Is our model of the universe wrong?
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There's a crisis happening in cosmology. You always know how old you are, you know when your birthday was, but we don't know when the birthday of our universe was. And that all rests on how fast the universe is expanding today. That current speed of expansion is characterised by a value known as the Hubble constant.

And as Astronomer Royal for Scotland, Catherine Haymans, points out, the Hubble constant is the key to understanding the age of our universe.

The issue is, cosmologists can't agree on what that number actually is. First, there's the one we get by looking at the late universe. That's stars and galaxies and supernovae. They give us a figure of about 72 kilometres per second per megaparsec.

I've translated into human terms for you. So if you take two points in the universe that are separated by the distance that exists between us and the Andromeda galaxy, which is two and a half million light years, they are moving apart from each other at a rate of 125,000 miles per hour.

But then there's the value we get by peering all the way back right to the very early universe, the cosmic microwave background.

And that tells us the Hubble constant isn't 72, it's about 67. The measurements of the cosmic microwave background, as an astronomer, I wasn't involved in that project, but, you know, I feel like weeping with joy. They're absolutely beautiful, pristine measurements, incredibly accurate. And the same goes for the direct measurements that are being made today.

So, two precise, beautiful measurements that give different answers for the Hubble constant. This discrepancy leaves a painful problem. Is there something wrong with the measurements? Or is there something wrong with our whole model of the universe? We've got these two dark components, dark matter and dark energy, that we don't know what they are. So for me, two is too many already. That's why we're in this crisis mode.

Scientists might have been hoping that the incredible images coming from the James Webb Space Telescope would help resolve the so-called Hubble tension, but instead the plot has thickened. Just last week, results from JWST confirmed that there doesn't seem to be anything wrong with our observations of the late universe. So, does the problem lie in our fundamental understanding of the cosmos?

We do love a good crisis because it means that there's something for us to discover. It's something that we can prod and probe and hopefully learn more about the dark side of our universe, which is this major unknown in science. So today, the crisis in cosmology. Why can't we find an answer to the Hubble problem? From The Guardian, I'm Madeleine Finlay, and this is Science Weekly.

Edwin Hubble was at Mount Wilson Observatory and he published a very influential paper in 1929 that showed that the galaxies in our universe, no matter which direction you looked, were moving away from us and that the further they were away, the faster they were moving.

Catherine Haymans is a professor of observational cosmology at the University of Edinburgh. She is Astronomer Royal for Scotland and Director of the German Centre for Cosmological Lensing at Ruhr University, Bochum, Germany. As she explains, Hubble's observation that galaxies are moving away from us led to a very profound conclusion, one that seems unimaginable.

Utterly matter-of-fact today. This meant that our universe was expanding, which was...

incredibly influential because if you reverse time, if you set the clock going backwards and reverse that expansion, that means that in the past, the universe was very small and very hot and very dense, which gave evidence for the birth of our universe happening in a big bang. How did Hubble work out that things are moving away from us and the further they are, the faster they're receding?

For that, you need to understand two things, the distance and the speed. Let's start with the speed. I'm sure you've been on the side of your high street when a police car has gone past you. As the police car is coming towards you, the pitch of that siren goes up. And as it goes away from you, the pitch of the siren goes down. This is known as the Doppler effect.

And that's how it works with sound, something that we're familiar with. But it also works with light. So if the galaxy is moving away from us, then the light of the galaxy becomes redder.

And the faster it's moving, the redder the light becomes. So this is known as redshift and that's how we measure speed in our universe. The faster something is moving away from us, the more stretched out the wavelength of light becomes and the redder it appears. So what about distance? This is quite hard as we don't have giant rulers to measure the distance. We

We use something called a standard candle. So I'd like you to imagine that you and your friend are out at night and one of you has got a torch. And when they're standing close to you, you know how bright the torch is. But as they back away from you, shining the torch towards you, you see that their torch appears to get dimmer.

But you know how bright the torch actually is. And so you can work out how far they are away by how bright you see the torch. And that's exactly the same trick we use in the universe. If we know how bright something is, then we can measure the distance to it by looking at how bright we see it. And there are certain things in our universe that have a standard brightness. There's something called a Cepheid, which was discovered by astronomer Henrietta Leavitt back in 1912.

And this is a pulsating star that sort of pulsates at a standard brightness. And we can also use a certain type of supernovae. These are stars that are exploding as they die. A special type of supernova always shines with the same brightness when it explodes.

So there are cosmic yardsticks known as standard candles. We can work out the distance to them by measuring how bright we perceive them. And we can work out the speed that they're moving by looking at how red the light is when it reaches us. With both of those numbers, we can work out the value of the Hubble constant.

When Hubble first calculated the constant that eventually took his name, he put the rate of expansion at around 500 kilometres per second per megaparsec. Over the years, the value fell as astronomers improved their observations. But then along came the Hubble Space Telescope in 1990 to get a definitive number.

One of the primary science goals of the Hubble Space Telescope was to finally resolve this question. How fast was our universe expanding? And they made very, very detailed measurements of lots of cepheids in nearby galaxies. They used exploding supernovae in further away galaxies. And it totally nailed it. Or at least we thought it did.

But in 2013, cosmologists' peace and quiet over the Hubble constant was interrupted by data from the European Space Agency's Planck satellite.

This had been mapping out the cosmic microwave background, a relic left over from the Big Bang. If you look across the whole sky, everywhere you look, you see this very faint emission now at the microwave wavelength. So it tells you about the universe as it was right after the Big

Bang. Cosmologists took the data from the Planck satellite and combined it with the current theoretical model of the universe, made up of matter, that's us, and stars and galaxies, dark matter, the mysterious force of gravity we measure but can't directly see, and the

and dark energy, the invisible force causing the universe's expansion to accelerate. And together, the model and the data were used to get a prediction for how fast the universe is expanding today. So this is called an indirect measurement of the expansion rate, which we can compare to the direct measurements of the expansion rate using those standard candles. And unfortunately, they don't agree with each other.

Hubble had taken incredible images of the late universe. Planck had collected an exquisite dataset of our early cosmos. Both were very precise.

but the results they produced were not even within touching distance. Now, the difference between 72 and 67 sounds small, but for cosmologists, it has really big implications. So the age of the universe, if you take the cosmic microwave background data as our best understanding of the universe, plus this theoretical model with dark matter and dark energy, the age of the universe there is 13.8 billion years old.

But if you take the measurement of the Hubble expansion rate that we measure today using our direct measurements, then our universe is only 12.6 billion years old. So sort of a billion years younger. ♪

What's going on? What's behind this tension? Is the problem with the measurements? Or does the discrepancy lie in our theoretical understanding of the universe? We have a joke in the astronomical community that we have sort of lovely emeritus professors in our community who always fall asleep during our talks. And you can give an astronomy talk about absolutely anything and they'll no doubt nod off.

But then they'll always, at the end of the talk, either ask, but what about dust? Or, but what about magnetic fields? And those questions you can ask about any astronomical subject, because dust and magnetic fields always trip us up. And it turns out that those are two potential answers to what could be going on here. Right. Well, what about dust then? The question of dust. Right.

You've got dust around your house. There's dust in the universe as well, which makes it hard for us to see. And as you look further back in the universe, are these standard candles actually standard? Do they shine at the same brightness or are they surrounded by a haze of dust that makes it hard for us to see them? And so we're misinterpreting our observations. And

This is where the James Webb Space Telescope was supposed to come to the rescue. The Hubble Space Telescope mainly worked in the optical, but with James Webb Telescope we can use the infrared, which allows us to peer through dust. So you can look further back in time and it also has much higher resolution. So when you're looking for those standard candles, the Cepheid stars, they're very small, but

And so being able to see that more clearly with JWST allows us to make more precise measurements. What they found was that the measurements from Hubble, there was nothing wrong with them. So questions of dust and not being able to see clearly enough were unfounded. And the original results from the Hubble Space Telescope stand.

So probably not dust. What about the other question from the sleepy emeritus professors? Magnetic fields. The theorists are going crazy. There are some speculative papers about some primordial magnetic fields in our universe that is changing the universe in the very early stages after the Big Bang. And that's why we're misinterpreting our data that we've taken of the cosmic microwave background.

It isn't just primordial magnetic fields. Speculative theories abound that attempt to pin down the Hubble constant and end the crisis once and for all. And many cosmologists believe the problem really does lie in our fundamental understanding of the universe. I think the theoretical model that is really leading the pack at the moment is something called early dark energy. This is...

where the dark energy that we observe is causing the expansion of our universe to get faster and faster every day. This early dark energy model has an additional component very early on in the universe, and it kind of gives the universe a kick very early on, which could explain why you have this discrepancy between the predicted expansion rate today and what you observe.

And the reason why this one's leading the pack is it's also helping to solve another problem that JWST has raised. So the infrared vision of JWST allows us to look back to the birth of the very first stars and galaxies. And what it's seen is completely mind-blowing because there are these huge galaxies that exist forever.

very, very early on in our universe where our models say that they shouldn't have had enough time to form. They're just too big to be there so early on in the universe. And if you had this additional component, this early dark energy, then that changes the universe in the early days, allowing these massive galaxies to form as we're seeing them. And if I was to ask you to place a bet,

on your favourite theory that you think is kind of the most likely, is there one that you'd put some money on? Oh, well, now you see, my official title at the University of Edinburgh is Professor of Observational Cosmology. And that's definitely where I stand. I'm an observer.

So I don't place bets. I just see what the universe shows me. I often worry that by the time I reach retirement, that I'm going to sort of look back on all of the work I've done about dark matter and dark energy and laugh that I've spent my whole career working on these dark entities that don't exist. I really hope that that doesn't happen. But

They really explain a lot of things in our universe. They explain the cosmic microwave background beautifully. They explain the distribution of galaxies and the way galaxies form. But the more data we get, the better our facilities become, the more cracks that seem to appear in this model. And because we don't know what these dark entities are, you start to worry.

But we really need a breakthrough now, I think, to really resolve this. We need a breakthrough in understanding the dark sector. Well, trying to solve a mystery and being on the hunt for a breakthrough sounds like a pretty exciting task. So, Catherine, thank you so much. Oh, you're very welcome. The crisis continues. Thanks again to Professor Catherine Haymans. Today, I'd like to ask for your help.

The Guardian and Observer's 2024 charity appeal has partnered with three charities: Médecins Sans Frontières, Warchild and Parallel Histories,

To support civilians living in Gaza, Lebanon, Sudan and Ukraine as they struggle with the devastating impacts of war. And to help schools teach children about sensitive and contested conflicts. To donate now, go to theguardian.com forward slash charity appeal 2024. Thank you. This episode was produced by me, Madeline Finlay. It was sound designed by Tony Onuchukwu. And the executive producer is Ellie Burey.

We'll be back on Thursday. See you then.

This is The Guardian.

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