How Fast Is the Universe Really Expanding?
Introduction
One of the biggest mysteries in cosmology seems to keep getting bigger. Astronomers have known since the 1930s that the universe is expanding, but in the 1990s, the discovery that this expansion is accelerating rather than slowing down came as a huge shock to the field. Something had to be driving that acceleration, and dark energy was proposed as the cause. What began as a seismic shock in cosmology eventually led to a Nobel prize.
But this story has a sequel, and it comes with another major plot twist. The two main methods for estimating the universe’s present-day expansion rate are producing significantly different answers. As a result, how fast the universe is really expanding has become a matter of considerable debate — with no small amount of angst — and the discrepancy has become known as the Hubble tension. Perhaps most surprising of all is that one of the loudest voices raising concern is Adam Riess, the astrophysicist whose Nobel Prize-winning work helped ignite the acceleration debate in the first place.
Riess joined co-host Steven Strogatz on *The Joy of Why* to explain how we got to this point, what the Hubble tension might be telling us, and what may happen next.
Listen on Apple Podcasts, Spotify, TuneIn or your favorite podcasting app, or you can stream it from Quanta.
Transcript
[*Music plays*]
STEVE STROGATZ: I’m Steve Strogatz.
JANNA LEVIN: And I’m Janna Levin.
STROGATZ: And this is *The Joy of Why*.
LEVIN: A podcast from *Quanta Magazine*, where we explore some of the biggest unanswered questions in math and science today.
STROGATZ: Well, great to see you, Janna. How’s it going?
LEVIN: Hi, Steve. Yeah, great to see you. I’m excited to hear what you have to talk about today.
STROGATZ: Yeah, good, because I could barely contain myself. This was an especially fun conversation I had with Adam Riess, who I guess is someone you would know from the circuit.
LEVIN: Yeah. Definitely. I was speaking with him not too long ago.
STROGATZ: Uh-huh. Well, right, I would think that in the astrophysical world you would have crossed paths, so I’m not super-surprised to hear it. But actually, I myself had not spoken to Adam or seen him for many, many decades. It turns out he was a, uh, a student when I was just beginning as a professor.
LEVIN: Oh, wow. In your class?
STROGATZ: He was literally one of my students at MIT in my first job.
LEVIN: Wow, how amazing to think these young kids in your class, one of them’s gonna be a Nobel prize winner.
STROGATZ: Well, exactly. Yeah, very memorable student. Really a great guy. But, I would actually like to pick your astrophysics brain if we could for a second because one of the things that I talked to Adam about is the issue of the universe expanding and also its expansion accelerating.
LEVIN: Right.
STROGATZ: You know, like you were there back in 1998 when we got the news about the acceleration. Do you, um, remember anything about that time? Like what it felt like in your community?
LEVIN: Oh, yeah. Well, I was at Berkeley, and that’s where Adam was. And also Saul Perlmutter, the other group, right? These were two different groups working. So I absolutely remember, I was at the Center for Particle Astrophysics, and Saul was kind of up in the hill in Berkeley, and he was coming down with these results, and I was like, “Come on, man.” Then there was a lot of discussion, honestly, about how there’s huge sensitivity to the temperature of the supernova, you know, very high power.
STROGATZ: And so the reason you’re bringing up this thing about the power and the sensitivity is that the results were so shocking that everybody wanted to be very, very careful.
LEVIN: Very, very careful. And this is outside my field, this is very observational. I’m more on the theoretical side. But, I remember so well, though, just being in the room while those conversations were happening between the observers, and they were really being interrogated by the theorists who were very interested in data. You know, the theorists right on the cusp of data. It was really interesting times, no question.
STROGATZ: That’s exactly where we’re gonna go. So, for our listeners who may not know about Adam, in addition to having been a student at MIT, soon afterward, he shared a Nobel Prize for his work on the discovery of the accelerating expansion of the universe. That was in 2011. But since then, as we’re gonna hear, there have been quite a few plot twists in the story. As new data have come in, he’s had to revisit some of his findings and the findings of other people in the field. So if you’re ready, Janna, should we go cosmic?
LEVIN: Let’s do it. It’s a really interesting moment in cosmology, for sure.
STROGATZ: Fantastic. Okay. Well, here we go.
STROGATZ: Adam Riess is a professor of astronomy and physics at the Johns Hopkins University and a senior member of the science staff at the Space Telescope Science Institute. Among his many honors, Adam is the co-winner of the 2011 Nobel Prize in physics for his part in the discovery of the accelerating expansion of the universe. Welcome to *The Joy of Why*, Adam.
ADAM RIESS: Thank you for having me.
STROGATZ: Oh, well I am so excited to see you I think it’s something like, I’m guessing about 35 years from the time. When you were an undergraduate in a class that I was teaching in the math department at MIT, it was a course in complex analysis, but I’m wondering, as a couple of old men now, can you tell me what you remember from that experience?
RIESS: Sure, of course. MIT was very impactful to me. You know, being a student there felt like a big jump, and so each of those courses felt like, you know, Job’s trials. And I remember that class was a sea of tranquility in the midst of other things that were super tough and rough.
I think it was because of you, to be honest. I remember vividly sitting in that class. You were a young professor. And I remember one day sitting there puzzled and getting ready to raise my hand, and you said, “Adam, you look bothered by this concept. Is there something about it that bothers you?”
And I was amazed. No professor, first of all had ever said my name, let alone actually gauged the expression on my face and cared that something was disturbing me. And, you know, we got right into it. And, so that course was great. I’d like to say that I spent the rest of my career focused on that material, but I moved over towards physics.
STROGATZ: Well, thank you for sharing that memory. I have to say, I do sometimes look at students’ expressions on their face and I can see something. You had a very animated face. You still do.
I mean, I think I’ve taught a few thousand students now so I don’t remember all my students, but I do remember the ones that win Nobel prizes and I think you might be the only one. So it’s really very—I’m very proud of you. I’m thrilled to be able to talk to you now. Anyway, it’s great.
So you say you moved into physics after, or I guess you were already probably pretty intensely interested in physics at that point?
RIESS: Sure, I was. You know, at MIT, everything is numbers instead of names. So I was course eight, and I was taking the required courses in math, which was course 18. That course was 1804, not the year. Although sometimes we feel that old, I know. But, but, you know, really my passion was physics. It was really understanding the physical world.
STROGATZ: And so very quickly that interest led you into subfield or adjacent field of astrophysics, and into the study of very spectacular objects in the universe, called supernovas or supernovae — if we want to be in Latin about it. So I have to admit, I’m not an astrophysicist. You probably realize that, and I need some of the basics explained here. So what are supernovas and then why are they interesting objects to study?
RIESS: Right. I’ll take the second — why they’re interesting objects to study — and I’ll jump back and say, what is the question or what is the point? And so to me, when I went from physics to astrophysics and first learned what it is that we know about the universe, I was fascinated by the discovery that the universe is expanding. I just think that’s amazing. It’s, as a kid, it’s not what I would’ve expected.
I would’ve expected, the universe is just the thing that’s always been there. It’s like the bedrock, you know, it’s eternal, it’s unchanging. And so that was amazing to learn and then to further learn that we still don’t understand that much about the universe. And so by watching it expand, by measuring it expand, by seeing its expansion history, we can deconstruct the nature of the universe.
We can figure out how old it is, we could figure out what its ultimate fate is. We could figure out what it’s composed of. And so those are all really, to me, the questions that I wanted to have answered. It just turns out the means to do that is to have reliable tracers of the expanding universe. So objects in the universe that act like test particles that we could watch.
And it just so happens when I started graduate school that exploding stars, supernovae, became some of the best tracers right at that time. Because they’re so luminous, you could see them far away. So you could see far into the history, the past. But also, and this is the most important point, which I’m sure we’ll talk about, to measure the expansion of the universe, you need to measure two aspects of your tracer. You need to measure what’s called the red shift, which is essentially, the stretching of the wavelengths of light emitted by that tracer because of expansion.
STROGATZ: Well, that’s fascinating. So you’ve already anchored us in really fundamental questions here that although I was pitching it as, “Oh, you like astrophysics.” Maybe I was mistakenly emphasizing astro, like, you’re curious about supernovae as things in themselves. Like they are a tool for you. Sure they’re cool objects, but you’re using them because you wanna ask really big questions: How fast is the universe expanding? How do we know it’s expanding? That kind of thing.
RIESS: Right. This is sort of the dichotomy that we sometimes call astrophysics versus cosmology. Cosmology, the study of the structure, the shape, future, the past of the universe, as a whole entity, is really fascinating to me. And, you know, as a result, you have to study all the astrophysics and the physics and the math and the nitty gritties because, you know, you need to tease out that information.
It so happens, in my case, I tended to study various classes of stars exploding, pulsating as those critical tracers.
STROGATZ: So this question about the universe expanding and the — what you refer to as tracers — the exploding stars, and then estimating how far away they are using the redshift to tell us about how fast they’re receding. It’s really gorgeous thinking, I have to say.
I would love you to give us a little tutorial about how it is that people can estimate how far away galaxies are because it’s not an obvious thing and you have a lot of different techniques, whether using trigonometry — very near and dear to my heart — or your concept of standard candles. Just give me a little basics of the cosmic ladder.
RIESS: This is actually the meat and potatoes of what I do. I should have said, when we look at these tracers, we need to measure their red shifts and distances. And the redshift is the easy part. You just take a spectrum and you recognize certain colors or lines which have moved to a redder location. And it’s trivial.
The hard part is the distances. This is many have called the biggest challenge in all of cosmology is to figure out how far away things are. And, it’s a really fundamental problem. It goes back to, you know, almost being a little kid. You look out at the sky and everything looks very two dimensional, right? And you have no sense of depth. The depth perception is completely absent. And so, we’re in awe, but we’d be in more awe if we just understood how far away things were, how far back in time.
So how do you lick that problem? And as you said, we start out with the things we know, which is geometry. And so we measure parallaxes when we can. And so just like we have two eyes and we get a vantage point from each of them on nearby objects, and the changing perspective or angle of something nearby, relative to far away, allows our brain and eye to do the geometry and estimate how far away something is.
The problem is that space objects become very, very far away so that angle either becomes very tiny or you need a much bigger baseline, a separation between the two perspectives. And the best one, the biggest one we get is when the Earth goes around the sun and we could view a nearby star relative to a distant star, let’s say in January when we’re on one side of the sun and in July when we’re on the other.
And if you’re lucky, you might be able to tease out the little change in angle of that nearby star. And so you can gauge the distance to some of the nearest stars. And we do this with space satellites and things like that. The problem is we’re after deeper waters. This is only a technique we can apply within the Milky Way galaxy.
To be able to measure the distance to galaxies — which are thousands, millions, billions of times further away than anything in the Milky Way — that parallax angle would become imperceptibly small. So we have to switch to a completely different method.
And the method we use most commonly is what ship captains know to use at night, which is the brightness of a lighthouse, right? So, you know, if you’re a ship captain at night, you wanna make sure you’re far enough away from a rocky shore. So you look at a lighthouse and you look for it to be faint, telling you that it’s far away that the light attenuates or dilutes as one over distance squared. That’s geometry too, of course, but it requires some understanding of what it is you’re looking at. That it’s a truly luminous object, a lighthouse, not a little pen light. And then you can gauge distances.
And so that method astronomers call “standard candles,” the lighthouses. And they look for objects in space that can serve that role. And as I started graduate school, there was great recognition that a certain class of exploding stars, called Type IA supernovae could serve as outstanding lighthouses. They were very homogeneous, very uniform. They had small differences. But they gave you what was a pretty standard light source that was maybe 4 or 5 billion solar luminosity in total output for a few weeks. So that allows you to see very deep into space.
STROGATZ: So let me just underscore that last thing you said, because your whole subject is so mind blowing that you rattle off these things that, you know, you’re so used to by now. If the listener didn’t catch that, Adam just said that this one star exploding is about equivalent in brightness to about a billion stars shining normally like our sun. Okay. One star is as bright as a billion. That is a big boom.
RIESS: Yes, it really is. You know, it’s really because stars have a lot of fuel and they’ll spend billions of years putting that output at a kind of normal sun-like rate. But they have a lot of energy in reserve. And during an explosion event a fair fraction of the total available energy of the sun, instead of trickling out over billions of years, trickles out over hours, days, weeks, all the rest of it.
So, it is a big boom. It’s sort of like, you know, the difference between driving a car with a gas tank, little bit of gas at a time, versus lighting the gas tank on fire. It’s just all of the fuel just goes up.
STROGATZ: Wow. That’s amazing. And then also this idea of standard candles. I love the basic physics that goes into explaining these Type I supernovae. Whenever they blow up, they will blow up with about the same amount of energy. You said it’s not perfect, but it’s pretty darn close. So if you could just tell us a little about the theory that gives us confidence that’s true, ’cause that’s a really pretty thing too.
RIESS: So I would say throughout the last century of cosmology, cosmologists looked for the best standard candle they could find. So, you look at a distant galaxy and every galaxy looks different than every other galaxy because a galaxy is a crowd of stars and there’s no such thing as a standard crowd, right? Those are different numbers. So those are not gonna be good standard candles, right? So what you need are actually individual objects that are the same kind of object just located in different places in different galaxies. Stars work, but they’re too dim.
But this one kind of supernova, which there’s still some debate about this, but generally the general principle is it’s the center, the core of an old star called a white dwarf star, which is holding itself up against its crushing gravity by a kind of quantum mechanical pressure called electron degeneracy pressure. And this is something that the great Indian astrophysicist, Chandrasekhar first showed was stable only at a certain critical mass known as Chandrasekhar’s limit. So it’s 1.4 times the mass of our sun.
And so that means that if a star exceeds that limit — if it’s sitting there at Chandrasekhar’s limit, and let’s say it has a friend, another star orbiting it, and material gets transferred from one star onto the other — if it gets close to or even exceeds Chandrasekhar’s limit, then you will get runaway thermonuclear explosion because this electronic degeneracy pressure is no longer strong enough to hold back gravity. And it will compress and crush the star, giving you the property necessary for fusion and basically doing thermonuclear fusion over all remaining fuel.
So as I said, there’s some debate about exactly the details, but this is the general broad-brush picture that gives you a fairly uniform explosion, far more uniform than anything else we know on a kind of macroscopic scale.
STROGATZ: If I followed you correctly there, and correct me if this is too crude, it’s almost like saying rather than standard candle, it’s a standard hydrogen bomb.
RIESS: Correct.
STROGATZ: Because it sounds like that’s what you just said, that we know the amount of material in the star. It’s gonna be 1.4 solar masses approximately. And it’s gonna completely blow up through this thermonuclear, which is fancy talk for hydrogen bomb. Maybe I’m a little bit off.
RIESS: Yeah. With the one substitution of it’s carbon and oxygen instead of hydrogen, but yes.
STROGATZ: Okay, good. Great. So it’s not a hydrogen bomb, it’s a carbon oxygen bomb, but still it’s fusion, right?
RIESS: It’s fusion. And while some details about how the other star maybe donates the material or how fast the star is rotating can perturb a little bit the amount of energy you get out. And in fact that small variation, that was my thesis project, was to figure out how to account for the small variation.
That if you think of a kind of a standard light bulb, like a 60-watt light bulb — I don’t know if kids still know. But you could imagine some are 58 and some are 62, ’cause the factory doesn’t make them all the same. And that could fool you into misestimating the distance a little bit. ’Cause you may be seeing one that looks a little fainter, ’cause it was 58 watts, but instead you’re, you think, oh, it’s further away, but it isn’t.
How do you figure out from here which ones are intrinsically bright or intrinsically faint, given a little bit of distribution? And one of the things that was discovered in the early 1990s and became part of my thesis was the ones that are more powerful, rise more slowly and fall more slowly, to reach their peak than the ones that are more dim.
And another confounding effect, we sometimes have to look at a supernova through a galaxy that has dust in it and that dust can obscure the light. So going back to my analogy of a lighthouse, it would be like looking at a lighthouse on a foggy night, right? It could make the lighthouse look dimmer and fool you into thinking it was further away than it really was.
But the dust in galaxies also shifts the colors of the supernova light. And so if you could simultaneously measure the light curve shape that tells you whether it’s a bright or dim light bulb, and the colors that tell you how much dust is in the way, you could tease out these competing effects and get back to what you wanted, which was to figure out how far away the supernova was. And that was the subject of my doctoral thesis when I moved to Harvard, in the early 1990s.
STROGATZ: Well, I appreciate your going through that because this is such a bit of Sherlock Holmes work that you don’t have complete evidence. You have to do a lot of reasoning like they’re the use of the different colors. Or you didn’t use the phrase light curve yet, but you sort of hinted at this idea that the brightness — it’s not just one flash, like I said, a boom. But maybe you should tell us a little more about that, that when you’re looking at a supernova, what are you really measuring?
RIESS: I should point out that a supernova is incredibly rare like this. So it’s not like, you know, you could expect to see one on any given night when you look out at the sky.
There is one in a galaxy like ours about once a century. And so, you know, if you wanna find a supernova, you just pick a nearby galaxy and you stare at it, you’re very unlikely to find a supernova.
The breakthrough came in the 1990s, when astronomers began to build telescopes that had wide angles, and they had detectors that they could cover the focal plane with, so that you could take a single image that might contain hundreds of thousands of galaxies in one frame.
Then you take another image, maybe a month later. And by math, you’re sure that one of those galaxies will have had a supernova over that month. Because you’ve just bought so many lottery tickets that you’re bound to win the lottery. And in the 1990s we learned to collect such images, digitally subtract one from another and find a new point of light. And then the supernova, it’s what we call its light curve. It usually takes about two or three weeks to go from explosion, when you don’t see it at all, to reach its maximum output, which is the sort of standard candle, if you will. The distance indicator.