The universe we can see is only a fraction of the great cosmic beyond. Galaxies, stars, planets, humans, trees — all of it comprises just 5% of the energy and matter in the universe. Among tangible matter, as opposed to the mysterious cosmic rending force called dark energy, only about 15% is the stuff we can detect. As for the rest, it comes in the unknown form known as dark matter.
This substance cannot be seen or held, yet cosmologists are broadly confident dark matter exists, because it is a shepherd of galaxies. Vast halos of dark matter surround every galaxy, including ours, and this invisible material can act as a lens to redirect the light that emanates from other galaxies, warping our vision of deep space. Dark matter also guides galaxy clusters as they evolve and move through the cosmos.
But we have no idea what it looks like, what it weighs, or how it functions. For decades, physicists have searched for a particle of dark matter in locations that range from deep underground mines to the International Space Station. All efforts have turned up empty so far. In other words, we don’t know what the universe is.
This is all extremely complex, mathematically abstruse, philosophically profound and, to theorists like Cora Dvorkin, great fun.
As a young girl in Argentina, Dvorkin read Stephen Hawking and fixated on the grandest questions humans can ask ourselves. She moved to the U.S. to attend graduate school at the University of Chicago. Now a theoretical cosmologist at Harvard University, Dvorkin comes up with new ways to ask those grand questions and then tries to find the answers. For her, cosmology is like philosophy, but with data.
Dvorkin investigates the relationship between everyday particles and the mysterious particles that must make up dark matter. For many years, the favored candidates for these putative particles have been shy things called weakly interacting massive particles, or WIMPs. Experiments that aim to detect WIMPs look for their calling cards, rather than the particles themselves. If a WIMP comes by and knocks regular matter around, the regular matter will recoil in a way that can be measured — albeit very carefully, and with great difficulty.
In 2013, Dvorkin published a groundbreaking paper examining scenarios where dark matter does not behave as a WIMP, but actually bumps into ordinary matter. The dark matter and ordinary matter might sail the void together in ways we have yet to understand. She hopes to probe this theory with an upcoming experiment, the next-generation ground-based cosmic microwave background experiment, or CMB-S4, which will use a collection of telescopes in the Chilean desert and at the South Pole.
Dvorkin’s research sits at the nexus of particle physics and cosmology, which have both reached a strange plateau. Both fields are in need of new evidence, and some cosmologists are calling for new concepts. Since scientists found the Higgs boson eight years ago, experiments have not shown how to move beyond the Standard Model of particle physics, the main paradigm explaining our shared cosmic underpinnings. Likewise in cosmology, the Lambda cold dark matter model (ΛCDM) says that there’s dark energy — the lambda — and dark matter, but it doesn’t say what they are. Dvorkin says she is motivated by all these non-answers.
“We are living in the golden era of cosmology, of cosmological data. There are so many current and upcoming experiments, it’s just like being a girl in an amusement park,” she said.
Quanta talked with Dvorkin about philosophy, paradigm shifts in science, neutrinos, and the questions that remain to be answered in the golden age of cosmology. The interview has been condensed and edited for clarity.
How did you become interested in physics?
When I was very little, I used to read a lot. One of the books that really inspired my interest was A Brief History of Time. I read it when I was very little, and I was very excited about questions related to the universe. I also had an interest in humanities, in philosophy, and in math. I realized that in cosmology, I could sort of combine my interests in philosophical questions, applying math to study what happens in the universe.
How is cosmology like philosophy?
I love the fact that with cosmology, you can make steps to try to answer fundamental questions about the universe using cosmological observations. So using actual data, you get to understand some of the most fundamental questions of the universe. And that is what I find fascinating, as opposed to philosophy, for example, which also asks fundamental questions about the universe, but it doesn’t deal with data. We have so much data; that makes it fascinating.
You know, it’s the questions that I’m searching for. To me, that quest for the correct questions to ask — in the moment in which we are living — is what drives me.
I feel like this is sort of a golden age for cosmology. We have so much that we know, so we can ask really good questions, and yet the main things are still elusive.
Exactly. There is a model that explains the universe at a very base level, but we don’t really understand the main components of this model. Like, what is the lambda in Lambda-CDM, and what is the CDM in Lambda-CDM, right?
Why is Lambda-CDM the leading theory for what dark matter is and does?
Lambda-CDM explains very well the observations at large scales.
We have tested this theory with different observables — with the cosmic microwave background, with galaxy clustering, with the Lyman-alpha forest. And all of them are in agreement with what is predicted by the Lambda-CDM standard model for cosmology. By construction, different models of dark matter agree well with observations at large scales. They have to — otherwise we wouldn’t use them.
But small scales have not been measured so well, and that’s why they provide fertile ground for testing different scenarios.
So how do we test at small scales?
With my research group over the years, we have been looking at gravitational lensing as a probe of dark matter at small scales.
What we work on is galaxy-galaxy lensing. So what happens is that you have a background galaxy and a foreground galaxy that deflects the light because of gravity. The foreground galaxy deflects the light coming from the source, because it deforms the space-time fabric according to general relativity.
So from our perspective, what we see is extended arcs in the sky that are the lensed images of the background galaxy. And we see if the galaxy doing the lensing is clumpy or smooth. We try to look for clumps of dark matter in a statistical way. We have also worked on directly detecting these clumps using machine learning methods, and we have seen that these machine learning methods are quite successful at this.
You can map the detections that you make to the small-scale fluctuations of dark matter in the universe, and you can put limits to different dark matter theories. For example, when dark matter is “warm,” it moves faster. Warm dark matter doesn’t allow matter to clump at small scales, in small regions of space. Cold dark matter, by contrast, has more matter clustered at small scales. So you can start to put limits on different dark matter scenarios.
Do we need something new to get us out of this mindset that we’ve been in for the last couple decades on dark matter? Are we sort of stuck?
It’s not that we are stuck; on the contrary, there are many new ideas that have been coming up in the last few years. With cosmology, you can indirectly explore the nature of dark matter, and that drives a lot of interest in different types of models.
And so you create new theoretical models and new ways to test them?
Yeah, exactly. It’s very fun, especially for upcoming experiments like CMB-S4, where I was involved in designing the experiments so that we can achieve important answers to important fundamental questions in physics. That’s very fun.
When you talk to younger people about this stuff, how do you convey that it’s fun? What do you tell them about what you do and why this is so important?
I like giving talks in particular to either women or young girls who want to become scientists, or other underrepresented groups. In those talks, obviously I don’t use technical language, but I try to convey the quest for the questions that we’re asking, as well as the quest for the answers. We want to understand how the place where we live works. That’s just a quality of human beings. We like to understand things.
In the quest for an answer, you open up a million new questions, and that’s the way it will always work. We try to get an answer to something, and we will never get an exact answer. We will approach an answer. It’s like trying to make the pieces of the puzzle be exactly in the right place. It’s not going to ever fit exactly, but we approach it. And while we approach it, many, many more questions open up. That’s sort of the nature of doing science. It opens up a world of questions. The more you know, the more questions you can ask and the more and more curious you get.
I have written a lot recently about early science, ranging from cultures in Mesopotamia to Galileo and his friends. And it’s fun learning about how much they learned and how quickly they learned it. And I feel like it’s sort of the same way now. It’s easy to forget that we just learned about dark matter 50 years ago, and we learned about dark energy only 20 years ago. This is really new!
Yeah. I think it’s very difficult to put ourselves in context with respect to the moment we are actually living in. It’s difficult to discuss contemporary discoveries in the context of our time when we are living in our time. It’s easier to put the context in place when we are talking about the past. But as you say, dark matter was discovered 50 years ago, and dark energy was discovered two decades ago. So it definitely hasn’t been so long compared to the history of humanity.
Are there any theories that really are compelling to you about these things, and about the nature of dark matter, especially?
I don’t have a favorite theory. Most of my work these days, and probably from the beginning, really relies on model-independent ways of learning about dark matter. So, for example, these days I am trying to learn about dark matter at small scales using gravitational lensing. And the beauty of that is that it doesn’t rely on any particular coupling between the dark sector and our Standard Model. I’m trying to map it without relying on any theory in particular. And in general, most of my work relies on model-independent probes of dark matter, or as modeling-independent as I can make them. So, no, I don’t have a favorite theory.
I’m very fond of neutrinos. I led a paper on neutrino mass and cosmology for the decadal survey last year. One of the main interesting things about neutrinos is that it’s the only evidence that we have right now of new physics beyond the Standard Model.
Some of your work, dating to your 2013 paper, explores the possibility that dark matter may be something other than WIMPs. What would these different particles be like, and how would they interact with regular matter?
We consider lighter masses, and we consider higher cross sections. The cross section describes the probability that a particle will be deflected by a given angle during a collision.
The particle in this scenario would not be a WIMP, because its cross section with regular matter is much higher than that of the WIMP. It could be a particle that is produced by feeble interactions of standard-model particles that annihilate and produce dark matter, as opposed to the general mechanism, where dark matter annihilates to produce standard particles.
There is no acronym and no particular name, because this particle could be many things.
I wanted to end by asking where we are headed next. There are all these really important, fundamental questions that we don’t know the answer to. What do you think will be the next huge, captivating discovery that really answers some fundamental questions?
If I told you what the next breakthrough will be, I would be lying, but I’m definitely sure that with the number of current and upcoming experimental efforts that we have, we will learn much more about the dark sector in the next decade. I mean, it will be really impressive. I think we will learn a lot about the nature of dark matter. We will learn a lot about neutrino properties. So I think in the field of dark matter and the dark sector more generally, we’ll be making a lot of progress in the next decade. It’s a field in which progress can be made on many different fronts.