Particles of Thought

Hakeem Oluseyi:

We've known for a while now that the universe is filled with mysterious dark matter. But how far have we gotten on the path to discovering what this stuff might actually be? Maybe further than you think. My guest today is David Kaiser, physicist and science historian, and he's gonna break down how we know this stuff exists, the state of the dark matter search today, and a possibility that has some scientists really excited.

We are gonna talk about some amazing stuff today, man. I'm so happy to have you here. We're gonna cover dark matter. We're gonna cover black holes. But, before I lean too deep into your theoretical physicist brain, we're gonna go into your historian brain, 'cause dark matter has this amazing history. And I know what dark matter is. Okay, I know what we mean when we say dark matter. Nobody knows what dark matter is. Maybe you do, maybe you'll break the story today.

David Kaiser:

I have ideas.

Hakeem Oluseyi:

You have ideas.

David Kaiser:

I got favorites. Thank you. It's a great, great topic. A long history to the topic.

Hakeem Oluseyi:

Yes.

David Kaiser:

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.

Hakeem Oluseyi:

Yeah.

David Kaiser:

There were astronomers including Fritz Zwicky, who was a Swiss-American astronomer working mostly 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 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 he 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 energy of motion. These things having a fast speed. Why don't they just zoom away from each other? And the mass that he would infer from all those galaxies acting through gravity to keep them stuck together. So this 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.

Hakeem Oluseyi:

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

David Kaiser:

Exactly right. That's right. 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 free to wander through the cosmos alone, and they appeared not to be. So he says, as kind of an offhand remark, he says, 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 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.

Hakeem Oluseyi:

Sounds like a discovery of, which one was it? Neptune.

David Kaiser:

Very similar, that's right, yeah. So you infer gravitational effects from things you can track through light. It was 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 think about is Vera Rubin. Amazing American astronomer with her longtime 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 was 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 close by galaxy. And they were studying individual stars within that galaxy. 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.

David Kaiser:

And so, Vera Rubin and Kent Ford figured the same would be true of stars in an individual 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 correspondingly 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 been 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 careful in these measurements, more than once. That would be consistent with the idea, still a hypothesis, that maybe there's more stuff that's acting through 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.

Hakeem Oluseyi:

A bigger blob, so it's wrapped around the galaxy.

David Kaiser:

Exactly, and then you would expect the speeds to level off and not fall off the way people expected. Okay, now I wanna pause there. Again, that didn't alone convince the whole community either. But now you see something where someone like Fritz Zwicky's finding this on a scale of 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 this similar phenomenon across such a wide range of scales, length scales. Huge change and yet looked kind of consistent. And then one of my favorite examples comes from much more recent times. Why we really, really think this is the stuff filling our world right now. And that comes from the remarkably subtle patterns in the cosmic microwave background radiation, the CMB.

Hakeem Oluseyi:

Man, I thought you were going for gravitational lensing.

David Kaiser:

We'll get there.

Hakeem Oluseyi:

You went way complicated now.

David Kaiser:

You know, it's not complicated. It's beautiful.

Hakeem Oluseyi:

It's beautiful, I agree. But remember, intro, intro level, right?

David Kaiser:

This is the earliest light in the cosmos. All right, let's pause and enjoy that for a second. It's amazing, as you know. And so astronomers, again, really with great skill since the 1990s. Now it's 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 light that we receive is almost entirely uniform, the same energy from all directions of the sky, but with little ripples about 1 part in 100,000. And you can actually do a kind of careful analysis of the pattern of those ripples, the heights, you can make a kind of, we call the spectrum. And the pattern of those tiny bumps and wiggles should have been sensitive to the stuff filling the universe when that light was emitted. So it tells us like the kind of ingredients list. It's like a cookbook, 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've measured.

David Kaiser:

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

Hakeem Oluseyi:

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 millions of light years. Now you have the cosmic microwave background radiation, which is another scale in time, another scale in size.

David Kaiser:

In length.

Hakeem Oluseyi:

So how do you characterize-- the CMB is short for that light, right? Cosmic Microwave Background. So how do you characterize that scale?

David Kaiser:

Yeah, it fills the entire sky. So it's the entire observable horizon, even bigger than any individual cluster of galaxies. 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 can start simulating what we call large-scale structure, the distribution of stuff throughout the entire cosmos. Some of it's very densely concentrated. Lots of matter and activity and energy flow, and huge voids.

Hakeem Oluseyi:

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, matter was uniform, but then structure built at these super large scales of hundreds of millions of light years that we call the cosmic web.

David Kaiser:

Exactly, yeah, and so I often say gravity is the most aristocratic of forces, right? The rich really get richer, right? So 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, slightly more mass here than a neighboring spot, but in parts of much less than a percent. A fraction of a percent. And just as you say, over time, the parts that happen to have a little bit more, through gravity, start attracting more and more. 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 and paper, it works now really impressively with these computer simulations, to build universes that look like ours today. In the sense of having this huge range of scales, a hierarchy of, of structure. So massive concentrations here and sort of huge empty voids there, the cosmic web. Okay, 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.

David Kaiser:

The structure will either not form or you have different statistics of it. 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 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 beautiful desk and all the rest of the stuff we're here to do.

funder

Hakeem Oluseyi:

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program

David Kaiser:

So lots and lots of examples 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.

Hakeem Oluseyi:

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

David Kaiser:

Exactly right. And it certainly didn't have to be.

Hakeem Oluseyi:

Sounds like quite the coincidence.

David Kaiser:

No, or maybe it's just telling us. And so now we say with great confidence, Oh, the universe is filled with dark matter. We know how much per volume compared to-- We can say that now, but that was not so obvious before.

Hakeem Oluseyi:

Well, okay, 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? So in most cases, extra gravity means extra stuff. So we don't actually see stuff.

David Kaiser:

No. Perfect, that's right. Exactly right. No, what we see, what we infer, as you said in the beginning, 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 see what we assume are the effects of that, tugging and nudging the motion of stuff. So 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 needs study. That really came together as even a question only 50 years ago. I say only because some of this evidence was coming in long before that.

Hakeem Oluseyi:

That's younger than me, man. And I'm young as hell, so.

David Kaiser:

Well, that's a separate episode we'll cover by the way, Hakeem. Young at heart. I feel the same. So in our lifetimes, which is actually kind of amazing–

Hakeem Oluseyi:

Absolutely.

David Kaiser:

This is energizing so much of 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. 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 theoretical checks.

Hakeem Oluseyi:

Let me just put a point in that. What you basically just said is, if we assume gravity as we know it, and gravity as we know it is Einstein's general relativity, and we have tested that sucker throughout a lot of scales.

David Kaiser:

Exactly right.

Hakeem Oluseyi:

And it works every time.

David Kaiser:

We have reason to be confident. Yeah. But what people are rightly saying is, could dark matter be the first 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 will look normal if we had different laws of gravity.

Hakeem Oluseyi:

Right.

David Kaiser:

That's the sort of setup. Often called modified gravitational models. And you know, that's, again, that's 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, very smart people have worked very hard at this and still do. Sometimes they'll come up with a very cool model, an adjusted set of laws of gravity, and it can make the one kind of physical system make a bit more sense. Maybe we don't need dark matter. But then it tends to break when we apply it to these other things. That's why I'm so impressed, 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.

David Kaiser:

And so, so far, doesn't mean that this won't ever work out, but so far, at least my understanding, is 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 gotta shoot for the moon, right? So that work goes on, and it should go on, but that's one option that's still a live option. This is why we're not done.

Hakeem Oluseyi:

Okay, all right, so let's look at these 'dark matter is stuff' options. And you know, I was teaching observational astronomy back at the turn of the century. And you know, I used to talk about supersymmetric particles.

David Kaiser:

Yes, you did.

Hakeem Oluseyi:

You know. And we used to talk about WIMPs and MACHOs and these sorts of things. 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.

David Kaiser:

Or at least, 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.

Hakeem Oluseyi:

Okay, so now what are, you know, what have we considered and discarded? What remains? And are there experiments that can find— Oh, and I must mention that my very first physics experiment ever, research ever, was in the basement of LeConte Hall at Berkeley with Bernard Sadoulet at the beginning of what became CDMS. That's where I first did physics.

David Kaiser:

Yeah, no, so that's right. 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.

Hakeem Oluseyi:

1991.

David Kaiser:

Who's counting? What is time?

Hakeem Oluseyi:

What is time? Yeah.

David Kaiser:

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. And that's still the most popular answer. Though that doesn't mean it's right. And just as you say, the candidates that most people will kind of vote for are shifting. 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 were saying, we have all kinds of candidates for other reasons, for other theoretical modeling ideas. The universe, they thought, could be chock-full of all kinds of particles beyond those we now call the so-called standard model.

David Kaiser:

So the standard model is— a remarkable achievement. I always say it's the most impressive boringly titled theory in human history. If you get a committee to write the name, they got—

Hakeem Oluseyi:

Standard model.

David Kaiser:

It should be like the outrageous, unbelievable, super cool— I mean, it's amazing.

Hakeem Oluseyi:

Yeah, it's amazing.

David Kaiser:

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

Hakeem Oluseyi:

Mostly these large accelerators, right?

David Kaiser:

Mostly huge particle series. Smash stuff together and see the stuff that comes flying out, exactly. With increasing 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 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.

Hakeem Oluseyi:

Fame.

David Kaiser:

These two enormous groups based at CERN, international collaborations, announced they had really found, conclusively found, the Higgs boson. The last missing piece of this beautiful standard model. And people were poised, saying this machine works so well, we're just gonna find all the other stuff, right? We'll 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, you'll find it here. And people were really excited. The machine was working great, still working great.

David Kaiser:

And what's happened in the years since 2012 is a lot of kind of hurry up and wait. And so the machines are working great. The teams are dedicated, doing extremely precise science. What they haven't yet found is any evidence at all of anything beyond the standard model.

Hakeem Oluseyi:

And these are particles— some of these particles that are beyond the standard model might be the dark matter that people are looking for?

David Kaiser:

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 it the miracle. These particles were assumed to exist for other reasons and then could also play the role, kind of like straight out of central casting, perfect for dark matter. They're not there. 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. 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. What there's no evidence for at all is a single particle, as yet, beyond the standard model.

Hakeem Oluseyi:

So that means that we've given up? We're out of particle— we're out of options?

David Kaiser:

We haven't given up. There's a lot more to explore and some extremely dedicated colleagues We just continue to explore. But what it's done is it pushed the kind of obvious answer. Seems a lot less obvious right now.

Hakeem Oluseyi:

So now we've gone through all these potential microscopic particles. But there was also an idea that, oh, it could be like Jupiters out there, MACHOs, and they searched for gravitational or microlensing, right?

David Kaiser:

That's right.

Hakeem Oluseyi:

And how did that turn out? I haven't paid attention since I was in South Africa 15 years ago.

David Kaiser:

So the short answer is you, I mean, I don't want to say you didn't miss much. People still do the work very carefully. But in terms of the big answer, still no big answer. So the idea was, as you say, these are often called MACHOs, massive compact halo objects. They could be like Jupiters that have a lot of mass, but not so much that they actually became a star. They 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.

Hakeem Oluseyi:

So they'd be dark.

David Kaiser:

They'd be massive, like Jupiter has a lot of mass, right? But not lighting up. That's a cool idea. And so people were doing dedicated searches, observational searches, starting really in the 1990s with real focus. And it's very cool.

Hakeem Oluseyi:

No pun intended.

David Kaiser:

I mean, a little pun intended. And so you take some of these dedicated telescopes. Some in Australia were being used at the time. And stare at, again, like a galaxy, like say Andromeda. Or some nearby galaxy. They were often doing it for the Large Magellanic Cloud.

Hakeem Oluseyi:

Yeah, satellite galaxies.

David Kaiser:

Near us in cosmic terms, some galaxy nearby. And what's really amazing. I mean, there's so many things amazing about Einstein's general relativity, as you know. One of them, a core idea, is that mass will warp its surrounding spacetime. Fantastic.

Hakeem Oluseyi:

Beautiful.

David Kaiser:

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. So what if you have some large object like Jupiter, right? Some mass 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. Then that should act like a lens temporarily and focus light beams that would have passed us, would never have hit our telescope, but will bend them back towards our telescope. So, we'll see a temporarily brighter image. So, microlensing should make an object that we can see in the night sky temporarily get brighter—

Hakeem Oluseyi:

What are the timescales of that? Is it like seconds?

David Kaiser:

It depends. It depends on how big the object is and how far away. So it's the geometry of it, just like playing with lenses, you know, would be for—

Hakeem Oluseyi:

Okay, so look, so a Jupiter is one kind of smallish dark chunk.

David Kaiser:

Could be, yeah.

Hakeem Oluseyi:

But then you have these dead cores of stars, right? On a scale, you have your white dwarfs that you probably, you know, if it's super old, it was made, right? So you could have a neutron star—

David Kaiser:

Exactly, that's right.

Hakeem Oluseyi:

That's naked and not giving off any light. Or the ultimate tiny object, which is a black hole. And you notice I said these stellar leftover cores.

David Kaiser:

You did.

Hakeem Oluseyi:

Yeah, stars that died and left behind their core in some small compact form. So, the question becomes, compared to a Jupiter, it would be smaller and it would have a much higher mass, right? So how would that change the microlensing signal?

David Kaiser:

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

Hakeem Oluseyi:

You find the lensing first and then you, get out, yeah. But I guess the question begins, the question is, if I have a particular lensing event, I'll use that, no, I'm not gonna use the phrase. I'm not gonna say degeneracies. But sometimes it can be like, okay, it's not just one size and one mass. I can match them up in such a way that they all give me the same signal.

David Kaiser:

That's right, exactly, perfect. And so what they wanna do is not get one bright spot in the night sky. You do this over again. They did it for the better part of a decade 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, we'll find a range, right? And we can still map out the kind of bell curve or whatever it might be, the kind of distribution.

Hakeem Oluseyi:

It's always a bell curve.

David Kaiser:

Well, you know. And so even taking that into account, this group— what's amazing is 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 the temporary lensing. Then they did what they should have done. They took more data for many more years. And guess what? The signal kind of went away. So those had 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.

Hakeem Oluseyi:

It has to be a lot. It has to be a lot. Much more than the luminous matter we see in stars.

David Kaiser:

Much more luminous. If dark matter is mostly these kind of chunky bits that never lit up, like Jupiter-like MACHOs, then they should have seen many, many, many more of these microlensing events than 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.

Hakeem Oluseyi:

Okay, so it's almost like, okay, the particle models haven't been completely eliminated, but—

David Kaiser:

But pushed in a corner, right?

Hakeem Oluseyi:

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

David Kaiser:

That's right.

Hakeem Oluseyi:

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

David Kaiser:

That's right.

Hakeem Oluseyi:

So where does that leave us?

David Kaiser:

So there's a third possibility that gets people very excited these days. What if it's 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?

Hakeem Oluseyi:

What?

David Kaiser:

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

Hakeem Oluseyi:

What?

David Kaiser:

Yeah, yeah.

Hakeem Oluseyi:

So they're far beneath that mass limit.

David Kaiser:

Way below, exponentially below.

Hakeem Oluseyi:

Oh wow.

David Kaiser:

That's right. And so these are called primordial black holes. They would have formed in the very early primordial universe.

Hakeem Oluseyi:

Yeah.

David Kaiser:

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.

outro

Hakeem Oluseyi:

So we know something's out there. We just don't know what. One answer might be something called primordial black holes. Ancient, invisible, and as small as an atom. Well, check out our next episode where we dive in deep.