*The artwork featured in this piece was done in collaboration with digital artist Kendra Oliver, an educator and artist living in Pittsburgh.
O n a September evening in 1846, German astronomer Johann Gottfried Galle received a letter requesting that he point his telescope at one small patch of the night sky. The letter detailed that by performing this observation, Galle might solve a puzzle that vexed the scientific community for years. This puzzle concerned the orbit of Uranus, the only planet whose orbit couldn’t be predicted by Newton’s theory of gravity. Uranus would be expected to be in one place but show up in another. Spurred by this mystery, Galle’s letter-writer asked himself: is Newton’s theory not a description of our world, or is there something else responsible for this anomaly?
Prior to writing his letter, French astronomer and mathematician Urbain Le Verrier made a calculation he hoped would explain the orbit of Uranus and maintain support for the theory of gravitation. Le Verrier and others suggested that there was a dark companion responsible for the disturbance: an unknown, massive body orbiting near Uranus providing an extra gravitational pull. Supposing this massive body existed, Le Verrier predicted where and how heavy Uranus’s dark companion must be to explain the data. He sent his letter to Galle telling him where to look. On the night the letter arrived, Galle searched the sky and discovered a royal blue planet three billion miles from Earth, known today as Neptune, within one degree of Le Verrier’s prediction and after less than one hour of searching.
The mystery of Uranus’s anomalous orbit posed a serious challenge for nineteenth century science. The puzzle and consequent discovery of Neptune are exemplary of a recurring pattern in physics: an observation is made that doesn’t agree with a theory’s prediction, so scientists must either come up with a better theory or find an effect (like an undiscovered planet) which might explain the observation using a current theory.
Twenty-first century physicists and astronomers, however, find themselves in a similar but much more dire situation. Around 50 years ago, another observation led us to question our best and most well-tested theories. Like the disturbance in Uranus’s orbit, this observation suggested that there is a certain missing mass in our universe, only far darker than the dimly lit Neptune. That mass is what physicists and astronomers call “dark matter,” an incredibly elusive and distinctly unusual substance. As a mid-level graduate student pursuing a PhD in particle physics, I work with collaborators at UC Irvine and around the world to help us better understand dark matter: how to find it, where to find it, its secrets, and what effects it might have on the world around us, or above us.
As a result of more than a century’s worth of research, physicists today have a grand theory of particle physics which is (unimaginatively) called the Standard Model. The theory holds that there are merely 17 indivisible particles that interact with one another and constitute the building blocks which form everything in our universe. The Standard Model describes how these particles interact, has been tested against countless experiments, and we can predict the outcome of these experiments with remarkable precision. In fact, the Standard Model is the most predictive theory that humans have ever come up with. We know, however, that the Standard Model is incomplete as dark matter cannot be any of these 17 particles. We know in great detail how these 17 behave, and dark matter does not behave quite like any of them. One of these known particles, called the neutrino, is similar to dark matter and can give some insight into how dark matter behaves. The neutrino is a ghost-like particle that passes through matter undisturbed. It was predicted to exist based on experiments that could not be explained, and the scientist who theorized it lamented, “I have done a terrible thing: I have postulated a particle that cannot be detected.”
From the Italian for “little neutral one”, the neutrino is an elusive particle that rarely interacts with other particles. Countless neutrinos are constantly being produced in the sun. These neutrinos travel through the vacuum of space, and more than 100 trillion of them pass through you unnoticed each second. Despite the neutrino’s near complete detachment from other particles and our experiments, it was eventually discovered two decades after it was predicted and is a crowning achievement of theoretical and experimental physics. Like the neutrino, dark matter is passing through you unnoticed each moment. Unlike the neutrino, however, we do not yet know how many dark matter particles are passing through you nor where they come from, and its identity still eludes us more than five decades after we gathered the first conclusive evidence of dark matter’s existence.
The story of Neptune’s discovery mirrors the story of how we learned about dark matter. By observing the motion of a body like Uranus, one can calculate the forces causing that motion, such as the gravitational tug of an unseen companion like Neptune. A half-century ago, astronomer Vera Rubin used this same tactic on stars within the nearby Andromeda Galaxy, two million light years from Earth. Starting with the stars at the edge of the galaxy, she measured their velocity as they orbited the disk, the galaxy’s “rotation curve.” Here again the tactic proved useful: just as one could predict the motion of Uranus around the sun using the known planets, Rubin could predict the motion of the stars around the galaxy using the other known stars.
Rubin painstakingly analyzed data and made a stark conclusion: all the stars at the edge of the Andromeda Galaxy were rotating much faster than expected. So fast, that the gravitational pull of the galaxy wasn’t enough to keep these stars from flying apart. There had to have been something else keeping Andromeda intact. Once more, observation suggested that there was some kind of dark companion, a missing mass providing the needed gravitational interaction. Just as Le Verrier predicted the mass and location of Neptune, Rubin could predict the mass and location of her dark companion.
Rubin and her collaborators predicted that about 85% of Andromeda’s mass was unaccounted for. Meaning, if you were to tally up all the stars, gas, and planets in Andromeda, add up their weight, and compare this with the mass required to keep the galaxy from flying apart, you would come up 85% short and conclude that you are not counting most of the galaxy’s actual mass. This conclusion wasn’t unique to Andromeda, but true of almost every known galaxy. Rubin’s work and follow-up studies predicted the location of this missing mass: it fills a large bubble or “halo” with the halo’s host galaxy embedded in it’s interior. We call this 85% missing mass “dark matter” because it does not emit, absorb, or strongly interact with light like stars, planets, and most particles do. To this day, while we’re confident that it exists, we have little idea of what dark matter actually is, even though it dominates our universe.
There are many ways that we study dark matter: astronomers and experimentalists scour the sky and design intricate experiments to find the dark matter needle in the haystack, while a theorist might develop a grand theory which unites dark matter with the successful Standard Model. My interest in this search is somewhere between these two in the field of dark matter phenomenology. To explain by way of our guiding example, the story of Neptune: Newton as a theorist developed the theory, Le Verrier as a phenomenologist used the theory to make a prediction to test the theory, and Galle as an astronomer searched the sky for the effects of the prediction.
A study in dark matter phenomenology usually begins down one of two roads, either the theoretical or experimental. When a theorist develops a new model of dark matter, it’s not always clear how this theory can be verified nor how it will affect our experiments. A theorist might come along and suppose there is a dark matter particle which only interacts with the elusive but well-known neutrino, for example. How this type of dark matter could subsequently be discovered with current experiments is highly uncertain.
Or a study might start with the experimentalists who report that they’ve developed an exciting new experiment, or they’ve obtained an experimental result that cannot be explained with the Standard Model. Maybe this anomalous result is dark matter, maybe it’s something else altogether, or maybe it is something more mundane like an error in the experiment. In each case, my collaborators and I have the job of predicting the phenomena from a theory and determining how these phenomena might be observed. Let me give an example of a project I worked on to describe how this process looks in a type of search called an “indirect detection” of dark matter.
How do you search for something you can’t see? By starting with something you can see. The Galactic Center Gamma-Ray Excess is a longstanding anomaly, in which there is light emanating from the center of the Milky Way which we can’t fully explain. The stars in the Milky Way glow of course, but there is a small excess of light above the glow we expect from conventional light sources. One popular idea is that this excess is the result of dark matter annihilation. Annihilation is a process where two particles crash into and destroy each other, converting their energy into other particles like light rays in the process. Annihilation happens with Standard Model particles all the time. In this proposed solution to the Galactic Center Excess, two dark matter particles would occasionally annihilate each other and give off the observed excess of light. As a phenomenologist, my job would be to find a model where two dark matter particles can annihilate each other, and calculate the properties of the dark matter particle such that it would give off the observed glow, thereby explaining the mysterious excess of light and learning more about this dark matter model.
The goal is to gather evidence for dark matter being a certain type of particle with a certain mass, interacting in a certain way with other particles. If enough evidence like this is gathered, scientists become more and more sure of what the dark matter particle actually is. Alas, no such gathering of evidence has been assembled. We currently have disparate pieces of evidence, and wide swaths of particles that we know dark matter isn’t.
There are countless particles which have been suggested to be dark matter, and each has been or will be investigated thoroughly. The history of the search for dark matter involves decades of experiments, theories, and mathematics that takes years to grasp, even at a basic level. I recall when I was a younger student when academic papers read like a Dr. Seuss book filled with made-up words. Consider this handful of actual theorized particles and see if you can guess which one I just made up: gluons, glueballs, inflatons, quarks, squarks, quirks, winos, neutrinos, neutralinos, tops, downs, charms, bottoms, simps, wimps, and chimps. For most physicists (though most may not care to admit), our time is frequently spent discussing and pouring over papers trying to make sense of the findings while deciphering language like this. You never know, it could be a decades-old study that one day gives you a clue on dark matter, or an idea a collaborator put forth at lunch that merits further investigation. It can sometimes be disorienting, but at a fundamental level, dark matter is a hypothesis that begs the most familiar and child-like questions: What are we made of? What kind of world do we live in?
To convince yourself that every galaxy, and thus the universe, is dominated by a mysterious and virtually undetectable dark matter requires more evidence. As Carl Sagan put it, “Extraordinary claims require extraordinary evidence.” Still, the suite of evidence for dark matter is overwhelming. More evidence is found in studies that examine groupings of thousands of galaxies called galaxy clusters. As Vera Rubin studied the rotation of stars around a galaxy, galaxy clusters allow us to study the orbit of galaxies around other galaxies. We’ve discussed this same phenomenon before, but sure enough the galaxies are found to be rotating so fast that they should fly apart. Notably, when the amount of missing mass is calculated, it is roughly the same 85% amount that Rubin found.
Another terrific observational technique which can be used to infer the existence of dark matter is called “gravitational lensing.” As a ray of light passes near a very massive body such as a star or galaxy, the ray’s path is bent by it like a lens. By measuring how much the light deviates from its path one can calculate how much mass is doing the bending. When these observations are performed on certain systems, the light appears to be bent more than it should be. You can probably guess what is responsible for the bending. Once again, these studies imply that there is a missing mass distributed as a diffuse halo around galaxies that outweighs regular matter by 5:1. How these analyses are performed is a longer discussion, but all of these observations plus countless others suggest the same missing mass. The interesting thing to note is that virtually every piece of evidence for dark matter relies on its bulk gravitational effects as opposed to the effect of one dark matter particle, obfuscating the particle’s identity.
But why should you care? What does dark matter have to do with you, at the end of the day?
One of the most significant signs of the existence of dark matter--if not the most significant--is the existence of you. Without dark matter our universe would be a much different place, and it’s entirely possible our universe would not exist at all without it. The full reach of dark matter's influence on our universe is not well understood, and its mysterious nature lures us toward a deeper understanding of our world.
But it is not just some substance light years away that you’ll never encounter; in fact, you are actually engulfed in dark matter right now.
Wherever you go, you are immersed in a sea of atmosphere, standing on an earth that orbits a sun you are familiar with. You might remember from chemistry class that the atmosphere is composed mostly of nitrogen and oxygen molecules, that the earth is a variety of metals like iron, and that the sun is almost 100% hydrogen. These atoms and molecules can absorb light, bump into each other, and their mass is well determined by countless experiments. But as you live in this atmospheric sea of regular matter, you are a speck in a much vaster ocean of silent particles. This silent dark matter does not absorb much light, if any light at all. The dark particle does not seem to bump into regular matter very often, and its mass is highly indeterminate. There is somewhere between one and more than a trillion-trillion dark matter particles all around you. Meaning, while we are confident of the total mass of all the dark matter in the room with you, we are so unsure of an individual particle’s mass that we can’t say whether it is a few heavy particles or trillions of light ones.
With all the evidence we have for dark matter, a mystery like this captivates the scientific investigator: so sure that something is there, and yet so unsure of what it is. But our studies and experiments have not been in vain. While we don’t yet know exactly what dark matter is, and questions abound more than answers, we at least know by studying dark matter’s effects what it isn’t. For example, we have no conclusive experiment that has detected even a quiet nudge of dark matter, so we at least know that it doesn’t interact very strongly.
If dark matter interacted strongly with regular matter, then as the Earth-sun system sweeps through the Milky Way’s dark matter bubble, I would feel a dark-matter wind against my face, even as I sit and type in my office. I don’t feel this, so it must not interact with me (or our experiments) very much, if at all. But one might ask: can dark matter occasionally bump into regular matter and give off a faint, detectable signal? This is what some experimentalists do in the study of dark matter “direct detection.”
At the Homestake Mine in South Dakota, there’s a retired gold mine that had previously been the largest and deepest gold mine in North America. It sports a large quarry carved into the brown earth and some repurposed buildings left over from the mining days. One mile beneath the surface, however, paints a different picture, a place anything but forgotten. It was here where a $10 million dollar detector sought to detect the anomalous bump of an undiscovered dark matter particle. Until its recent retirement in 2016, the Large Underground Xenon Experiment (LUX) was an effort that tried to observe the direct detection of dark matter, whereby the detector would give off a signal that it had been bumped by something. The detector is shielded by nearly a mile of rock to prevent Standard Model cosmic rays and other particles of our atmosphere from reaching the detector and giving a false-positive for dark matter.
This mile of rock is thick enough that the only thing that should be able to reach the detector are neutrinos and the elusive dark matter. Neutrinos are not of signal interference concern, however, because they’re known to be so weakly interacting that the detector is not (yet) sensitive to them. Under mountains and in mines across the world, direct detection experiments like LUX have not reported any conclusive signal, which is not the most exciting result. And yet, these results do rule out certain candidates of dark matter, which in turn focus our research efforts on more promising candidates. As these null results come in every few years, the detectors are upgraded and become more sensitive to a dark matter particle that might interact more weakly. These direct detection experiments are constantly being improved and eventually will become sensitive to neutrinos, which cannot be shielded by any mountain. These detectors will then hit the so-called “neutrino floor,” below which it will be difficult to distinguish a regular neutrino bump from the nudge of dark matter on the detector. While there are studies trying to figure out how to get around this neutrino floor -- some prefer the term “neutrino fog” -- the ceiling is rapidly coming down. Aside from a few minor signals which have not yet convinced physicists, the dark matter particle has evaded every test of direct detection thus far.
But even if we pursue all these experiments, even if we discover dark matter, what is the point of it all?
The first and easiest answer I might give is that it matters for technological applications, which is the most concrete and potentially useful outcome of knowing more about dark matter. While it is not immediately clear how useful dark matter might be for humans, the source of various technologies has been famously unanticipated in the past. For example, as NASA studied robotics and developed new materials for space flight, countless technological applications unexpectedly came about, from innovations in artificial limb research and cochlear implants to the improvement of solar panels. In the 1980s, particle physicists were smashing various particles together for experiments and a few of them invented the world wide web so they could share data quickly. Technology has a habit of sneaking up on us and improving our lives from places we least expect; who is to say what applications dark matter might yield?
Dark matter has even been studied as a potential rocket fuel for deep space missions. It is expensive to bring anything to space, let alone fuel, but if you are constantly submerged in dark matter fuel then that is no longer a problem. The dark matter fuel could even give off neutrinos as a byproduct when burned, for all we know, which makes it hard to imagine humans guzzling it at a dangerous rate reminiscent of fossil fuels here on Earth. While a dark matter fuel is an exciting prospect, it is difficult to foresee how or if this could ever be realized.
And yet, the realm of the possible is sometimes limited the most by our imagination.
Beyond the technological reasons for studying dark matter, there are other exciting possibilities that can be fun to imagine. In some models, the dark matter can form dark atoms or dark molecules, and one can envision dark planets, dark stars, or even dark beings. There might be an entire dark sector secluded from our realm but living in the same space. The world we are used to shows all these complex structures: from DNA to entire galaxies, and regular matter is only a small fraction of the universe compared to dark matter. About fifteen percent, remember? Isn’t it only natural to assume the same complexity extends to our mysterious friend?
My reason for studying dark matter is based on the profound realization that humans can understand this grand and ancient universe we live in. Only 300 years ago, with a pen and paper, Newton realized that what kept his feet on the ground was the same phenomenon that kept the planets in motion, and in less than a lifetime he united the Earth with the heavens. Newton passed down a technique for his descendants to use and make spectacularly correct predictions, like Le Verrier did with the discovery of Neptune. Discoveries like this bolster a belief that our theories must be describing something that must be true about our universe.
Today we have soared far beyond what Newton and Le Verrier could have ever dreamed. We now understand that we live in a universe that is 14 billion years old, and on a planet that formed 4 billion years ago as a species that appeared less than half a million years ago. We know that less than twenty particles comprise every human that has ever lived. These particles are the building blocks of our world, and we understand how these particles assemble themselves into complex structures. But despite the success of the Standard Model of particle physics, we know it is not complete.
We live in an interesting valley of time, where we are so sure of dark matter’s existence yet have so little idea of what it is. Dark matter has quietly evolved alongside us over the past fourteen billion years; without it, our galaxies would not have the gravitational glue needed to stay intact and intelligent life may never have formed. And so, we have rediscovered ourselves as particles that have been guided by an unknown dark matter: a dark companion which has guided the particles in us to search and eventually ask, “What is dark matter?” If a question like this doesn’t compel us to search in every dark corner of the universe for an answer, then what question does?
P.S. The answer to the Seussical particle challenge I posed is “chimps.”