One Constant, Two Answers

17:10 minutes

Once upon a time, everything in the universe was crammed into a very small space. Then came the Big Bang, and the universe has been expanding ever since. But just how fast is it expanding? Calculating that number is a challenge that dates back almost a hundred years, when Edwin Hubble used data from Harvard astronomer Henrietta Swan Leavitt to try to answer that question. His value came to be called the Hubble constant, H0.

But the exact value of that constant has been hard to pin down. And now two different approaches to measuring the Hubble constant have come up with close, but different answers—and each team says they’re pretty confident in the accuracy of their measurements.

Science writer and author Anil Ananthaswamy and Nobel laureate Adam Riess of the Space Telescope Science Institute join Ira to discuss the discrepancy, what it means, and how it might be solved.

View infographics and Hubble images that explain the Hubble constant (click the images for higher resolution).

an infographic that reads "three steps to measuring the hubble constant."
This illustration shows the three steps astronomers used to measure the universe’s expansion rate to an unprecedented accuracy, reducing the total uncertainty to 2.3 percent. Astronomers made the measurements by streamlining and strengthening the construction of the cosmic distance ladder, which is used to measure accurate distances to galaxies near to and far from Earth. Click on the image for a downloadable PDF version. Credit: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU)
a bright oval, swirling galaxy. there are many yellow circles round stars
A galaxy located 65 million light-years from Earth. The yellow circles in the galaxy represent the locations of pulsating stars called Cepheid variables. Astronomers use Cepheid variables to calculate the galaxies’ distance from Earth, which helps them refine the universe’s expansion rate. Credit: NASA, ESA, A. Riess (STScI/JHU)
a circular, swirling galaxy with a bright center. there are yellow circles around particular stars in the photo
Here’s another galaxy, located 118 million light-years from Earth, that helps astronomers calculate the expanding universe. Credit: NASA, ESA, A. Riess (STScI/JHU)

Further Reading

  • Read Anil Ananthaswamy’s article about the Hubble constant and measuring how fast the universe is expanding in Scientific American.
  • Find out more about Adam Riess’ research at the Space Telescope Science Institute
  • Read about the discrepancy and how the Hubble constant is measured by NASA.

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Segment Guests

Adam Riess

Adam Riess is a 2011 Nobel Laureate in Physics, 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 in Baltimore, Maryland.

Anil Ananthaswamy

Anil Ananthaswamy is a science journalist and author based in Berkeley, California. His latest book is Through Two Doors At Once (Penguin Random House, 2018).

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. 

You know the story. Once upon a time, everything in the universe was crammed into a very small space, and then the Big Bang, and the universe has been expanding ever since. Well, so the theory goes. 

But just how fast is it expanding? Calculating that expansion rate is a problem dating back almost 100 years, when Edwin Hubble used data from Harvard Astronomer Henrietta Swan Leavitt to define the expansion rate has a number that came– well, it came to be called the Hubble constant. But the value of that constant has been hard to pin down. Two different approaches to measuring the Hubble constant have come up with close, but different– significantly different– answers. 

Joining me now to talk about why that matters are my guests. Anil Ananthaswamy is a science journalist and author based in the Bay Area. He recently wrote about this for Scientific American. Welcome to Science Friday, Anil. 

ANIL ANANTHASWAMY: Hi, Ira. It’s good to be on the show. Thank you. 

IRA FLATOW: Nice to have you. And Professor Adam Riess, he is a 2011 Nobel laureate in physics, a professor of astronomy and physics at Johns Hopkins University, senior member of the science staff at the Space Telescope Science Institute in Baltimore. Welcome back, Dr. Riess. 

ADAM RIESS: Hi, Ira. Nice to be here. 

IRA FLATOW: Nice to have you. All right, Anil, let me start with you. What is the Hubble constant in lay terms? 

ANIL ANANTHASWAMY: The Hubble constant tells us about how fast the universe is expanding. And it’s related to the– if you’re looking at some galaxy, then if you know the distance to that galaxy, and you know how fast that galaxy is receding from us, you can use those two numbers to calculate the Hubble constant, which tells you how fast the universe is expanding. 

IRA FLATOW: And Dr. Riess, why is it so hard to measure then? 

ADAM RIESS: Yeah, well, it’s just the universe, after all. That should be pretty simple. 


But no, really, it’s not that it’s so hard. It’s that it’s so hard to measure it so precisely. And particularly, when you see a disagreement that may tell us something really interesting about the universe, you want to be really right about your measurements. So we’ve been measuring and remeasuring for about a decade, using the Hubble Space Telescope to calibrate the universe. And we have, we think, the most precise answer to date, which is the answer we presented last week. 

IRA FLATOW: But that answer is in disagreement with other measurements, is it not, Anil? 

ANIL ANANTHASWAMY: Yeah, so that measurement is quite significantly in disagreement with another way of measuring it, which is the value that comes from the European Space Agency’s Planck satellite. So they measure the cosmic microwave background, which is radiation left over from about 380,000 years after the Big Bang. And that radiation has features in it that you can use in order to correlate what happened in the early universe with what’s happening now and use that to calculate, sort of estimate, the expansion rate now. And those two ways of doing it– Adam’s group’s and the Planck data– are in disagreement. 

IRA FLATOW: So Adam, how do you measure it? 

ADAM RIESS: How do I measure it? Well, I use different kinds of exploding and pulsating stars in the nearby universe. And by calibrating their luminosity using geometrical methods, like building triangles in space, we can determine how far away they are from how bright they appear. And then we measure what’s known as the redshift. And as Anil described, that tells us how fast the universe is expanding today. 

IRA FLATOW: Our number, if you’d like to talk about it, is 844-724-8255, 844-SCI-TALK. Or you can tweet us, @scifri. 

So why do you think there is this discrepancy? And if it seems to be a significant one, why is there, Dr. Reiss? 

ADAM RIESS: Well, let me give you an analogy. Imagine you were trying to measure your height. And one person started at your feet and measured to your head. And the other person started at your head and measured to your feet. And if they got different answers, you’d say, well, you know, somebody’s made a mistake. That’s not so interesting. 

But let me imagine it a different way. Instead, somebody measured your height when you were two years old. Now, when you’re two years old, if you’re like an average person, you reach about half of your eventual height. And so, you know, you double that number, and you say, OK, I’m 3 feet tall when I’m two years old. I guess I’ll be 6 feet tall when I’m all grown up. 

Then we go out and we actually measure. And, oh my gosh, you turn out to be 7-foot-2. You’re going to play center for the Knicks. But what’s exciting is something has gone awry in our understanding. 

So what we failed to mention was that the other measurement, the one made from the cosmic microwave background, is not really a measurement as much as a prediction. It is a measurement when the universe was very young, and then we use our understanding of the physics of the universe to figure out how fast the universe will be expanding today. And just like that child who is grown up, we then go out and measure how tall the child is, and we find a very different, very surprising answer. And that’s why this is really so exciting. 

It’s not just like two people measuring the same thing and getting a different number. It’s two scientists, or two teams of scientists, measuring at opposite ends of the history of the universe and finding that those two do not connect, that there is something missing or another wrinkle in how we understand the universe. 

IRA FLATOW: And both teams of scientists are dug in on– they are really pretty sure that they’ve got the right number. Right, Anil? 

ANIL ANANTHASWAMY: Yeah. I mean, both teams have reduced their errors substantially. So the Planck team is very confident of its measurements. And as Adam said, they’re measuring the cosmic microwave background with very high precision. But then they have to use this thing called the standard model of cosmology to extrapolate to the universe as it is now and make estimates of the Hubble constant. But the actual measurement– you know, the precision of the measurements is very high– and as is the data coming from Adam’s team. 

IRA FLATOW: So, Adam, what does it matter? Why does it matter if it’s 67 or 74? 

ADAM RIESS: Aww, right. 

IRA FLATOW: You know? 

ADAM RIESS: Right. It’s true. I don’t actually care what the number is. What I care about is that these two ways disagree and that it might tell us something interesting about the universe. So for your listeners, who may or may not know, about 95% of the universe, we believe, is made up of dark matter and dark energy. These are these mysterious components that we see only by their gravity. But we don’t really have a detailed understanding of their physics. They’re very mysterious to us. 

So, as Anil said, when we extrapolate from the youthful universe to the present universe, we take some very naive guesses about the nature of that dark matter and dark energy. And so if those naive guesses are failing, if we’re seeing this important difference, it may be teaching us something, something really crucial, an important clue about the nature of dark matter or dark energy. And that’s why we make these kinds of measurements, really to learn more about these mysterious components. 

IRA FLATOW: Well, you know, if you don’t know what 95% of the universe is made of, I mean, you don’t know anything. 


ADAM RIESS: Hey, we know it’s there. 


IRA FLATOW: Isn’t that right? I mean, why should this little– you know, what could these numbers teach you about learning what 95% of the universe is made out of? 

ADAM RIESS: Yeah. Well, you know, this becomes a quantitative game at some point. I mean, as I said, we use gravity. This is sort of the new science of cosmology in the last few decades is to recognize, hey, most of the universe is not made out of what we’re made out of. And in fact, most of the universe doesn’t emit light like things we’re familiar with. We have to use gravity, like a telescope, to see parts of the universe– dark matter, dark energy. You may have heard of gravitational waves, which we are just getting the ability to see. 

And so these are our new observing tools that we’re using to learn about gravity. I mean, if you were a blind person walking through a room, you know, you’d have to use a different set of senses to figure out what’s in the room. And so that’s where we are. 

IRA FLATOW: Let’s go to the phones– 844-724-8255. Let’s go to Terry. Hi. Welcome to Science Friday. In Sioux City– 

TERRY: Hi, how are you? 

IRA FLATOW: Hi there. 

TERRY: Hi, how are you? 

IRA FLATOW: Fine, go ahead. 

TERRY: I was wondering if it isn’t the word constant that might be the issue here. 


That assumes that it’s the same throughout the entire universe. And maybe the universe is expanding at different rates in different places. 

ADAM RIESS: Right. That’s a great question. We actually have used these same tools to determine if the universe is expanding at the same rate in different directions. And in fact, that has been confirmed to very high precision. But you’re also right that constant is kind of a funny misnomer anyway, because it is a number that will change as the universe ages. It’s just we think constant at any one point in time but the same in all directions. 

IRA FLATOW: Could the dark energy that is pushing the universe further apart– could that have something to do– you know, we don’t know anything about it– with the Hubble, the number being wrong? 

ADAM RIESS: Yeah. That is, in fact, one of the possibilities. You know, we take a very, I would say, vanilla guess at what the nature of dark energy is. And we tried to measure that. And you know, it roughly looks like that vanilla guess. But that could be part of the story of what’s going on is that we have kind of a turbo-charged dark energy that makes the universe accelerate and expand even faster today. 

IRA FLATOW: And Anil, I’m going to bring another one of our favorite topics into this, and this is like black holes and gravity waves. Is there a LIGO measuring, you know, that could be the referee of these two other measurements? 

ANIL ANANTHASWAMY: Yes, absolutely. I think that’s going to be the exciting thing over the next five years. So your listeners will know that LIGO, which is the Laser Interferometer Gravitational-Wave Observatory, detected gravitational waves from the merging of black holes in the past few years. But in August of 2017, it had another event that it observed that was actually very interesting, which was the merger of two neutron stars. And so these two neutron stars, when they merged, they generated gravitational waves. But they also generated a burst of electromagnetic radiation. 

And in order to calculate the Hubble constant, you need two things. You need the distance to some astrophysical object. And you also need to measure its recession velocity. So the gravitational wave signature from the neutron star merger gave the LIGO team a way to tell the distance to this merger event. And the electromagnetic radiation, which was captured by other telescopes, gave them redshift information, which then allowed them to figure out how fast this is receding from us. Putting those two together, they came up with a Hubble constant value of 70, which sits bang in the middle of the Planck and the data from Adam’s group, except that there are error bars because it’s just one event, and they just don’t have enough data to have good statistical significance. The error bars are so big that they can accommodate both Planck and the result from Adam’s group. 

So what’s exciting is that just this week, LIGO announced that they have upgraded their instrument and have restarted observations. And they’re going to, I think in about a year’s time, close it down and upgrade again. So over the next five years, they’re hoping to observe about 50 such neutron star mergers. And that will give them enough data to pin down this number to within 2%. And it will be really exciting to see which way they move– whether they move towards Planck or whether they move towards the data from Adam’s group. 

IRA FLATOW: Very interesting. I have a tweet from Sara, who asks a question that’s been asked of everybody for the last 100 years, and I want to repeat it. If the universe is everywhere– you know what I’m going to say– how is it expanding? Is it becoming more infinite, Adam? 

ADAM RIESS: Oh, I was hoping you were going to ask Anil this question. 


ANIL ANANTHASWAMY: And I was going to punt it to Adam. 

ADAM RIESS: Yeah. What we really mean when we say the universe is expanding is we mean locally, around us. And as far as we could tell everywhere else, that whenever two things are separated, like galaxies, that separation grows. We can’t really verify what happens much further than we can see. There’s a limit, and we can only see as far as the age of the universe times the speed of light. But we believe that the universe continues to expand. 

And so if it’s infinite, it’s infinite and getting bigger. As my father used to like to say, what’s bigger than infinity? Well, infinity plus 1. 

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios. Now you’re getting into aleph and aleph-one and Cantor and all those sort of things. 

We have a lot of callers who are interested in– whenever we talk about the universe, people want to talk about the universe and expansion. And that tweet reminded me of a letter I saw many years ago that was sent to Einstein himself about what happens if you poke your thumb through the finite universe, where does it go? That sort of stuff has been around all the time. 

You worry? Here’s another tweet. Let’s go to the phones. A lot of– let’s go to line four. Hi, welcome to Science Friday– Charles in Dayton. 

CHARLES: Hello, Ira. 

IRA FLATOW: Hi, Charles. Go ahead. 

CHARLES: Yes, Ira, thank you. I would like to know if the CMB’s boundary is expanding? Or is it too far to tell if it’s expanding? 

IRA FLATOW: The Cosmic Background Radiation that’s left over from the Big Bang. 

ADAM RIESS: Right. Well, the cosmic microwave background radiation is actually a tiny slice in time. It’s a snapshot of the universe at a certain moment– the moment when the universe went from being a fog, where light couldn’t really propagate very far, until it suddenly became thin enough, diluted enough by the expansion of space that it suddenly gets out. And so it’s not really expanding. But it is getting more distant from us in time, as time goes on. But because we can always look back to it, and it’s in all directions, it’s always available to us. We can always see it. 

IRA FLATOW: Well, let me just wrap up because we’re running out of time. Is it possible that we need new physics here? Could there be particles– unknown particles, unpredicted particle? 

ADAM RIESS: Yes, yes. I would say, you know, on our menu of possibilities are new particles, like what we would call a sterile neutrino, exotic dark energy, a dark matter that interacts or decays, another episode of dark energy– all kinds of interesting possibilities. And that’s why we’re so excited about this discrepancy. 

IRA FLATOW: But do you have the tools to discover those new particles? 

ADAM RIESS: I think so. I mean, new tools are coming online. The various predictions of what would happen if you had exotic particles make specific predictions of what other signatures you should see. So sometimes this is just the way science works is, you know, you might get your first clue, and your first clue leads to a hypothesis. And that hypothesis recommends another experiment. And so we may be going through a generation of that now. 

IRA FLATOW: Let me see if I and get a quick caller in from Johnny in Beacon, New York. Hi, John. Welcome. 

JOHN: Hi, thanks. How’s it going, guys? So great discussion. My question was could these potential differences in measurements from these two groups be based off of– we always assume that the initial Big Bang caused an ever-expanding universe as a constant in one direction. What if the universe hit a point where then it were to retract and almost come back to, let’s say, that initial plant based off of gravitational pull? Is there any evidence why that is not possible? And I think we always just assume that it was ever-expanding. 

IRA FLATOW: OK, I got about 30 seconds for an answer. Adam? 

ADAM RIESS: Well, I could jump in here on this one. You know, we don’t really get a good glimpse of the universe between the time shortly after the Big Bang to, oh, the last 7 or 8 billion years. And so there’s kind of a Dark Ages in there. If the universe took a break– you know, took a snooze during that– we wouldn’t know. And that really could also mess up these calculations, of course. We would look for what is the exotic new physics that explains why that would happen. But you know, just generic, as a statement, we don’t think the universe took a break. 

IRA FLATOW: Thank you, Adam. Adam Riess, 2011 Nobel laureate in physics talking with us. Also Anil Ananthaswamy, a science journalist author based in the Bay Area, working for Scientific American every once in a while.

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