The universe is expanding. But it moves at very different speeds depending on where we observe it.
This problem is called the Hubble tension, and at its core is the calculation of a number for the expansion rate of the universe, called the Hubble constant. To find it, scientists pored over tiny fluctuations in the cosmic microwave background (CMB) – the ancient remnants of the universe’s first light – and built a cosmic distance ladder to distant pulsating stars called Cepheid variables.
But the best experiments using the two methods are inconsistent. The difference in results may seem small, but it is enough to trigger a major crisis in cosmology.
University of Chicago astrophysicist Wendy Friedman has spent four decades studying the Hubble constant.
Now, she’s using one of the most powerful tools in astronomy—the James Webb Space Telescope (JWST)—to obtain the most precise measurement of the Hubble constant to date. Her team is observing several objects that are the same distance from Earth. Hopefully, through multiple measurements, the tension can eventually be resolved in some way.
Live Science spoke with Friedman about how this tension arises, why it matters, and how she uses JWST to find answers.
related: Two years later, the James Webb Telescope revolutionized cosmology. Can it be fixed?
Bendtner: You have spent most of your scientific career measuring the Hubble constant. What drew you to study it? Why is it such an important measurement for cosmologists?
Wendy Friedman: The Hubble constant measures the size of the universe and is probably the most fundamental parameter we can measure that tells us about the evolution of the universe.
What attracted me was the fact that you could take measurements in our local community – which is, of course, a large community in astronomical terms – and use them to learn something about the early universe and its growth. This really piqued my interest.
BT: How important is the Standard Model of Cosmology? What is Haber tension?
working team: Standard model [which explains how the universe has expanded since the Big Bang] This is an interesting model because we are made up of a small fraction of the total amount of matter and energy in the universe.
So there are some very basic things we don’t understand. We don’t know what dark matter is yet. We also don’t know what dark energy is, only that it causes the universe to accelerate. But given that we don’t understand its basic structure, the model works very well.
The Hubble constant gives us the opportunity to learn more about the universe in this way. We test the Standard Model by making measurements locally, and then compare them with what we find in the early universe by measuring temperature fluctuations across the cosmic microwave background.
You can match the Standard Model to those cosmic microwave background measurements, and it fits very well. Because the Standard Model is a predictive model, you can use information from the cosmic background radiation to predict what the Hubble constant should be today.
But if we compare the predicted value of the Hubble constant to the value we measure using stars called Cepheid variables, they don’t match – this is the Hubble tension.
BT: If we accept that the Hubble tension is real and not some systematic error, how big of a challenge is this to the Standard Model of Cosmology?
working team: At this point my mind is completely open [on whether it’s real]. I don’t know where this is going. But yeah, it’s going to be significant. How important is it? Probably not as important as the Standard Model itself. But if it leads to newer, fundamental understanding and improves our understanding of something that currently remains a mystery, it could be profound.
BT: So let’s dig into how to measure this. In addition to fluctuations in the cosmic microwave background, Cepheid variables are the other main way astronomers find the value of the Hubble constant. What are Cepheids and how do we use them to measure astronomical distances?
working team: Cepheid variables were used by Edwin Hubble when he discovered the expansion of the universe. They are 5 to 20 times more massive than the Sun, and their atmospheres actually pulsate over time—in and out. They do this in a very regular way, lasting several days, going through up to 100 or so cycles in light levels.
In the early 1900s, Henrietta Leavitt discovered a correlation between the pulsation speeds of Cepheids and their brightness. This gives us a way to measure distances and is one of the most accurate methods astronomers have today.
What if we could somehow measure nearby stars to determine their distances, for example through geometry.We can then look at the Cepheid variables in the galaxy and compare their brightness at a given period using the periodic luminosity relationship and then use the inverse square law of light [light dims from a source in proportion to the square of the distance to its viewer] We got the distance.
BT: However, although measurements of Cepheids are very accurate, there are still many uncertainties. what are these? What are researchers doing to account for them in measurements?
working team: There are complications.There is dust between us and the Cepheids, which dims them; their atmospheres contain varying amounts of heavy elements, which can change their brightness [meaning they have a high metallicity]; And there are uncertainties in the measurement results.
Additionally, when we get to more distant galaxies, it’s difficult to measure Cepheids individually because the light from other stars in the galaxy is difficult to separate from the Cepheids themselves.
We’ve been improving the accuracy of these measurements for decades.Before the turn of the century, we argued that Cepheids had Hubble constants between 50 and 100 [kilometers per second per megaparsec] ——Actually, there are two uncertain factors. As of 2001, our group has published 72 [km/s/Mpc] With 10% uncertainty. This value has stood the test of time: if we looked at Cepheids today, we would get numbers like 72, 73 and 74.
BT: But when we look at recent measurements of the cosmic microwave background from the Planck satellite, we get a value of about 67. At first glance, this looks like a difference of at most 7 km/s/Mpc, maybe even less. At first glance, this doesn’t seem like a big deal, so why does it matter?
working team: The nerve-wracking part is that over the past few years we’ve been able to make really precise measurements of tiny temperature differences in the cosmic microwave background. We’re talking very small – like one in a thousand.
You can measure these fluctuations accurately, and you can fit the Standard Model of cosmology very well to this range of temperature differences. From this, you can deduce that the Hubble constant is 67.
Now, there seems to be this difference between 67 and 73. That doesn’t sound like much considering we started with between 50 and 100. In fact, when Haber first took measurements, he started with 500. But since the accuracy of measurements is improving, it seems likely to be important.
BT: So how do you find out?
working team: The reason I’m excited right now is that we have the opportunity to use the James Webb Space Telescope to measure Cepheids and other types of stars.
We have already discussed systematic errors caused by dust, metallicity, etc. Each method we are going to use has its own set of system uncertainties. No matter how many times we make measurements more accurate – if you don’t understand what they are, these systematics will eventually kill you.
So what we used to do was make precise measurements of the stars at the tip of the red giant branch [which also pulsate regularly] As a comparison. We get a result of about 70. Within the uncertainty range, they are very consistent with Cepheid variables, but they are also very consistent with the cosmic microwave background.
Our current JWST program is measuring Cepheid variables, the tips of red giant branch stars, and a third star called the JAGB star [aging carbon stars with a near-constant brightness] In the same galaxy, all within a certain distance. We’ll see how well we agree, which will give us a sense of the overall system answer.
BT: In short, why do the tips of red giant branch stars provide useful comparisons with Cepheid variables?
working team: They are older stars or lower mass stars – they don’t have much dependence on metallicity. We don’t know much about the metallicity dependence of Cepheid variables, and it remains an open question.
And Cepheids are still young, so they haven’t had time to spread out of the region where they formed. They are located in crowded regions of high surface density, whereas red giants are isolated. Therefore, measuring their luminosity is very simple.
BT: Are there any results you can tease about? How soon will you receive them?
working team: Not yet, our team is blind right now so we won’t do an absolute calibration in the distance field until we measure and analyze all the data. We have to measure the periods and luminosities of Cepheid variables, create period-luminosity relationships and measure these luminosities (together with JAGB stars). We will not unblind until all analyzes are complete. We would sit in a room and we would know.
So I don’t know absolutely [distance] calibration. But what I can say about our database and the reason we made the big proposal to use JWST is that it has four times the resolution of the Hubble Space Telescope at infrared wavelengths. This means that the stellar crowding problem is greatly alleviated, and we tested using different filters to look for metallicity effects directly where we observed them. So I think we’re going to be able to get a lot of systemic effects.
I don’t know yet where the Hubble constant drops off. But we’re really excited because I think we’re going to have some really interesting things to say.In our first galaxy, we see many differences from Hubble [Space Telescope] Measurements – Those stars are really crowded. Now we’re looking at less crowded galaxies.
Like I said, I’m totally open. I don’t know where this will fall. But this is a problem. This is a question of experience.
Editor’s note: This interview has been edited and condensed for clarity.
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