The universe is expanding. But depending on where we look, it's doing so at bafflingly different speeds.
The problem is known as the Hubble tension, and it centers around figuring out a number for the universe's expansion rate, called the Hubble constant. To find it, scientists have pored over tiny fluctuations in the cosmic microwave background (CMB) — an ancient relic of the universe's first light — and built cosmic distance ladders to remote, pulsating stars called Cepheid variables.
But the best experiments using these two methods disagree. The difference in results may have seemed small, but it was enough to spark a major crisis in cosmology.
Wendy Freedman, an astrophysicist at the University of Chicago, has spent four decades studying the Hubble constant.
Now, she is using one of the most powerful tools in astronomy — the James Webb Space Telescope (JWST) — to get the most precise measurements yet of the Hubble constant. Her team is looking at several celestial objects at the same distance from Earth. The hope is with several measurements in hand, the tension can finally be resolved one way or another.
Live Science spoke to Freedman about how the tension arises, why it matters, and how she's using the JWST to hunt for an answer.
Related: After 2 years, the James Webb Telescope has broken cosmology. Can it be fixed?
Ben Turner: You've been measuring the Hubble constant for a large part of your scientific career. What drew you to studying it? And why is it such an important measurement for cosmologists to know?
Wendy Freedman: The Hubble constant gives you a measure of the size of the universe, and it's probably the most fundamental parameter that we can measure that tells us about the evolution of the universe.
What drew me to it is the fact that you could make measurements in our local neighborhood — which, of course, astronomically speaking, is a large neighborhood — and use them to learn things about the early universe and how it grew. It really intrigued me.
BT: How important is the standard model of cosmology? What is the Hubble tension?
WF: The standard model [which explains how the universe has expanded since the Big Bang] is an interesting model in the sense that we are made of what is a very small fraction of the overall amount of matter and energy in the universe.
And so there are very fundamental things that we don't understand. We don't know yet what dark matter is. Neither do we know what dark energy is, except that it's causing the universe to speed up. But the model works remarkably well, given that we don't understand its fundamental structure.
The Hubble constant gives us an opportunity to learn something more about the universe in that way. We test the standard model by making measurements locally, and then compare them with what we find in the early universe by measuring fluctuations in temperature across the cosmic microwave background.
You can fit the standard model to those cosmic microwave background measurements, and it's an astoundingly good fit. And because the standard model is a predictive model you can work forwards, using data from the cosmic background radiation to predict what the Hubble constant should be today.
But if we compare that predicted value of the Hubble constant to what we measure using stars called Cepheid variables, they don't match — that's the Hubble tension.
BT: If we accept that the Hubble tension is real and not a systematic error somehow, how big a challenge is it to the standard model of cosmology?
WF: At this point I have a completely open mind [on whether it's real]. I don't know which way this is going to go. But yes, it would be significant. How significant? Probably not as significant as the standard model itself. But if it led to a newer, fundamental understanding that improves our knowledge of these things that really remain mysteries at the moment, it could be profound.
BT: So let's dig into how we measure this. Besides the fluctuations in the cosmic microwave background, Cepheid variables are the other main way astronomers find a value for the Hubble constant. What are Cepheid variables, and how do we use them to measure astronomical distances?
WF: Cepheid variables were what Edwin Hubble used when he discovered the expansion of the universe. They're stars that are five to 20 times more massive than our own sun, and they have atmospheres that are actually pulsating — moving in and out — with time. They do this in a very regular way for periods of a couple of days, going through up to 100 or so cycles in their light levels.
In the early 1900s, Henrietta Leavitt found that there was a correlation between how fast Cepheid stars were pulsating and how bright they are. That gives us a means of measuring distance, and it's one of the most accurate means that astronomers have today.
If we can measure nearby stars in a way that we can determine their distance, say from geometry. Then we can look at Cepheid variables in galaxies, compare their brightnesses at a given period — using the period luminosity relation — and then by the inverse square law of light [light dims from a source in proportion to the square of the distance to its viewer] we get the distance.
BT: And yet despite being very accurate, there are a lot of uncertainties associated with Cepheid measurements. What are they? And what are researchers doing to account for them in their measurements?
WF: There are complications. There's dust between us and the Cepheids that make them dimmer; their atmospheres contain different amounts of heavy elements that can change the brightness [meaning they have a high metallicity]; and there are just uncertainties in the measurements.
Also when we go to more distant galaxies, it's very difficult to make a measurement of a Cepheid on its own because other stars in the galaxy contribute light that's hard to separate from the Cepheid itself.
We've been improving the accuracy of these measurements for decades. Before the turn of the century, we were arguing about Hubble constants from Cepheids being between 50 to 100 [kilometers per second per megaparsec] — literally a factor of two uncertainty. As of 2001, our group published a result that gave a value of 72 [km/s/Mpc] with 10% uncertainty. That value stood the test of time: If we view Cepheids today we get numbers like 72, 73 and 74.
BT: But when we look at recent measurements of the cosmic microwave background taken by the Planck satellite, we get a value of around 67. At first glance, that looks like a difference of at most 7 km/s/Mpc, maybe even less. At a casual glance, that’s not very big, so why does it matter?
WF: Where the tension has arisen is that, in the last several years, it's been possible to make really accurate measurements of small differences in temperature in the cosmic microwave background. We're talking really small — like a thousandth of a percent.
You measure these fluctuations accurately and you can fit the standard model of cosmology incredibly well to this spectrum of temperature differences. From that, you can infer that the Hubble constant is 67.
Now there seems to be this discrepancy between 67 and 73. That doesn't sound like a lot given that we started between 50 and 100. In fact, Hubble started off at 500 when he first made his measurements. But because the measurements are improving in their accuracy, it appears as if it might be quite significant.
BT: So how are you looking for an answer?
WF: Why I'm excited right now is because we have the opportunity with the James Webb Space Telescope to make measurements of Cepheids and also other kinds of stars.
We've talked about systematic errors from dust and metallicity and so on. Each method we're going to use is going to have its own set of systematic uncertainties. No matter how many times we make the measurement more accurate — those systematics are going to get you in the end if you don't understand what they are.
So what we did in the past was take precise measurements of stars at the tip of the red giant branch [which also pulsate regularly] as a comparison. We got results for that coming in at around 70. Within their uncertainties they agreed pretty well with the Cepheids, but they also agreed pretty well with the cosmic microwave background.
Our current JWST program is to measure the Cepheids, tip of the red giant branch stars and a third star known as a JAGB star [aging carbon stars with a near-constant brightness] in the same galaxy, all at one distance. We'll see how well we agree and that will give us a sense of an overall systematic answer.
BT: Very briefly, why are tip of the red giant branch stars a useful comparison to make with the Cepheids?
WF: They’re older stars or lower mass stars — they don't have much of a metallicity dependence. We don't understand the metallicity dependence of the Cepheids well, that's still something that remains unsolved.
Also Cepheids are young, so they haven't had time to diffuse away from the regions where they were formed. They're in crowded high surface density regions, whereas the red giants are isolated. So it’s very simple to make a measurement in terms of their luminosity.
BT: Are there any results you can tease? How soon will you get them?
WF: Not yet, our group right now is blinded so we're not going to do an absolute calibration in the distance field until we have all the data measured and analyzed. We have to measure the periods and luminosities of the Cepheids, create a period-luminosity relation and (along with the JAGB stars) measure these luminosities. We're not going to unblind until all that analysis is done. We'll sit down in a room and we'll know.
So I don't know in terms of the absolute [distance] calibration. But what I can say, about our database and the reason we put in this big proposal to use JWST, is that it's got four times the resolution of the Hubble Space Telescope at infrared wavelengths. This means the star crowding issue is alleviated enormously and we have a test using a different filter to look for metallicity effects directly where we're observing. So I think we're going to be able to get at many of these systematic effects.
Where the Hubble constant is going to fall from this I just don't know right now. But we're really excited because I think we're gonna have something really interesting to say. In our first galaxy we see a lot of differences from the Hubble [Space Telescope] measurements — those stars were really crowded. Now we're looking at galaxies that are not quite as crowded.
As I said, I'm just completely open. I don't know where this is going to fall. But it is a question. It's an empirical question.
Editor's Note: This interview has been edited and condensed for clarity.
