

And so on, there's a little wiggle room in between.

If someone's really devoted to these things, as I tend to be.

Okay, maybe these tiny black holes are clustered.

And so the odds of having one pass through the solar system are not what you calculated, because they might be hiding out somewhere else.

What about primordial black hole galaxies?

A dark matter only galaxy that consists of primordial black holes?

I assume it would be a dwarf galaxy.

Very light.

Do you?

Is that something that is proposed, or would they have coalesced into one long ago?

Not necessarily.

So again, I have very dear colleagues looking, pursuing that.

It's a great question.

And again, with all the new telescopes that we have now, we can find out many things.

So there are so going back to the thing you mentioned before, what if a typical star swallows up on these tiny black holes?

Yeah, that will change very subtly.

It turns out you can change the evolutionary path of that star.

And so that should change how the star would look later, right?

Change the color.

All blue stragglers.

That's right.

Yeah.

They actually often called red stragglers.

They are.

They are called red stragglers.

Yeah.

Because they're off the typical evolution.

Right.

There are a couple of these kind of anomalous things found in one of the, dwarf galaxy.

Drago, something or other.

I'm not the astronomer, but it is a tiny dwarf galaxy.

It is assumed to be highly dark matter.

Rich, right?

And it has more than typical number of these anomalous things.

Oh, is that right?

So the dark matter rich dwarf galaxy has more stragglers.

That's my understanding.

Exactly.

That is interesting.

That's pretty cool.

And so now that doesn't of course-- That doesn't say anything.

But that is interesting.

But that's awesome.

And so now we get more and more data, more and more reliable statistical data on things like that that don't look like our Milky Way galaxy don't look like Andromeda.

Yeah, right.

So I have, again, very dear colleagues who are who are chasing that down.

Yes.

So what really would the capture rate be, which stars really capture enough black holes, would there be a nudge, and we have enough theoretical understanding, we think, computing power to say, let's really model how that star behaves.

So that's that's another thing to go look for.

And it's intriguing.

It's not proof but boy it's a hint that's worth, you know, sitting with.

Dave.

This has been amazing.

I accept your job offer.

You'll start on Monday, thats great.

Well, you know, it's got to be remote.

Yeah.

But, man, this is fascinating work.

Thank you so much for coming in.

Great.

Dave Kaiser, thank you so much.

No association with the medical facility?

Correct.

Yeah.

You have been awesome, sir.

Thank you so much.

Thanks for having me on.