Babbage: The hunt for dark matter

Summary

Babbage: The Hunt for Dark Matter – Detailed Summary

Episode Release Date: February 21, 2024
Host: Alok Jha
Title: Babbage from The Economist

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Introduction: Unveiling the Universe's Mysteries

In the February 21, 2024 episode of Babbage, host Alok Jha delves into one of the most profound mysteries in modern astrophysics: dark matter. Despite making up approximately 25% of the universe's total mass-energy content, dark matter remains elusive, as it neither emits nor absorbs light, rendering it invisible to conventional detection methods. Understanding dark matter is pivotal for comprehending the universe's structure, evolution, and ultimate fate.

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Understanding Dark Matter: Insights from Fermilab

Don Lincoln, a senior scientist at Fermilab, elucidates the foundational concepts surrounding dark matter. He explains that dark matter was postulated to resolve discrepancies observed in galaxy rotation speeds and the gravitational binding of galaxy clusters. Specifically, galaxies rotate faster than can be accounted for by visible matter alone, suggesting the presence of additional, unseen mass.

“Dark matter is a hypothetical or at least a theoretical idea. And what it does is it solves astronomical problems.”
— Don Lincoln [03:44]

Lincoln emphasizes that over the past five decades, extensive research has led scientists to conclude that dark matter likely consists of particles that do not interact electromagnetically but exert gravitational forces. Despite numerous detection efforts, dark matter has yet to be directly observed, leading to a consensus that its nature remains one of science's greatest enigmas.

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Indirect Detection: Gamma Rays as Dark Matter Signatures

Christopher Carwin from NASA's Goddard Space Flight Center specializes in indirect dark matter searches using gamma-ray observations. Utilizing the Fermi Large Area Telescope (Fermi-LAT), Carwin seeks the characteristic gamma rays produced when dark matter particles annihilate each other.

“So what's dark matter is particles and they're interacting with each other, they might be annihilating, producing gamma radiation, which is very high-frequency light.”
— Christopher Carwin [10:53]

Carwin discusses the challenges in distinguishing potential dark matter signals from astrophysical sources like millisecond pulsars. Despite observing an excess of gamma rays from the galaxy's center that align with dark matter predictions, the ambiguity remains whether these signals are indeed from dark matter or other cosmic phenomena.

Looking ahead, Carwin highlights the upcoming Cosmic Origins Spectrometer (COSI) mission, slated for launch in 2027, which aims to enhance gamma-ray detection capabilities and better constrain background noise, thereby improving the chances of identifying dark matter signatures.

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Space Missions Enhancing Dark Matter Research

The Euclid spacecraft, launched in July 2023 in collaboration with NASA, represents a significant advancement in mapping the universe's large-scale structure. Joseph Aschbacher, the head of the European Space Agency (ESA), remarks on Euclid's role in constraining dark matter parameters by creating a detailed 3D map of a substantial portion of the cosmos.

“We cannot measure directly today dark energy and dark matter, but Euclid should really be the mission that is providing a 3D map of the universe... making it possible that some conclusions can be made.”
— Joseph Aschbacher [15:56]

Euclid's data will be instrumental in understanding how dark matter influences the universe's geometry and expansion over billions of years, potentially paving the way for breakthroughs in dark matter detection.

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Direct Detection: Exploring the Depths of Earth

To obtain direct evidence of dark matter, scientists like Jody Cooley, Executive Director of Snow Lab in Ontario, Canada, conduct experiments deep underground. Snow Lab, situated 2 kilometers beneath the Earth's surface, provides a shielded environment that minimizes interference from cosmic rays, allowing for the sensitive detection of rare dark matter interactions.

“The laboratory is located 2 kilometers under the earth and it is a 5000 meter squared clean room science laboratory... half of our experiments are actually searching for dark matter.”
— Jody Cooley [19:51]

At Snow Lab, a variety of detectors—ranging from germanium and silicon detectors to superfluid-based bubble chambers and charge-coupled devices (CCDs)—are employed to identify potential dark matter particles. These detectors are meticulously maintained at near-absolute zero temperatures to reduce thermal noise, enhancing their sensitivity to faint signals indicative of dark matter interactions.

Similarly, the Xenon Experiment in Italy's Gran Sasso mountain leverages a massive tank of liquid xenon to detect scintillation light and free electrons produced when dark matter particles interact with xenon nuclei. Michael Murrah of Columbia University explains the sophisticated mechanisms that enable the detection and reconstruction of such rare events.

“Our experiment, the xenon experiment, is located in Italy, in central Italy, close to Rome... measuring the time between the S1 and the S2, the second signal, and you can know the depth of the interaction.”
— Michael Murrah [26:24]

SuperCDMS, another prominent experiment overseen by Cooley, focuses on detecting weakly interacting massive particles (WIMPs) within the 1 GeV to 1000 GeV mass range. By utilizing ultra-sensitive, cryogenically cooled detectors, SuperCDMS aims to identify lower-mass dark matter candidates that have been challenging for previous experiments to detect.

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Collider Searches: Creating Dark Matter in Particle Accelerators

When indirect and direct searches yield no definitive results, physicists like Deborah Pinner at the University of Wisconsin turn to particle colliders to potentially create dark matter particles. Working with the Large Hadron Collider (LHC) at CERN, Pinner explains how high-energy collisions can produce dark matter particles, which escape detectors without direct observation.

“We might produce dark matter, so we are not looking for what is already there in the universe, but we might be producing dark matter.”
— Deborah Pinner [34:37]

By meticulously analyzing collision events and accounting for all visible particles, deviations in energy conservation can hint at the presence of unseen dark matter particles. This method relies on detecting the "missing" energy and momentum that would indicate particles escaping the detector.

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Future Prospects: Optimism Amidst Challenges

The quest to uncover dark matter continues to inspire scientists despite frequent setbacks. Don Lincoln reflects on the intrinsic human drive to understand the cosmos, emphasizing that each incremental discovery, even in the absence of direct detection, contributes valuable knowledge.

“Looking for dark matter and other cosmological or astronomical things is really just a next step in a century-long journey by scientists who are able to better and better understand the cosmos.”
— Don Lincoln [40:26]

Joseph Aschbacher remains cautiously optimistic, suggesting that dark matter discovery could occur within the next 10 years, while Jody Cooley posits a broader timeframe of one to two decades, balancing scientific ambition with the practical realities of experimental and technological advancements.

“Maybe we are lucky. Of course, we are looking for this WIMP thing, but maybe it's something completely different... it's really a very, very valuable information.”
— Jody Cooley [43:10]

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Conclusion: The Endless Pursuit of Knowledge

The hunt for dark matter epitomizes the relentless pursuit of scientific understanding. As Babbage concludes, the collaborative efforts spanning space-based observations, underground laboratories, and particle accelerators underscore the multifaceted approach required to solve this cosmic puzzle. While the journey is fraught with challenges and uncertainties, the dedication of scientists like Lincoln, Carwin, Aschbacher, Cooley, Murrah, and Pinner embodies the enduring human spirit to unveil the universe's hidden secrets.

“Understanding the universe is a millennial long journey of humanity. And so the best each of us can do is to add one little stone into this edifice of knowledge.”
— Don Lincoln [41:43]

As research progresses and technology evolves, the hope remains that one day, dark matter will no longer be a mystery but a well-understood component of the cosmic tapestry.

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Notable Quotes

1. Don Lincoln
   “Dark matter is a hypothetical or at least a theoretical idea. And what it does is it solves astronomical problems.”
   [03:44]

2. Christopher Carwin
   “When you say annihilate, what do you mean by that?”
   [10:14]

3. Joseph Aschbacher
   “We cannot measure directly today dark energy and dark matter, but Euclid should really be the mission that is providing a 3D map of the universe...”
   [15:56]

4. Jody Cooley
   “The laboratory is located 2 kilometers under the earth and it is a 5000 meter squared clean room science laboratory... half of our experiments are actually searching for dark matter.”
   [19:51]

5. Don Lincoln
   “Looking for dark matter and other cosmological or astronomical things is really just a next step in a century-long journey...”
   [40:26]

6. Don Lincoln
   “Understanding the universe is a millennial long journey of humanity. And so the best each of us can do is to add one little stone into this edifice of knowledge.”
   [41:43]

---

Babbage continues to explore the frontiers of science and technology, bringing listeners closer to the breakthroughs shaping our understanding of the universe.

Transcript

[Alok Jha]

Hello. This episode of Babbage is available to listen for free. But if you want to listen every week, you'll need to become an Economist subscriber. For full details, click the link in the Show Notes or search online for Economist podcasts.

[Jody Cooley]

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[Alok Jha]

The economist There's a mystery in the sky. Most of the universe is missing. The stuff that we're made of and for that matter, everything else we can see on Earth and also the stars only makes up around 5% of the universe. Most of the universe, around 70%, is made of something called dark energy, a substance that pushes space apart. The rest, around 25%, is dark matter. Dark matter doesn't emit light, so it's impossible to see. But we know it has played an important role in the past 14 billion years in bringing stars together to make galaxies. And it's also responsible for the large scale structure of the universe that we see today. Scientists want to know what this stuff is because understanding it will be crucial in working out what the future size and shape of our universe will become. But although scientists have searched for decades, so far they've found nothing. However, a new generation of researchers with new instruments, techniques and new ideas are stepping up. How do you find something that you can't see? The answer is to get creative. I'm Alok Jha and this is Babbage from the Economist. Today, the hunt for Dark matter. This week we're in Denver, Colorado at the annual meeting of the American association for the Advancement of Science. This is one of the world's longest running scientific gatherings. The first meeting took place in 1848 in Philadelphia as a way for the then small number of scientists in America to come together and share their ideas. In Denver this year, scientists have come together to discuss everything from potential cures for diabetes to the various impacts of generative artificial intelligence. For this show though, we wanted to turn our gaze to the heavens.

[Don Lincoln]

Alright, I'd like to thank you all for coming to this session on how to Search for dark matter. Each one of them will talk.

[Alok Jha]

That's Don Lincoln. He's a senior scientist at Fermilab in Illinois, America's flagship particle physics lab. I sat down with him at the meeting in Denver to get the basics on what dark matter is thought to be and how we really know that it's there.

[Don Lincoln]

Dark matter is a hypothetical or at least a theoretical idea. And what it does is it solves astronomical problems. So when astronomers look out into the night sky, one of the things they study is how fast galaxies rotate. And what they find is that if they try to predict the speed at which galaxies rotate using the known laws of physics, it rotates too quickly. And if they rotate too quickly, this is a problem. There are additional problems. If they look at clusters of galaxies bound together, the galaxies themselves should not actually hold themselves together. They should fly apart. But they don't. And again, it's a mystery. So there's several astronomical mysteries that are all leading us to believe that either we don't understand the laws of physics, or there's invisible matter that causes more gravity than we can see. And if there is some matter that makes that we have a name for it, we call it dark matter.

[Alok Jha]

And so how much of that dark matter do you suspect there is?

[Don Lincoln]

By looking at the sky, we can actually predict, and it appears that there is approximately five times as much dark matter as there is the ordinary matter. So for all of that majestic glory that you see in the sky, dark matter is much, much more.

[Alok Jha]

And just explain for listeners, if we don't know what it is and it doesn't shine, hence the dark, how do we know it's there?

[Don Lincoln]

We don't know that it's there. And we have to be very clear about that. What we do know is that galaxies rotate too quickly. So now there needs to be an answer. And either the answer is we don't understand physics, and there is a significant amount of reason to believe that we do, in fact, understand physics, or there's something there we can't see. And it's not that we haven't looked. We have looked for dark matter for a very long time in a myriad of ways, and we've not seen it. So over the course of now, something like 50 years, we've come to the conclusion that the answer to all of these mysteries is most likely a form of matter that neither absorbs nor gives off light, but yet it exerts gravity on the things around it.

[Alok Jha]

Astronomers found that the edges of galaxies spin too fast for the amount of mass that we can see inside them. The mass is fundamental to the amount of gravity that holds the galaxy together. If there's not enough mass, Stars should be flying off out of the galaxy and into outer space. So the theory goes that there must be some kind of extra matter, some invisible glue Providing the extra gravity to make sure the galaxies stick together. That's dark matter. So the idea that dark matter exists has been around for some decades.

[Don Lincoln]

Yes.

[Alok Jha]

Talk us through how the sort of theories around what it could be have evolved over that time.

[Don Lincoln]

The search for dark matter is long because when you think about the possibility that there's something out there that we can't see, you, have several options, One of which is there's some sort of exotic matter out there. And one of the idea is that there is a particle form of matter, Like a proton or a neutron, except unlike a proton, which will give off light, it's just a small, heavy subatomic particle. And the idea is that there's a gas of these subatomic particles throughout space. If what I just said is true, Then there is dark matter in the room that all of your listeners are in. Dark matter would be flowing through them like a wind. And so we have built particle detectors looking for dark matter, because every so often, even though dark matter doesn't interact very often, it should occasionally bump into an atom, and we should see that atom move. And we've looked really hard, and over the course of 40 years, we've made better and better and more sensitive detectors. And now we can see things with a million times better capability Than we were in the 80s, and we still don't see it. So that's one way we look for it. Another way we look for dark matter Is we look out in space, because if dark matter is out in space, Then dark matter particles can interact with one another and give off familiar particles. So we look for, say, gamma rays, and we do see them. The problem with that particular method Is it's very difficult to determine Whether it's dark matter or it's just ordinary things. Neutron stars, pulsars.

[Alok Jha]

So you're just never sure.

[Don Lincoln]

Yes. And then finally, colleagues of mine, we have huge particle accelerators. We collide particles together, and we try to generate dark matter, and still we've not seen anything. So what we have done is we've set significant limits on what dark matter can be. Now, we have by no means exhausted all of the possibilities, because there are other ideas where dark matter particles are very light, and they need to be tested with other methodologies and we're looking at those too. The problem is the range of possible masses of dark matter is enormous. And each one requires a specialized technology to look at that particular type of dark matter. So slowly but surely, we're knocking them off one by one. And hopefully one day one of the methods of looking for it will find it. So I'd like to introduce you to our speakers. We have Christopher Carwin, Saint Goddard, NASA.

[Christopher Carwin]

Okay, so today I'm going to tell you about indirect dark matter searches and specifically with the Fermi Large Area Telescope called Fermilat. So we can start with an overhead view of the Milky Way. So this is a schematic, and here on earth we're about 8.5 kiloparsels.

[Alok Jha]

The obvious place to look for dark matter is in space. That's where Christopher Carwin focuses his research. He's a fellow at NASA's Goddard Space Flight center in Maryland. His specialty is looking for gamma rays coming from the center of the galaxy.

[Christopher Carwin]

I work with what is called indirect detection. So the basic idea is that dark matter is made up of particles and something that particles do. Something that we know is that particles interact and they annihilate.

[Alok Jha]

When you say annihilate, what do you mean by that?

[Christopher Carwin]

So they kind of interact and they turn into other particles and they turn into light and just turn into other things. And one of the things that they turn into theoretically is predicted that they turn into gamma rays. And so basically with indirect detection, the whole idea is that for any given description of dark matter, there's a very characteristic and unique kind of signature in the gamma ray sky that you can look for. And so that's what we're doing with these indirect searches is looking for these specific kind of like fingerprints, if you will, of this dark matter annihilation process.

[Alok Jha]

So if dark matter is particles and they're interacting with each other, they might be annihilating, producing gamma radiation, which is very high frequency light.

[Christopher Carwin]

Right.

[Alok Jha]

Which you can detect because we can't detect dark matter particles by themselves, essentially.

[Christopher Carwin]

Yes, exactly. So we're not directly sensing this dark matter, but just kind of the byproducts of the interactions that are taking place.

[Alok Jha]

Tell me how you detect these gamma rays then. What's the instrument you use?

[Christopher Carwin]

Yeah, so I work with an instrument, it's called the Fermi Large Area Telescope, or fermilat. So this is a space based instrument. It's in low Earth orbit. The thing with gamma rays is it's a lot different than viewing visible light. You know, the way you have to detect it. It's basically like you have a particle detector in space. Because these photons have such high energies, you have to use a completely different method to even detect them. That's what Fermi does.

[Alok Jha]

And so how long has Fermi been working and what kinds of things have you been doing to try and understand the parameters of these particles?

[Christopher Carwin]

So Fermi was launched in 2008, and it's been observing ever since then. And it sees the entire sky every day. And so we have tons and tons of very good data. And indeed, gamma rays come from many different sour sources, many different processes. And so when you're looking for dark matter, you really expect it to be a small fraction of the total gamma rays that you see.

[Alok Jha]

In terms of the data that's been collected since Fermilat was launched, what have you managed to do to understand dark matter better?

[Christopher Carwin]

Right. So it's a very interesting situation with Fermi right now, and it has been for a long time. So back in the early days, right when it launched, there was reports of an excess signal coming from the center of our galaxy that kind of looked like what we expected dark matter to look like. You're excited, so the community was very excited. And of course, though, people are very careful. And so ever since then, this has been still today a matter of debate. These characteristics of the signal that we see are kind of like what we expect for dark matter. But the problem is that dark matter is not the only possibility. And so the other main possibility is that this signal could be coming from an unresolved point source population known as millisecond pulsars.

[Alok Jha]

And a pulsar is a rotating star, isn't it?

[Christopher Carwin]

It's a rotating neutron star. That's right, yeah, yeah. And so that can generate gamma rays. And because of this, it's still, even today, unclear exactly what the nature of the signal is. And again, it's really complicated because, you know, we're looking towards the center of our galaxy, and when you look in that direction, most of the emission that you actually see is coming from this diffuse emission from the interaction, very energetic particles. So basically these particles interact with the gas and the light in the galaxy, and that ultimately gives you gamma rays, and you're looking for a very low weak signal. So first you have to kind of.

[Alok Jha]

Get rid of the background, get rid.

[Christopher Carwin]

Of all the backgrounds. That's very challenging to do. But nevertheless, there's a couple of things that people tend to agree upon. And one is that this signal does exist. It's not just something that you know, maybe we're a group is doing wrong or something, but what people don't really agree on is kind of the interpretation or the nature of the signal.

[Alok Jha]

What's the next generation of instrument then in space that's going to be doing what Fermi has been doing in terms of getting better data and so on?

[Christopher Carwin]

Well, for gamma rays this is a big issue now in the community. And so the next gamma ray telescope is actually currently being developed. It's called cosi. And so COSI is going to observe actually in the energy range right below where Fermi observes. So this is quite exciting actually the main thing that I work on these days. So COSI is going to be a space based instrument and it's scheduled to launch in 2027. And it's going to also be able to look for different interesting signatures of dark matter and it will also help us to better constrain the backgrounds that we have to deal with. So that's the next Gamma instrument that's being developed. This is a NASA mission. But also as a community there's really a big effort to try to get the next instrument. We would like to have a large scale gamma ray instrument for the dark matter case. You know, we want something that maybe we can get very high angular resolution to try to look at the center of the galaxy better to kind of maybe resolve some of these point sources.

[Alok Jha]

Searching for the gamma ray fingerprints of dark matter isn't the only way that scientists are looking Beyond Earth. In July 2023, the European Space Agency, in collaboration with NASA, launched an observatory called Euclid.

[Joseph Aschbacher]

Euclid spacecraft separation confirmed Euclid is measuring the unmeasurable.

[Alok Jha]

That's Joseph Aschbacher, the boss of the European Space Agency, who we spoke to a few months after the launch of Euclid.

[Joseph Aschbacher]

We cannot measure directly today dark energy and dark matter, but Euclid should really be the mission that is providing a 3D map of the universe going back to about 10 billion years, mapping a large portion of our universe and therefore really getting a much better picture of how the universe is composed, how it develops over time, and therefore making it possible that some conclusions can be made.

[Christopher Carwin]

On dark energy and dark matter.

[Joseph Aschbacher]

I'm pretty sure that the measurements coming out of Euclid will create a lot of Nobel Prize winners because the knowledge that we get out of it will be fundamental. Of course, I cannot tell you what it is because we have yet to take the measurements, but I'm really looking forward to this kind of measurements.

[Alok Jha]

The Euclid probe will aim to create a three dimensional map of a third of the universe. By understanding the geometry of outer space, it hopes to put constraints on what dark matter could be. But it's unlikely that these space based missions will, on their own, find a smoking gun for dark matter.

[Jody Cooley]

I always think that people searching for dark matter have to be the most positive people in the universe because, you know, we are constantly not finding it, but we keep thinking it's around the corner. We do have some handles on what we think it could be, how we think it interacts, what size it could be, but ultimately we haven't found it yet. So that means that we need to kind of search everywhere, if you will.

[Alok Jha]

Searching everywhere, as physicist Jody Cooley will explain, includes looking thousands of meters under the surface of the Earth. She'll tell us why next. But before we dive deep underground, just a quick reminder that you're listening to a free episode of Babbage. To enjoy our content every week and to listen to all of our other award winning specialist weekly podcasts, you'll need to become a subscriber. For the month of February, we've put Economist Podcast plus on sale. Why not enhance your daily commute or session at the gym with our podcasts for less than $2.50 a month for an annual or two year subscription? This offer ends on February 29th, so don't miss out. To sign up, search online for Economist Podcasts or click the link in the show notes. We'll be back in just a minute.

[Jody Cooley]

Imagine the merging of trusted intelligence into a unified experience. Imagine collaboration amongst teams and across continents. Imagine an empowered ecosystem designed to deliver actionable insights that inspire growth and sustainability. That's the power of the Connect Industrial Intelligence platform to help you see further, innovate faster, accomplish more. That's the Connect effect. Learn [e-mail].

[Alok Jha]

Nice to meet you.

[Deborah Pinner]

I'm a little bit early, but I.

[Jody Cooley]

Thought I was like, no, no, come in, come in.

[Alok Jha]

Early's fine. It's good for us. So far on our hunt for dark matter, we've gazed into the night sky. But those space based methods would only ever find indirect evidence for dark matter. To find more direct evidence, scientists need to find ways to observe dark matter on Earth. And one way they're trying to do that is deep underground.

[Jody Cooley]

Snow Lab is one of my favorite places in the entire world.

[Alok Jha]

Jody Cooley is the Executive Director of Snow Lab in Ontario, Canada. It's a laboratory that's dedicated to investigating the subatomic particles that are Continually streaming through the earth, including, they hope, dark matter.

[Jody Cooley]

The laboratory is located 2 kilometers under the earth and it is a 5000 meter squared clean room science laboratory. In order to get to the laboratory, you first take an elevator ride in a mining cage. So it's in an active nickel mine in northern Ontario. So you take this two kilometer ride down the cage with the miners and then you walk one and a half kilometers to the lab. And then you get to the lab and you go inside. Like I said, it's a clean room and mines are really dirty. So the first thing you do in the morning when you get to the lab is take a shower and then get into your clean room garb. It is plum full of experiments. Half of our experiments are actually searching for dark matter.

[Alok Jha]

Okay, so dark matter is somewhere between the luminous stars in the universe as you look up in the night sky. Why are you looking for it 2km or 3km underground?

[Jody Cooley]

That's a great question. What we know today is that we have a halo of dark matter all around our galaxy. And it's really quite extensive. There's some kind of non luminous particle out there, adding that extra gravity, adding that extra glue so that the galaxies would stick together. And so as it is that the Earth is rotating around the sun and our sun in the galaxy is moving towards the star Cygnus, we're actually moving through dark matter all the time. And so if we build a detector here on Earth, we could hope to see dark matter interacting in those detectors. The reason that we have to put them underground is that on the surface of the Earth, we're constantly being bombarded by cosmic radiation. So these cosmic rays, they're just a huge nuisance to dark matter detectors. And so we use the Earth as a filter.

[Alok Jha]

So you're underground. And the idea is that the cosmic rays and the daughter particles of those are filtered out to the large extent. So what is it that in your detectors you're looking for as the signature of a dark matter particle being there?

[Jody Cooley]

So that's actually the thing I love best about working at Snow Lab is that we don't have just one dark matter detector. We have a whole host of them, a whole suite. And each one operates slightly differently. But the general idea is that each of the detectors has some kind of target in it, whether it be a germanium or silicon detector, or whether it be some kind of superfluid like in some bubble chamber experiments we have, or whether it be actually a CCD detector. So CCD stands for charge coupled device. And essentially this Is a little piece of silicon. It's just like the hardware you would have in your camera to take a picture. And so when charged particles go through the ccd, they leave little deposits of charge, essentially, that can be read out. And then we can determine what kind of particle it is and try to decide whether or not it's consistent with dark matter. Some of the detectors I was talking about scintillate. And so what scintillation means is essentially when the particle is traveling through the detector medium, it gives off light, essentially. So you can read this light from the particle passing through the material with all sorts of different types of detectors. And then the technologies of the detectors, depending on which one it is, read out that energy to determine whether or not it's a dark matter particle or whether it's one of these background particles.

[Alok Jha]

It would be a very rare event.

[Jody Cooley]

It is very, very rare because the.

[Alok Jha]

Definition of dark matter Is that it doesn't interact with normal matter.

[Jody Cooley]

Right. So we know there are four forces in nature. So you have gravity. That's the charge that keeps you on the earth. You have the electromagnetic force, which I used to always like to tell my students is what keeps us from falling through the earth, because it's the repulsive force between your shoes and the floor. You also have the strong force. The strong force is the force that binds the very center of an atom, the nucleus, together. And if you think about this, so the electromagnetic force says that opposite particles attract, and same charges repel. And if you think about a nucleus, like, as you get to a heavier element, you could have multiple protons, which all carry a positive charge packed together in that nucleus. The electromagnetic force would try to push those protons apart. The strong force keeps them together. And then there's the weak force, and that is what allows radioactive decay. That's actually the force by which we think dark matter interacts. And as you can tell, weak force kind of gives you the sense that it's not the strongest of the forces. It is one that is more rare and happening. But we have a lot of models of different candidates that could be dark matter that would interact via this weakness force.

[Alok Jha]

So how is the search going? Is there a sort of parameter space you're looking through, or do you just have to wait? Because these interactions are so rare that you just have to collect lots and lots of data and look through the stats, and that's how you do it.

[Jody Cooley]

Yeah. So we have made tremendous gains in excluding what the dark matter can't be. And so I like to make an Analogy of searching for dark matter. Kind of like looking for your lost key. Okay, so you lose your keys, and what do you do? You systematically think about going through your house room by room, searching for where they could be and excluding it. And then the longer and longer you search in your house, the smaller and smaller the space where those keys could be, you know, happens to be. So the problem we have in dark matter is that our house is so big, so we've excluded a lot of rooms, but we still have quite a few to look in.

[Alok Jha]

Yeah, yeah. Well, the thing about keys is it's always in your pocket. So that's a piece of advice I can give in that case. Snow Lab isn't the only facility that's searching deep beneath the Earth's surface for dark matter. Underneath the Gran Sasso mountain range in central Italy is the xenon experiment, a dark matter detector centered around a giant tank of very cold, very dense liquid.

[Joseph Aschbacher]

So I'm in the field of direct dark matter detection.

[Alok Jha]

Michael Murra is a physicist at Columbia University.

[Joseph Aschbacher]

So the idea is that if we just find a material that can interact with the dark matter in some way, that dark matter interacts with the material and creates some signature by depositing energy on this particle, then we want to see this faint signature in our detector, basically.

[Alok Jha]

So dark matter, by definition, doesn't shine, doesn't produce any light. So you're hoping that it interacts with real matter in some way.

[Joseph Aschbacher]

Right.

[Alok Jha]

So describe your experiment for me then.

[Joseph Aschbacher]

So you cannot just put it on Earth. So, unfortunately, the problem is with this. If you put in some math, you can calculate that you maybe expect only five to ten events per year in your detector. So the problem is, now, if you put the detector on the surface of the Earth, you have cosmic rays, like a muon, for example, that heavy sibling of the electron. So if you look at your thumb, and you take your thumbnail once per second, one of those muons is passing through your thumb. So you can imagine if that's like just on my thumbnail once per second over the year, how many, like, hundreds, millions, billions of signals you would see in your detector. And you have to find one specific detector.

[Alok Jha]

So you get lots of dark matters.

[Joseph Aschbacher]

Exactly. You get a lot of background events. We call that background. And our experiment, the xenon experiment, is located in Italy, in central Italy, close to Rome.

[Alok Jha]

So what does it literally look like, and what is it made of?

[Joseph Aschbacher]

So in the center of the detector, you have a core of liquid xenon. We put that in the detector, then when it is hit by a dark matter particle, it can create scintillation light, so it produces photons and it also creates free electrons. And what we do then is we have photosensors that are able to detect the xenon light, and we call this the S1 signal. Very innovative. And with the electrons, they would usually recombine and get lost. But we do a neat trick. So we apply an electric field in our detector, and then the field makes the electrons drift upwards in the detector. And at the very top, there's a very thin, few millimeter layer of gas, xenon. And these electrons, they scatter a lot with the gas atoms of the xenon, and they create again scintillation light. So they create a second light signal that then can be detected again. And then you measure the time between the S1 and the S2, the second signal, and you can know the depth of the interaction. So you have a fully 3D reconstruction in the detector, which allows you then to select in the detector where you can do your analysis.

[Alok Jha]

So the mechanism you've described of a potential dark matter particle, and there's infinite numbers of them flying through the Earth right now, if they exist, but occasionally, very rarely, they would interact with this massive nucleus of xenon. Xenon, which is a noble gas, so it doesn't react with anything.

[Joseph Aschbacher]

Exactly, and that's also one of the good points.

[Alok Jha]

So the idea would be that this particle would interact somehow with this nucleus and produce light.

[Joseph Aschbacher]

Yes, exactly.

[Alok Jha]

Which is detectable in the detection. You gave a really good description of it. I'm assuming it's not happened yet, because otherwise we found dark matter.

[Joseph Aschbacher]

Exactly, yes.

[Alok Jha]

So your job is to define, differentiate those things from the background.

[Joseph Aschbacher]

Yeah, exactly.

[Alok Jha]

How does xenon fit with these sort of other global experiments that are going on to detect dark matter?

[Joseph Aschbacher]

Right. You mean the xenon experiment?

[Alok Jha]

Yes, of course.

[Joseph Aschbacher]

They're all kind of complementary and all these different searches, they look slightly for different kind of interactions of the dark matter particles.

[Alok Jha]

You're looking at different parts of the parameters, right?

[Joseph Aschbacher]

Exactly. This is how you, you could state that.

[Alok Jha]

So what is the energy level of the dark matter particles you're looking for?

[Joseph Aschbacher]

We are looking for WIMs, weakly interacting massive particles. The energy region is about one GeV to a thousand GeV. So one proton has about the energy of one GeV. So it's about the mass of an hydrogen atom at the lower end and a couple of lead atoms at the higher end. Let's say the mass range.

[Alok Jha]

These direct searches for dark Matter underground still have lots of promise, though. As Jodi Cooley explains, scientists are already building upgrades.

[Jody Cooley]

Right now, we actually have a number of experiments that are under construction for dark matter. Probably the one that I personally am most excited about, because I've actually spent a great deal of my career on it, is an experiment called super cdms. These are silicon and germanium detectors. They're about the size of a hockey puck. And the idea is that when a particle goes through and interacts with the nucleus in one of these detectors, the detectors start vibrating. And what I should point out, actually, which is actually a key point to understanding this, is that we keep these detectors really cold. And by really cold, I mean like near absolute zero. We operate at something like 20 millikelvin. So this is really, really cold.

[Alok Jha]

This is very, very small amounts above absolute zero.

[Jody Cooley]

Very small. So absolute zero is minus 273 Celsius. So the lattice in your crystal is very, very cold. The structure, it's very still. And so when a particle comes in and interacts with the nucleus, the whole crystal starts vibrating. And we measure those vibrations in order to try to determine whether the particle might be dark matter or whether that particle might be just ordinary matter that we already know about.

[Alok Jha]

The temperature you're describing is colder than outer space, isn't it? Technically. Because outer space is three kelvins.

[Jody Cooley]

Yes, you're right. So it is colder than a photon from the cosmic microwave background, which are those photons that come in the Big Bang. I had never thought of that before.

[Alok Jha]

So Super CDMS is going to look for a specific mass, isn't it?

[Jody Cooley]

Yes.

[Alok Jha]

That's different to previous experiments. Tell us about that.

[Jody Cooley]

Super CDMS is looking for particles that have a mass sort of under 10 GeV. This is where it's most sensitive compared to other technologies. And just to give you an idea of what that is, is a proton has a of one GeV. So essentially, we're looking for something that could be as big as 10 times the mass of a proton, but as small as like, 1/100th of a proton's mass. So it's actually still a pretty big range, but it's actually kind of a small range compared to, you know, what's out there. Exactly.

[Alok Jha]

And other previous experiments really have been looking for dark matter particles that are much, much heavier.

[Jody Cooley]

Yes, they've been looking essentially for particles that are greater than mass above 10 GeV. So a lot of what we call direct detection, which are the ones where they're here on Earth looking for the dark matter that is, in our own galaxy, they typically are looking up to masses of like 100 or say 1 TeV. So that would be 1000 times the mass of the proton. And then the ones that look up in space, I mean, they're going up to 10 TEV or more. So it's really a lot.

[Alok Jha]

If you're looking with CDMs for particles that are potentially the mass of a proton or of that order of magnitude, doesn't it just make your life a lot harder? They're going to be a lot less interacting, right?

[Jody Cooley]

So I hate to, like, date myself, but when I started back in this business, I was a postdoc on an experiment called CDMS2, which was the second generation of our experiment. And so this would be in the early 2000s. And at that time we were like, it's impossible to run these detectors in such a way that we could look for something that would be lower than 20 GeV in mass. So, like, we were just like, that's pie in the sky.

[Alok Jha]

Never predict the future.

[Jody Cooley]

And now I know, it's amazing. Like, the detector technology just evolves so fast. And this is, I think, one of the really exciting things about science is, like, you just never know what you're going to do and what the next new generation of people coming through the program, you know, the ideas they have for expanding the analyses, for improving the detectors. I think that's part of what makes science so exciting.

[Alok Jha]

But what if both the indirect search for dark matter in space and the direct searches underground don't yield any results? Fortunately, physicists are pursuing one more approach. If you can't find dark matter particles that already exist, why not try making them instead?

[Deborah Pinner]

So now we just play a little game, right? We want to make some assumption on how dark matter can be produced in this collision. So we do this guess who game. We have to understand what is the figure that our opponent has. So what is your strategy to try to win this game?

[Alok Jha]

Deborah Pinner is, is a physicist at the University of Wisconsin. She also works at the world's largest and most powerful particle accelerator, the Large hadron collider, or LHC, at CERN in Switzerland.

[Deborah Pinner]

It was 2013 when I started my PhD, and if you think just a year before CERN discovered the Higgs boson. So that was really one of the missing pieces for our understanding of the nature. And after that, we had still the data there. So what was the next puzzle that we could discover at lac? And dark matter was one of those. So actually, nobody thought that we could look for Dark Matter colliders. But then this possibility became real, and so I started to do this, and I found it very interesting because you can think about LAC being a sort of telescope to look how we can produce dark matter. So I think that that's very cool.

[Alok Jha]

I've heard about the lhc, the Large Hadron Collider, being described as a microscope before, where you're looking very closely at what's inside atoms. And that's what we've talked about on this podcast lots and lots of times before. We've been to see the Large Hadron Collider and talk to physicists there about, you know, what's beyond the standard model of particle physics, what's everything made of. So I'm intrigued by what you said about it being a telescope, too. So how do you actually look for dark matter in something like a particle accelerator? I mean, dark matter is meant to be all around us and in the universe. So what's a particle accelerator doing involved?

[Deborah Pinner]

Well, actually, the cool thing is that we might produce dark matter, so we are not looking for what is already there in the universe, but we might be producing dark matter. And also, dark matter is very challenging. And for now, we just know that interacts to gravity, and so this is very challenging for a collider. So what we usually say is that we see the invisible through the visible, because we have to measure all the visible particles that we have in our collisions very well, just to understand if there is something that escape our detector.

[Alok Jha]

So let's talk about how you normally make detections at the detectors in the lhc. You have protons going around the accelerator ring, and they collide and they smash together. And the idea is that you look for what's inside there, and there's lots of energy involved. So describe for me normally how you interpret that cloud of energy into particles. And then specifically, how do you look for a dark matter particle in that?

[Deborah Pinner]

So basically, what we try is to take sort of pictures of these interactions. Of course, there are many. So first of all, we have to make, isn't it? Basically, yeah, you have plenty. So it's a very, very busy environment. So first of all, you try to choose and select only the pictures that they might be interesting for what you wanted to look at. And then you have different parts in our detector that help us to distingu between the particles that were produced. And so these particles will leave some sort of energy. They will interact with these detectors in different ways. And then based on this information, we can reconstruct. Okay, here A particle passed with this energy or like with these, you know, trajectory. And once that you have all this information, you do the math, you add all the energy that you measured from your detector and maybe you see, okay, but this does not conserve. So this is a hint that a particle that did not interact with your detector was there. So you can kind of like infer it. So you didn't see directly, but you can see its presence from these non conservation of the energy.

[Alok Jha]

So when you say using the invisible to see the visible, what you're meaning is that you have to add up the particles that are created in the collision and all of their energies. And if it doesn't match the energy that went in from the protons that collided, then you know something's going a bit funny.

[Deborah Pinner]

Exactly. And if you think this is very challenging because it's not only seeing the invisible through the visible, but the visible needs to be very well measured because otherwise you can also say, oh, maybe I didn't measure it well. And that's why like the sum doesn't adds up. But we can achieve this level of precision at colliders and in our detectors to say, okay, something was produced, but it just went out without, you know, leaving any information for us.

[Alok Jha]

So you're looking for that thing that doesn't quite fit, basically. And you're hoping and keeping your fingers crossed that you can't explain it. I mean, that's basically what's going on. Right. And so as soon as it does fit something else, it's disappointing, I'm guessing each time.

[Deborah Pinner]

Well, it's disappointing, but I would say that it's not also like, you know, a failure because we really have such a few information about dark matter. So even knowing what is not, I think that it's a very valuable information.

[Alok Jha]

Uses the space that you.

[Deborah Pinner]

Exactly. Because so far we know dark matter interacts with gravity, but we don't know anything about its nature, how does it interact and everything. So it's like a very huge space of possibilities. And even reducing that and knowing, okay, this was not the right direction, but okay, I've excluded this, so now I can go in this other direction. I find it that it's a very, very valuable information. So there is a disappointment because of course everybody wants to find something new, but I think that it still has a very important values. And so that's why we do all of these type of searches.

[Alok Jha]

The hunt for dark matter has been going on for decades. And with so many places to look, whether it's the near infinite physical Space of the universe or the enormous range of energies in the subatomic world. No one thinks that finding this mysterious form of matter will be easy. For the scientists who've dedicated their careers to it, though, it must sometimes get frustrating. Before I left the meeting in Denver, I asked our guests what kept them motivated in their search, despite not knowing when or if they'd ever find answers.

[Don Lincoln]

It's always a tricky question when people ask what is the value of dark matter? Because after all, realistically, studying a black hole is not going to change my life. It's not going to buy me a better car or anything like that.

[Alok Jha]

That's Don Lincoln again.

[Don Lincoln]

But the truth is, if you look at the earliest records written down by humans, we have been inherently curious about how the universe came to be, the laws of nature, been fascinated by the heavens. And so what these sorts of studies do is they fill a deep set at almost a primal need by humanity to understand the world around us. And in some sense, looking for dark matter and other cosmological or astronomical things is really just a next step in a century long journey by scientists who are able to better and better understand the cosmos. And I personally am just sad I'm not going to be alive to see the end of it.

[Alok Jha]

You never know.

[Don Lincoln]

But there'll always be a new mystery, whatever it is. Even if someone figures out dark matter tomorrow, then we'll have to study it in detail and we'll find out that we don't quite understand it. And the sad truth is understanding the universe is a millennial long journey of humanity. And so the best each of us can do is to add one little stone into this edifice of knowledge.

[Alok Jha]

When do you think we'll find dark matter then, given what you know about this subject?

[Joseph Aschbacher]

Oh, that's very, very hard to predict.

[Alok Jha]

I have a go.

[Joseph Aschbacher]

Yeah, I don't know, maybe in the next 10 years.

[Alok Jha]

Michael Murrah again, maybe with the next generation.

[Joseph Aschbacher]

Maybe we are lucky. Of course we are looking for this wimp thing, but maybe it's something completely different. And there are like so many different experiments and I think we just have to keep going if we really want to understand it. And maybe our experiment might be the lucky one and find it or maybe someone else.

[Alok Jha]

Well, there's a Nobel Prize waiting for somebody.

[Joseph Aschbacher]

Yeah, yeah, absolutely, absolutely, yes. And the parameter space of potential dark matter candidates is so huge. Right. It spans from the tiniest energies that we can imagine to like almost black hole size material. Right. So that's insane in terms of parameter space. So we have to start somewhere and we have to look for like promising theories. And I think even to rule out most of the parameter space to nail down what it could be, I think is evenly important as just finding it. Let's say.

[Alok Jha]

Well, I never predict the future, but I'm going to make you predict the future now. When do you think we're going to find dark matter then?

[Jody Cooley]

Oh, gosh, I really hope in the next decade.

[Alok Jha]

That's sooner than a lot of people think.

[Jody Cooley]

Maybe I should say two decades.

[Alok Jha]

So that's Jody Cooley again.

[Jody Cooley]

So the question is, are we working on the timescale of scientists or the timescale of our funding agencies? If it's on the timescale of scientists, I say a decade. If it's on the timescale of funding agencies, maybe two decades. We're going to make tremendous gains in the search in the next decade in this sort of lower mass regime, the under 10 GeV regime. Unfortunately, to make much progress going above 10 GeV, we really need to do a next generation of experiments. So the generation three dark matter experiments. And it's going to take a long time. These Zix experiments are huge now because the easy space has already been searched and so now you're just left with the harder to reach space. So you have to build these giant experiments and it requires international cooperation. It requires people with a lot of different expertise, and it just takes time to build it takes time to organize the funding for it. It takes time to come over some of the technology challenges that you have as you scale something to something that massive. But I am really feeling good about the next one to two decades.

[Alok Jha]

Well, we'll be sure to come back in 10 years or so to see whether our scientists were right. That's all from us. Thanks to Don Lincoln, Christopher Carwin, Michael Murragh, Jody Cooley and Deborah Pinner. And thank you for listening to this free episode of Baby. Search online for Economist podcasts to subscribe and listen to us every week. Babbage is produced by Jason Hoskin with mixing and sound design by Nico Rofast. The executive producer is Marguerite Howell. I'm Alok Jha and in Denver. This is the Economist.

[Jody Cooley]

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[Alok Jha]

Day and said he was a big roas man.

[Jody Cooley]

Then he told everyone how much he.

[Joseph Aschbacher]

Loved calculating his return on ad spend.

[Christopher Carwin]

My friends still laugh at me to this day.

[Don Lincoln]

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[Jody Cooley]

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[Don Lincoln]

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[Jody Cooley]

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[Don Lincoln]

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