Jo Dunkley is a leading researcher into the origins and evolution of the universe and professor of physics and astrophysical sciences at Princeton University. In her work, Dunkley uses the Atacama Cosmology Telescope to probe the history of the universe and study the ancient radiation known as Cosmic Microwave Background. She has received numerous awards and honors, including the James Clerk Maxwell Medal, the Royal Society Rosalind Franklin Award, and, most recently, the New Horizons in Physics Prize. In October 2019, following the publication of her book Our Universe: An Astronomer’s Guide, she took part in a wide-ranging conversation with Zócalo editor-in-chief Lisa Margonelli at a salon in Los Angeles, California. They discussed how to conceptualize the vastness of space, the startling legacy of women in astronomy, and the possibility of life outside Earth.
This transcript has been edited for clarity and length.
Lots of people start off in astronomy by looking at the stars. Is that how you started?
No. So I grew up in London and the stars are not so plentiful there. I loved doing math, and I thought that I would actually study that. I realized gradually that I could use it to understand how the world around me worked. That seemed amazing—that you could use these big, clean, nice equations to actually answer real questions about the real world; why is it like it is? But it did take me a while. I went to study physics in college, and then I slowly realized that the questions that I found most fascinating were the ones actually about space.
Did you actually have a moment where you suddenly were like: stars?
There were two moments: One, I remember, was actually in my first year in college, when we studied special relativity for the first time. It blew my mind, because we had examples like, ‘How fast would you have to run to fit a 20-foot pole into a 10-foot garage?’ And there was an answer to that; there was a speed. That's what special relativity is. I remember thinking: This is so cool.
I had another epiphany about stars while backpacking across South America after my degree, actually. I thought I didn't want to be a scientist. And then I backpacked across from Ecuador to Brazil and passed through the most wonderful skies you could imagine.
So it was stars?
It was stars.
This has been an incredible decade for astronomy. Tell me about some of the things that have happened in the last 10 years.
So many things have happened, and it's all come from being able to have better observations of space.
I would say one of the things that's happened just recently is this amazing wealth of knowledge about the planets around different stars. In the 1990s, no one knew if the stars in the night sky had any planets around them. If you look up in the sky, you see the stars. Astronomers had long said, ‘Well, yeah, but do they have solar systems around them? We live in ours, and there are planets going around the sun. Do they have that? Do they have any?’
Planets are really hard to find, but astronomers finally figured out how to do it, and they discovered that they are there. Just in the last decade, we found thousands of new planets and solar systems around us. Now we think that probably most of the stars that you see twinkling in the night sky have their own solar systems around them.
Which is wild. How could we have missed them all these years?
They're really hard to see. The thing about a planet is, it doesn't send any light. So, it's actually really hard to find a planet, because you look up and you see the twinkling stars, but you don't actually see the planets themselves. Astronomers had to figure out really clever ways of finding them. Actually [the three scientists who] won the Nobel Prize just a couple of weeks ago, they used this really neat effect—if a planet is going around a star, even if it's quite small, the gravity of the planet pulls the star toward it. As the planet goes around, the star slightly wobbles backwards and forwards, because of the gravity.
So it's the wobble?
It's the wobble, yeah.
The universe is 50 billion light-years across. We are just such a crumb within this. How do you imagine such spaces?
It can blow your mind. As an astronomer, I have to figure out ways of not getting completely overwhelmed. We do it by trying to break things down into different scales. We don't try and think of it all at once because it's too much.
We'll think of our little solar system—where we live—and that's one scale. Then you step out, and you find the stars around us and that's a whole other scale. And then you can step out and find stars swirling around in galaxies. That's another object. Then we step out, and we find a universe for the galaxies, and we don't try and think of all those different scales at once.
It's a little bit like having an atlas of the world. You don't look at a map of a city at the same time as a map of the globe.
How has our sense of the whole universe changed in the past 10 or 20 years?
One of the big changes is that 20 years ago we thought that the universe might collapse back down to a thing called a Big Crunch.
What was the Big Crunch? We think that the Big Bang happened about 14 billion years ago, and it started [the process of] space growing. Einstein, who was a clever person, had a theory that told us that space should start shrinking, eventually, and it should pull back together. So, all the galaxies that are currently moving apart, should actually come back toward each other and crunch back down. Twenty years ago, astronomers went out to say, ‘Okay, how much is the universe slowing down? Is it going to eventually stop growing and turn back into this Big Crunch?’
Then, what they found was that space is actually growing faster and faster. So now it's this really weird thing that space seems to be growing faster and faster and faster, and we have no idea why.
What does it mean that space is growing? What's it growing into?
That's a really good question. First, I should go over my little tour of the universe. I'll ask you to imagine each step. As with many things in physics, simplifying things to an easier example is usually a good thing.
Let’s imagine making a model of the universe, the modern universe, that's just a long piece of elastic. So, imagine if you take a piece of elastic out of your clothing and you stretch it out long, and now put some stickers on it. The stickers, imagine, are going to be your galaxies; they're going to be your galaxies full of stars. Put them on your piece of elastic, and now stretch that piece of elastic. We don't think that someone is actually stretching the universe, but the really special thing about stretching a piece of elastic is it stretches everywhere. It doesn't stretch from the middle.
The whole thing is stretching out.
The whole thing grows. Yeah, exactly.
So everything is growing out?
Everything is growing. But the thing about the universe is that we don't think the piece of elastic has any ends. We think it might be infinitely long. And the real universe is three-dimensional. That's like having elastic in that direction and in that direction, and in that direction.
So it's this giant exploding ball of sorts?
Yeah, except it's not a ball. So we think it just keeps—there is this real misconception about the Big Bang, that it happened in one place, and it was things flying up from one place.
It's more like this kind of sudden stretching out of that piece of elastic, but like a three-dimensional version of that. But the thing is that because everything gets stretched out, wherever you are, you feel like you're in the middle of that stretch. So imagine you put yourself at one point on your stretch elastic and you peer out, you would see everything appear to be away from you, and you'd feel like you were in the middle of this kind of bang. But jump somewhere over there—
And you're still going to feel the stretch.
Exactly. You still feel like you're kind of at the middle of everything. So we think the Big Bang is more like a Big Stretch. Because we think it happened everywhere throughout space and everyone would feel like they're in the middle of it, but they weren't; they were just the middle of what they were looking at.
If it's stretching out, what is it stretching out into?
Well, so one thing is that we don't think this room is growing right now.
We can agree on that.
But actually it's an important point because I think there's a kind of conception that maybe it's growing a little bit, because the whole universe is growing. It genuinely isn't, because the gravity in our Milky Way galaxy is just way stronger than the gradual stretch of space. We really do think that it's the galaxies themselves that contain maybe a 100 million stars, a billion stars. They’re not growing. So it's really just the spaces in between them.
The spaces in between the galaxies are growing.
Then the galaxies themselves…
Are not growing. They're just moving further and further apart from each other. So from where we are on Earth or in the Milky Way, in our galaxy, we actually see galaxies move out away from us because we're living in this growing space.
That was actually something that was measured here, in Pasadena, at the Mount Wilson Observatory.
You’ve said that looking at starlight is like looking at a time machine. Tell me about that. How is it when I look up at the stars, I'm actually sort of time traveling?
It’s this really cool thing. Basically, whenever you look out anywhere you're looking back in time. So even in this room, actually, if you look at a light, you see the light, but the light had to travel from the light bulb to your eyes for you to see it. The light doesn't travel instantaneously, it just travels really fast. Light is the fastest thing we know of, but it's not infinitely fast. It has to get to you. When you see the light, you see it as it was a split second ago.
It's old by the time it gets here.
It's old, that's right. Now you go further, to the moon; you look at the moon in the night sky, it took about a second for the light to get to you. So you actually see the moon as it was one second ago whenever you see the moon. Now the sun is about eight minutes, so you'd see the light from about eight minutes ago. The solar system itself—light takes hours to cross from the furthest reaches to us. So if you look at the furthest reaches of the solar system, you're talking about looking back hours in time. But now you go to the stars. For the nearest stars, the light has taken years to get to you.
So it could be gone, but you would still be seeing it?
Yes, that's right. The further you look out, the further you're seeing back in time, because you're seeing the object as it was when the light set off from it. If we step through into our Milky Way galaxy, we're seeing things that the light set off hundreds of thousands of years ago. But if we go out to distant galaxies, we see the light from millions and billions of years ago. It's this amazing thing, that the further you look right out, the further back in time you see.
So you're seeing some deep, deep history out there.
Yes, and it's incredibly useful. We would like to figure out how the whole of the universe behaves, but all we can do is sit here on Earth and look. We can't go anywhere else in the universe to look. But the fact that we get to see successively distant parts of it in the past means we can sort of piece together what its lifespan has been. The analogy is: Imagine an alien arrived on Earth and their job was to figure out how humans evolve through their lifetimes to get up to their job. Imagine you gave them a room full of eight-year-olds. They might have some clue, but they won't do a very good job. Now give them a room full of humans of all ages, a baby, a child, a teenager, an older person—all the way through, from zero to 80. Now, they could do a pretty good job of figuring out how humans evolve, because they get to see them at all stages.
That's actually what we do with the whole universe, we see the local parts of it as it is now today. But we see the further-out parts as it was back in time, billions of years ago, throughout its history. Even though we can't see our bit as it was today, we can see someone else's. You don't need to be able to track one human for its whole life to figure out how humans grow and change in general.
You study the very beginning of time with the Big Bang. So tell me what is primordial light?
It's the very earliest picture we can take of the universe. The further we look out into space, the further we look back in time. Since the Big Bang happened about 14 billion years ago, the furthest we can look back then is at light that basically set off 14 billion years ago, at the Big Bang itself. We think that when the Big Bang happened, that light was actually made during the Big Bang. That's completely different from stars: The stars that you see in the sky came a lot later, and that's the thing we think of as twinkling light in the sky. But there was light made during the Big Bang itself, and that's still around.
This was what blew my mind. That light is still hanging around from 14 billion years ago.
Yeah, because the universe is really big. Imagine turning on a lamp in this room. Now imagine turning on a lamp in this room in the middle of space. Its light will set out and travel indefinitely until it hits something. We think that there was this time, very early on in the universe, which was a bit like switching lamps on throughout the universe. This thing happened, and that light just set off and traveled through space forever until it hit something. We are today just being hit by some of it right now. We are all right now being hit by some rays of light that set off 14 billion light-years away from us.
And it’s called cosmic microwave background. What do the waves look like? What are the waves?
They are waves, so you can't see them with your eyes. The microwave is a hint; the wavelength of this light is the kind of wavelength that you might find in your microwave oven, which is about, well, the light we measure, it's about a millimeter long in wavelength.
So these little tiny waves of light are coming at us. How hot are they?
They're incredibly cold. This light has been traveling for 14 billion years, and as the universe has grown, it's stretched and cooled down. It's almost absolute zero. It's minus 270 degrees.
So your telescopes have to be frozen?
Yeah, we have these cameras and inside them we have these things called fridges, and they cool things down to minus 270 degrees.
And when the little lights hit them, they can measure the little tiny fluctuation?
That's right, and the thing about it is when you see an image—you might have seen an image of this in the press, which is this oval of blue and red splodges—it looks like nothing, but it is actually a picture of what some of the universe looked like at the beginning. Why do you care? Well, those little blobby features were actually these very, very tiny, tiny, over-dense regions of space, but over millions of years, gravity would pull lumps of matter together to eventually form the very first stars and galaxies.
One of the questions I have as a cosmologist is: How did we get to be here? Why are we here at all? The amazing thing is that we can actually track, using just laws of physics, how things got from tiny bumps 14 billion years ago to actual planets and stars today. You can actually follow that process.
What I want to talk to you about is the process of figuring this stuff out. What's it like to wake up as an astronomer and start your day looking for 14 billion-year-old light chunks?
Well, the first thing is that we work in big teams. So you can't do this by yourself. The thing I love is that everyone does a different piece of it. Some of my colleagues will actually build these incredible instruments that can cool things down to almost absolute zero. The thing that I and my team of postdocs and students do is take these images from the telescopes and decode it. We basically match it up to models of the thing we get at the end. The thing that I'm trying to do right now with my new data is to say, how fast is the universe growing? We're trying to find out which universe looks like the thing that's in our data. So we generate billions of possible universes.
So you take the data that you get, and then you model it and you create potential universes and compare it to the real one?
It's very computational. The thing I do is I write computer codes to study the data and match it up to say, which universe fits? Well in practice, mostly I'm advising students and postdocs who are doing that.
But you're communicating then with people all over the world?
Yeah, this morning actually I had a phone call at 7 a.m. here, which was the one time when we could get our collaborators to meet. We have collaborators on the West Coast, East Coast, Europe, and Japan. Figuring out when we can all speak is definitely a challenge. But it is incredible how we're all working the same problem. That is a real aspect of astronomy.
So it's this global conversation between scientists, between the astronomers, who are the mathematicians, the physicists, the cosmologists.
Yeah, that's right. I do love that about our community. We're all asking the same questions and then we build these teams with people from all over the world. We just say, "Hey you, you're the best at doing that. We need you to answer our questions," and that makes it really fun. It's very international.
It strikes me that it is almost philosophical.
It is. I think one thing that's really changed is, even a hundred years ago, no one had any idea if there even was something beyond our own Milky Way galaxy out in space, and that there'd even been a beginning of the universe. People were speculating about things much more in terms of ideas of cosmology. Thoughts about how our universe could be were very philosophical because you didn't have an ability to look at it.
It's really changed in the last 50 years or so, where I would say the philosophical questions are still there, but they don't dominate what we do because we actually have data. We can see stuff, we can see the universe evolving, we can see how it happened. So we can actually push the philosophical questions actually almost outside the realm of astronomy.
The other thing that's struck me is it seems like we’re kind of in a golden age of women in astronomy. Tell me a little bit about how having more women in astronomy is changing things.
Well, here our field has not got as many women as it should. I don't know how many women would be doing astronomy if we had a genuine culture where it was completely accepted that men and women both do science and like science. I don't how many would be still doing astronomy, but it'd be more than we have now, because we are losing tons of very smart women to the kind of cultural misconceptions about what a scientist is. But it's really changing. There are more of us in the field than I think ever before. There's a realization that you need to get the best minds.
You need to tap into all possible resources, and also that people come with different skills. You want, the kind of science we do nowadays needs people with very, very many different contributions and skills and ideas, and the better diversity throughout, the better. But yeah, we have, astronomy has this wonderful history—this is when I discovered more in writing this book than I knew before starting it—this history of the wonderful women doing, making these incredible discoveries. One of the key figures was Vera Rubin.
Tell us who Vera Rubin was.
She was an astronomer who sadly died just recently. I'm at Princeton University now. She applied to graduate school at Princeton, but was not accepted because she was a woman, and at the time women were not accepted. She went to Cornell, and then actually followed her husband down to Washington to do her PhD. The end of her story is that she basically figured out—well, really, established—that there's five times as much invisible matter, stuff in the universe, than the stuff that we're actually made of. It's this thing called dark matter. It's one of the biggest issues.
She started working on it in the ’50s?
Yes. She had a pretty challenging time. Even to do her PhD, she had young kids and she went to do her classes in the evening. It was just an incredible logistical challenge getting to go to the classes in the evening and looking after the kids at a time where it was not really an accepted thing to do—to have a young family and to also do a PhD in astronomy. She kept having these challenges but she's just doing her work. Then she realized that one of the things she wanted to do was to go and look at how galaxies spin around.
She realized that the key is, how fast does a galaxy of stars spin around? Because if you can figure out how fast it moves around, you can figure out how much it weighs.
There are all these things that you can't usually do in space. You can't measure the distance of things in space because you can't go there, and you can't weigh things easily because you can't put them on a weighing scale. But Newton told us that the heavier something is, the faster the thing spins around. If we made our sun 10 times heavier, we would orbit around it more quickly. If you can measure how fast a galaxy spins around, you can weigh it.
That's such a cool breakthrough.
It's really cool. It's very convenient. You can actually kind of do weight scales in space. Vera Rubin realized that was one of the things that she wanted to study, and she had the same idea as the wiggling stars with the planets around them. In the same way that you would look for a planet because the star wiggles, a rotating galaxy, also, you look for the squeezing and stretching of the light. The stars that are coming toward you, the light gets squeezed to a shorter wavelength, and the light coming away from gets stretched.
So some are blue and some are red?
That's right. She was looking for that. But to measure that, you need to have a really good telescope that can look across the whole of the galaxy and see different parts of it, and how fast different bits are moving. So she realizes that she needs to use the best telescope available at the time, which was at Palomar Observatory here in California. But women were not allowed to use this telescope. It said—on the application form—it said women are not allowed. She applied anyway. I think she edited it to say, “most are not allowed.” One of the reasons given why they weren't allowed was that there were no restrooms. I mean, come on. I think it was just one bathroom and how could you possibly have a woman using the telescope if there was no bathroom?
Anyway, she was the first woman to use the telescope. It was a magnificent telescope. She found something that had been seen before by Fritz Zwicky here at Caltech. The galaxies were all spinning around too fast. They were all going much faster than they should be given how many stars you could see in them. The explanation for that is this group of stars that we think of as a galaxy is actually five or ten times bigger than it looks. Actually, each galaxy that you might have seen in pictures, the swirling stars, actually is surrounded by and threaded through with invisible matter. It's kind of crazy.
Can you explain what invisible matter is?
Invisible matter is something that—it might be a particle that would fly through your body right now. In fact, we think that if this invisible matter is going through our bodies right now, there'd be particles that just flow through us, but they have gravity. So, imagine we could turn our sun into dark matter. We would not be able to see it, but we would still orbit around it. We think that the stuff is just like everywhere throughout space.
This is fantastic. You raise so many unnerving questions. In addition to dark matter, there's also dark energy, which suggests that we may be missing whole parts of our theories.
On the one hand, it's kind of extraordinary that we're in a place now where we can approximately say we think we understand how the universe evolved over its whole lifetime from the Big Bang to what we see around us today, the stars in the night sky. We think that kind of makes sense, which on the one hand is amazing. But it has these really fundamental issues. We say, yeah, there's always invisible matter throughout it, and there's some weird energy that's making it grow faster and faster, and we don't know why it began to grow in the first place.
When you have that many questions, you do start to wonder whether there's something really big in the physical description, the physical laws, that we've got wrong. If we look back in history, this just happened. There's no way we’ve got it right, right now. That just doesn't happen. We're always updating.
We were always wrong.
We were always wrong. But we're right enough in a way we need to be, and then where we're wrong, we push it. Then you put in new laws of nature that explain where it's a bit wrong.
So we may quickly have to learn a whole new thing?
Yeah. I mean, I think there may be big new physics out there. We might've got it quite wrong.
I wanted to hit one of the important questions before we go. We've got all these thousands of planets that we're finding now. Do we have to assume that there are other life forms out there?
I think this is such a fascinating question, and it's why this field is just blooming. Most planets that are out there will definitely not be habitable. Many of them are really up close to their stars and orbit around their stars in four hours or something. Just insane places that you could never live. But people are increasingly finding planets that are in what you call the habitable zone, which means they're just far enough from a star that you could maybe have life on it, and they're rocky, because you need to be able to stand on a planet. Well, for having life like us, anyway.
We're just speculating now about how many of these planets might have life. I think in the next ten years, 20 years, 30 years, we're going to see enormous advances there, where people will be looking at the atmospheres of the planets and saying, what elements do we see there? Do we see hints of things like oxygen and ozone that might be signs of life? I think we genuinely don't know right now how many of those planets will show any signs of life. But this is possible. I think that in our lifetimes we could see; it depends—most of these planets are so far away from us that we wouldn't be able to tell, but maybe the closer ones we'd be able to see hints where we might say, yeah, maybe that's a sign.
Maybe we're not alone?
We're not alone, yeah.
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