Theoretical solutions to the very real dark matter problem.
There’s something bizarre going on with over 80% of the matter in our universe: we don’t know what it is. The normal matter we touch, see, and interact with every day is only 20% of the universe. The rest – that other 80% – is a mystery.
We call this stuff “dark matter” because we can’t see it; it doesn’t emit light the way that normal matter does. From our cozy Earth-bound perspective, we are faced with the weird task of trying to study a huge chunk of the universe that we know alarmingly little about. It’s as if we were ants trying to figure out what a cloud is only by observing its shadow. Even equipped with sophisticated telescopes and lots of hardworking scientists, “what is dark matter?” remains one of the biggest questions in physics and astronomy today. Dr. Chanda Prescod-Weinstein is a theoretical cosmologist/particle physicist/astrophysicist who is determined to be part of the answer.
Seeing the invisible
If we don’t know what dark matter is, how did scientists discover it in the first place? We’ve actually known about dark matter for a while now. In the 1930s, astronomers noticed that galaxies within galaxy clusters were moving around more quickly than expected. This odd observation was kind of forgotten until the 1960s when Dr. Vera Rubin noticed something weird about the way stars were moving around the edges of galaxies. Her measurements showed that stars were moving faster than we expected them to move based on the amount of mass we could see. In other words, there was way more gravity than there were stars that could be generating the gravity. (Read more about Dr. Vera Rubin, her incredible discovery of flat rotation curves, and gravity here.)
After this discovery, more astronomers and physicists began to detect similar strange results. They all pointed to the bizarre conclusion that there is a LOT more mass in the universe than just the mass we can see. Over and over again, calculations show that normal matter only makes up 1/6th of the universe’s matter.
MISSING: non-luminous, massive, spherical halo
As for that other 5/6ths, all we can observe about it at the moment is its gravity. Gravity is like dark matter’s shadow: we can measure the effects of its presence, but we can’t get a direct observation of the source. We can tell what its size and outline is, but not its full shape, color, or composition.
So where can we start in the search for dark matter? Scientists often need to find a way forward despite having very few clues to build on. When we’re totally lost, we have to start at the beginning with what we do know. In this case, all the evidence gathered in support of dark matter indicates that it should fulfill three requirements:
It should be massive. The dark matter problem began when extra gravity was observed throughout the universe, and gravity is generated by mass. So whatever dark matter is, it should definitely have a gravitational effect and mass.
It should be dark. In this case, “dark” means it shouldn’t emit light. What’s more, dark matter’s interactions with all the other forces in the universe (the electromagnetic, weak, and strong forces) are either non-existent or are so hard to detect that we haven’t seen any of them yet!
It should be distributed in a spherical halo around a galaxy. Normal matter is concentrated in a flat disk throughout a galaxy, but dark matter has a much rounder distribution. We know that dark matter is shaped like this because we can see its gravitational effect— its shadow.
Every galaxy in the universe was made possible by dark matter’s gravitational force, which drew normal matter together and ignited the very first stars. Our Milky Way, Solar System, and Earth wouldn’t be here without dark matter, but we don’t even know how to address our Thank You card. Trying to face down a big mystery such as this requires courage, and Dr. Chanda Prescod-Weinstein is staring that challenge in the face!
Creative names for creative solutions
Ever since the discovery of dark matter, physicists have worked around the clock to figure out what it could be. Their guesses have some pretty fun names: RAMBOs (Robust Associations of Baryonic Objects), MACHOs (Massive Compact Halo Objects), MOND (Modified Newtonian Dynamics). These are all creative attempts to explain the dark matter problem using physics that we already know about. However, none of these guesses can fully explain the weirdness of dark matter– none of them meet all three requirements we listed above. If the physics we know about can’t explain dark matter, then it must be made of something new, something we haven’t been able to imagine before. It’s time to look beyond the standard model of physics, toward the vast and wild unknown…
One possible explanation for dark matter is that it's made of some brand new particle that we haven’t discovered yet. It’s possible that there are tons of new particles out there that we just haven’t been able to detect yet, and it’s the job of scientists called theoretical physicists to imagine what those particles might be like. But how do they come up with new particles anyway? Do they just choose random properties and give it a funny name? It’s a bit more rigorous than that! Physicists should only imagine particles that fit within what we know for sure about the universe. For example, we’ve measured exactly how big an electron is, so the theories that physicists propose can’t contradict that fact. This is an abstract process, but has a higher success rate than you might expect. (Mathematician Dr. Emmy Noether’s research was super important for this kind of physics.)
Some of the theoretically proposed dark matter candidates also have silly names: WIMPs (Weakly Interacting Massive Particles), SIMPs (Strongly Interacting Massive Particles), Sterile Neutrinos, Axions. None of these particles have been proven to exist yet, but we also haven’t been able to rule out the possibility of their existence. It might seem hard to prove or disprove the existence of something theoretical, and that’s because it is. Physicists test their theories either by detecting the proposed particle (this is super hard to do since we’re talking about brand new physics here), or by finding something that contradicts the theory and rules it out (this is more common, and we call these constraints on a theory).
Chanda Prescod-Weinstein is one of the theoretical physicists trying to propose and understand potential dark matter candidates. She studies the theoretical particle called the axion. Something she finds cool about the axion is that if it exists, it would actually solve two physics mysteries at once.
Axions were originally proposed to solve a completely unrelated problem in particle physics called the strong CP problem. This problem is a complicated and subtle mystery of particle physics, but it would be answered if this new type of particle existed. Axions were actually named after a popular laundry detergent at the time, since they “clean up the problem”.
It turns out that axions may also be able to “clean up” the problem of dark matter. If the theoretical predictions are right, then axions would have all the properties we know that dark matter has to have – they would be massive, dark, and potentially could be distributed in halos around galaxies, just like we expect!
And is the “axion” in the room with us right now?
There are a lot of great dark matter candidates that are massive and dark, but can’t replicate the dark matter structures we observe throughout the universe. Dr. Prescod-Weinstein works to check if axions could form those structures, like the outer halos of the biggest galaxy clusters or the central cores of the smallest galaxies. She also studies how the universe has evolved and changed over time. Dr. Prescod-Weinstein combines these perspectives on the universe to ask the question “if axions are dark matter, can they explain how the universe evolved into what we live in today?”
Answering such enormous questions takes collaboration with many different people. Dr. Prescod-Weinstein works with experts in math, astronomy, and particle physics to understand what the universe would be like if dark matter is made of axions. If their predictions match the universe we can see with telescopes, then that’s more evidence that dark matter may be axions. For example, Dr. Prescod-Weinstein works with astronomers who observe super dense objects called neutron stars. If axions exist, they could collect around the super strong gravitational fields of neutron stars. And maybe these huge clusters of axions could have some observable effect, letting us confirm axions’ existence.
Because it’s so hard for us to detect axions (if they exist!), it’s theoretically possible for axions to collide with us on Earth without us even noticing. Axions could lump together to form structures anywhere from the size of an asteroid (about ~500 kilometers, or ~330 miles across) all the way to the size of a galaxy’s core (about 100 parsecs, or 326 light years– way way bigger). We wouldn’t be able to directly feel the effects of a galaxy-sized lump, because we’re constantly swimming in it. However, an asteroid-sized lump could (on a very very very rare chance) collide with the Earth, but it would pass right through us and all we would feel is a teeny tiny gravitational tug!
Science is a human endeavor
Chanda grew up in east Los Angeles and decided she wanted to be a theoretical physicist when she was 10 years old. She majored in physics and astronomy at Harvard University, got a masters degree in astronomy and astrophysics at the University of California, and finally earned her PhD in Theoretical Physics from the University of Waterloo in 2011. She was the 54th Black American woman to earn a PhD from a physics department. She’s now a physics professor at the University of New Hampshire, and is also a faculty member in the Women’s and Gender Studies department there. Dr. Prescod-Weinstein is a brilliant physicist, author, and essayist, but she also has expertise in tons of other fields like sociology, Black studies, and feminist thought. She draws on this expertise – as well as her life experiences as a Black, Jewish, agender person and a child of immigrants – to drive her research in a field she’s titled “Black Feminist Science, Technology, and Society Studies.” She’s also a powerful advocate in STEM. She works tirelessly to promote Black students in physics and to broaden participation of other underrepresented minorities in the sciences.
Chanda believes science is a collective human endeavor, and that together we can expand our knowledge and reassess the things we think we know. She says, “I believe the Universe is always more amazing than we think it is.” And through Dr. Prescod-Weinstein’s work, we all get a glimpse of how weird, enormous, and amazing that universe is.
Written by Madelyn Leembruggen
Edited by Katie Fraser and Ella King
Illustrations by Sal West
Sources and Additional Reading
The Other Dark Matter Candidate from Symmetry Magazine
What is Dark Matter Made Of? These Are the Top Candidates from Discover Magazine
Do Dark Matter Axions Form a Condensate with Long-Range Correlation? by Alan H. Guth, Mark P. Hertzberg, C. Prescod-Weinstein
Relaxation times for Bose-Einstein condensation in axion miniclusters by Kay Kirkpatrick, Anthony E. Mirasola, Chanda Prescod-Weinstein
Explore the search for dark matter through these activities!
Define (20 minutes): Explore the Standard Model of Particle Physics, and take a virtual tour of the world's largest particle accelerator facility, CERN!
Deepen (15-90 minutes): Read about Dr. Vera Rubin's discovery of the first experimental evidence for dark matter. Be sure to check out some of the activities related to her research, like measuring star speeds, building a spectrometer, or simulating dark matter.