Instant Genius - The first stars in the Universe, with Dr Emma Chapman

Episode Date: May 8, 2022

Physicist Dr Emma Chapman tells us everything we know – and everything we don’t – about the first stars to exist after the Big Bang. Once you’ve mastered the basics with Instant Genius, dive deeper with Instant Genius Extra, where you’ll find longer, richer discussions about the most exciting ideas in the world of science and technology. Only available on Apple Podcasts. Produced by the team behind BBC Science Focus Magazine. Visit our website: sciencefocus.com Hosted on Acast. See acast.com/privacy for more information. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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From BBC Science Focus magazine, this is Instant Genius, a bite-sized masterclass in podcast form. I'm Sarah Rigby, online staff writer at sciencefocus.com. On this week's episode, I'm joined by Dr Emma Chapman. She's a physicist at the University of Nottingham, and author of First Light, switching on stars at the dawn of time. She tells me all about the very first stars that existed in our universe. So firstly, could you please just tell us a bit about yourself? Sure. So my name's Emma Chapman. I'm a lecturer at the University of Nottingham and I study the first stars to exist in our universe that popped up about 13 billion years ago and lit up the universe. That's my main area of study. I've recently got into SETI as well, which is searching for extraterrestrial intelligence. So a little bit extra on us, you've got to have a hobby. Yeah. And so that's my main thing, really. That's what

I do. Brilliant. And now can you tell us a bit about your book, please? My book is called First Light Switching on Stars at the Dawn of Time. It's out in paperback now. It's just been released in paperback actually by Bloomsbury Sigma. It is a book which covers the first stars, which is my main research area. But it really goes into detail of how the universe began, the history of how we discovered that there could be these extinct species of stars. and what that means for us today, so the real, really modern telescopes that are being built to look back in time. So why are you interested in the first stars in the universe?

I think it's because they are a real mystery. When we look at our timeline, our cosmological timeline, if you will, the evolution of our universe over 13 billion years, there's about 1 billion years at the beginning missing. So this is equivalent to missing everything, in your lifetime scrapbook from the day you're born to the first day you go to school. That's a huge amount of information to be missing, and it's a hugely formative time in our lives as well as the universe is like. And so never having observed a first star,

never having observed really anything from that time, that's very exciting to me. It's uncovering, it's opening a tomb that's been closed for a very long time, and that's why I love it. One thing I find quite funny about cosmology, really, is the vast differences in time scales that you have to care about. So after the Big Bang, you're thinking in nanoseconds, milliseconds, that sort of scale, aren't you? And then suddenly you're thinking in billions of years. So could you please just give us a brief kind of timeline of the early stages of the universe? Sure. So really, the universe began with a Big Bang.

We've got plenty of evidence to really trust in that theory now. I say trust because we don't fully understand it. I really believe our brains are just not able to comprehend the fact that everything in the universe was squeezed into this infinitely dense point that suddenly just came into being. Anyway, just, you know, trust me on that one. And then, like you say, on a scale that I can't even pronounce, like it's tiny, tiny, tiny fractions of a second, the universe suddenly inflates to a huge size. And then within minutes, about three minutes, three and a half minutes, pretty much everything interesting happens.

So you really settle down, you create these first very light elements. The universe is very hot, it's very violent. Everything's just hitting each other, including the light photons. So you can kind of imagine it as a jacuzzi. So if you're trying to look to the bottom of a jacuzzi, the light just can't get through. It's too many, too much violence, too much turbulence that to happen. But about 400,000 years after the Big Bang, someone switches off the jacuzzi. Basically, what happens is that the photons stop interacting with all of the other stuff,

like the hydrogen, the electrons, the protons and everything like that. Suddenly the light can go in a straight line and it can get on its way across the year. universe. And we see this as something called the cosmic microwave background, which this is not the topic of the day. But it's interesting because that's kind of our first long-term marker. You've got 400,000 years after Big Bang. Then you skip forward another 400 million years, and then you've got your first stars that are everywhere in the universe. And then, you know, you go to a billion years and you've got the galaxies. And then you go to four billion years. And then you go to four billion years. is and you've got the solar system. So you like you say, you're really going from these tiny,

tiny timescales to these huge timescales. And it really depends on where you study on what you're interested in for me, because I'm in the era of the first stars. I just round everything up. I'm always saying two and a half billion, ish. Give or take. So when was the era of the first stars? and when was the era of, I guess we call the modern stars or stars as we know them today? Yeah, it's vanishingly fast. So we think that the era of the first stars began around 180 million years after Big Bang. That's when we think they first really started. These gas clouds came together.

They started to fuse hydrogen, started to produce heat and light and become stars as we know them. So we think that was about 180 million years after Big Bang. I can talk about that later. because it is our single observation in this time. And they live very, very short lives because they are so massive. We're talking up to 100 times the mass of our sun, really massive stars, that they get through the fuel of the hydrogen that they have very, very fast. So they die within about a million, 10 million years.

That is tiny. And to give a little bit of time scale there, our son is projected to live more like 10 billion years, of which we're halfway through, so nobody panic. So after those first stars die, let's just say one million years, they explode in these incredible supernova, very, very, very bright, which we're hoping we can maybe one day observe. All of their stuff in the gas cloud comes back together over many, many more millions of years to form the second generation of stars. And we can still see those stars. We call them Population 2. We can still see them in our Milky Way. And then of course these

population 2 stars, some of them also then die and they form population 1 stars. And I can talk about what makes them different in a little bit, but our son is a population 1. So we're talking really quite fast on the order of tens to 100 million years after the Big Bang, very, very fast that we get our second population. So this lost population was a blink in the terms of the cosmological timeline, but they were vital to everything, to producing all of the heavier elements that make everything up that we see around us today in the universe. Right. So you said it's a bit like missing the first few years of our baby books. So how much of this is the theory of what we believe to have happened and how much of it is, how much have we actually observed about these population

three stars? Well, we've never observed them. So we think that the vast majority of them are probably extinct. We probably think that they went into supernovae, well, as I said, millions of years after they formed 13 billion years ago. But there are ways. of observing. There are ways of looking back in time and seeing the light from those first stars. We are trying to observe them today. I was slightly late to record this podcast because I was at a meeting for my telescope collaboration called Lofar. We've been looking for this light from the first stars for, gosh, we started in 2011. So 11 years now. We're very patient, but we're getting there. then there's the other side of it which is can some of these first stars or whether could they

have survived to the modern day could there be some that are small enough let's say about 80% the mass of our sun that guzzle through their hydrogen much slower and so that they could actually still be around in the Milky Way today lots of people are searching for them it's called stella archaeology amazing name but again they've been so searching for a very long time. Actually, that started in the 1950s. So, you know, a really long time there. But they're getting tantalizingly close. So we are, we're in this, what I feel like, is going to be a very exciting decade for this area. When we look into the sky, to look back in time, we have to look really distant. Is that right? That's correct. So we use a property of light,

which is that it has a finite speed. There is a speed limit to light. That's about 300 million metres per second. So it's very fast, which means that when I'm waving at you on screen, you see it instantly. You can wave back and I'm like, oh, what a friendly host. But if I had a friend on, let's say, Mars, then I could wave at my friend on Mars,

and it would take about four minutes for the light for me to get to, them for them to be able to see that light. So there would be a four minute delay. So they would be seeing me four minutes in the past. Now let's go to the sun. We see the sun as it was eight minutes ago. Now let's go to the nearest galaxy, Andromeda. We see Andromeda as it was 2.5 million years ago. So we look further and further and further back and we're tuning into light that has been traveling for longer. So for the first stars, all we do, you can't see me with quotation marks, but all we do is tune into light that has been traveling to us for 30 billion years. And then we see the universe as it was then. Right. Okay. So does that mean that if we wanted to see

these first stars in the universe, we'd be, we have to look beyond our own galaxy to find them because they would just be too old? Absolutely. If we're going to go, there's There's two methods here. So there's the stellar archaeology route where we look within our galaxy for kind of left over first stars, little ones. But my method, which is looking back in time, we have to look much, much further than our galaxy, much further than any observation of a galaxy has got to yet. So further than Hubble's gone, much further.

And this is why we do it in radio waves, because optical light, like, Hubble observes, it loses energy as it goes over 13 billion years. And it's too faint. It's too faint for us to see with any technology. I mean, there's not even any technology in the pipeline that would be able to be able to take a photograph of that time. So we tune in to the kind of lower energy, karma types of light, which is radio. And how many of these stars were there? That is a fascinating question that, you know, I can't give you an answer to. And that really shows how little we know about this time. So there are so many theoretical models covering all sorts of things like loads and loads and loads of first population three stars and they didn't last very long. And then there was lots of galaxies or lots of black holes. And then there's other ones which say, no, no, no, no. There was not many population three stars that really contributed. But there was lots of black holes, for example. And so there really are a zoo, a zoo of different possibilities here. And every detection we make, every observation we make, sorry, that we've been making for 11 years now,

is ruling some of those out. And so how do these early stars differ from the stars that exist now? Well, they are pristine. They are a pure example of what our universe was like after Big Bang. The Big Bang was so hot and violent that you couldn't form heavy elements above hydrogen and helium. So we have lots of different elements around us. We're breathing oxygen and nitrogen. We're made of carbon, all of these elements that you will be familiar with, titanium, silver, you know, all of these things. In the early universe, it was just hydrogen and helium.

And that's because to get to the heavier elements that have more protons, more neutrons, more electrons. It's like you're building a Duplo or Lego tower. If you've got lots of sugar-crazed toddlers running about, your tower will get knocked down. So you can try and build a heavier element. It will not last long. And that's what it's like in the early universe.

It takes a lot longer for the universe to calm down and be much cooler and calmer, really, for the gas to start to collapse into stars. But those gas clouds contain only hydrogen and helium. So we have stars that are formed of these two light elements. And it's only within those stars through the process of fusion. So that's pushing together two small elements, two small atoms, shoving them together. And then they fuse.

They form a heavier element and they produce a large, amount of energy. That's what powers the star. It's a stellar engine. And so that's how we get the heavier elements. And it's only when those stars explode, releasing all of those heavier elements, that the second population of stars can form from this polluted gas, if you will. So you can't form these first stars anymore in our universe because there's too much pollution of heavy elements. So they really are very rare, possibly extinct, and very, very interesting to study because they're unique. So we know that at the end of a star's lifetime, it can explode into a supernova and then collapse into a black hole. Would it be possible that any of the black holes that we know about

now are the remnants of these early universe stars? And would there be any way to tell? Yeah, absolutely. In fact, it's probable. And that's because what we see around us now at the center of most, if not all spiral galaxies, including our own, are what we call supermassive black holes. It's all in the name. I don't need it. Needsner explaining. But they are so large that we can't actually explain how they got so big.

Because if you figure out how much stuff, how much matter that they can accrete, which, you know, gets dragged into this black hole, if you figure that out over the whole 13 or so billion years lifetime of the universe, they couldn't have eaten that fast. They couldn't have gathered that much stuff, that fast. It's really confusing. It's a mystery we have. But if we have these first stars that are really massive from 100 times the mass of our sun, and there are some models which are saying they could be the size of a thousand times the mass of our sum,

if these collapse in such a way that they form black holes, those black holes would be very big. And so this is a way of kind of skipping a stage of growth, which would then explain the supermassive black holes at the center of spiral galaxies. Can we tell? No. Black holes don't come with an identity card. They are very difficult to pin down because they swallow pretty much every piece of information. and you can't get any information from that. But we can kind of say, well, how can they have got that massive? How are there so many? And when you look at the models of the universe, you start to rule things out. And it is looking like you kind of need those massive first stars to collapse in that way.

Okay, I see. So in your book, you also talk about an experiment called the Edge's experiment and how that had a really exciting result a few years ago. So could you just tell us a bit about that experiment? please. Yeah, this is a fun one. And I will bookend this by saying it is a tentative result that still needs validating, but it is hugely exciting. So I'm more than happy to discuss it. So in 2018, I get a call from a UK newspaper asking me to read an embargoed paper. I thought, sure, I'll read this. I get this paper. And it's in an experiment that I've kind of, I've heard of,

but I've not really been that interested in. And I read that it's made a first detection. of the first stars era, of when the first stars formed. And I was, I mean, I was absolutely shocked. I was euphoric, actually. I was bouncing around the house because it was like, yay, somebody's opened a tomb. We've got a first bit of information. I'm so excited. Like, I didn't care that it was a competing technically experiment.

So what happened is that they studied radio waves from this missing billion years. And what they did was they took the temperature. of the hydrogen that was all over the universe, all around these first stars as well. So there's lots of stuff in space, really, lots of gas floating around. And it emits radio waves. So what this experiment was, was a metal table, probably about the size of, oh, I'd say it would seat four if you had it in your dining room. It really does just like that, look like that. And they put it in the Western Australian Desert, where it's very quiet, something you

for radio experiments, radio quiet, I should say. And they measured this temperature. And what they found was that they measured the temperature of the hydrogen all very well, and then suddenly it started to get hotter. And that's because the first stars were forming, and we know from our sun, it's pretty hot. And so when these first stars formed, they started to heat the gas around them. And that's what we think they measured. We think they measured that point, which turned out to be 180 million years after the Big Bang. when these first stars had begun to form. That's a very exciting result in itself. There was a second part to this experiment,

which stopped my jumping around the house when they got to that part of the paper and made me sit on the floor and go, what? Because what they'd found was, without going into too much technical detail, they found that the temperature of the hydrogen was not what we expected it to be.

We've got lots of models that said, It should have been this temperature. It was actually twice as cold as we expected it to be overall. That's a bit strange. And it ruled out every single one of our models. So we were left with a gigantic question mark, which is still there today. We've got a lot of experiments all over the world in Cambridge, in India, Australia, the US, that have all built different kinds of experiments that look a little bit.

it like coffee tables. Some look like kind of bow ties, I guess. They're all different, very strange little things. And they're all trying to make the same measurement to try and validate it. It's not being done yet. But there was a lot of excitement at the time because there's not much that can explain how to get hydrogen cold at the early universe. There's not much to cool down the gas at that time. And the theory paper that came out with it made a very, very bold. claim that it might be something called dark matter, which was cooling the gas down. And again, I have to bookend this statement with the fact that it is very tentative. And actually, over the four years, we believe that this is getting very unlikely now. It's much more likely

to be a difficulty within the experiment that has made us, well, just take the temperature a little bit off where it should be. It doesn't mean that the result's completely wrong. It just shows you how difficult it is to listen to this tiny radio signal that's covered with 13 billion years of noise and then try and dig it out. But it was a very exciting result. Okay, brilliant. Thank you. And finally, what three things do you think we all should know about the first stars in the universe? I think we should thank them for the fact that they are responsible for everything in the universe, like planets and galaxies and us.

So, yeah, let's thank them for that, round of applause. The second thing I think we should take away is that these signals are very faint. And so we need to be very careful to keep the sky, which is what we call radio quiet. We've all heard of light pollution that interferes with optical astronomers. We have noise pollution in terms of radio. So that is actually all the satellites that are being launched right now. They all interfere with our things. So let's keep that in mind.

And the third thing, just look up. So these first stars, you might not be able to see the light from them with your eyes, but you can see their descendants everywhere. And it's free. It's free. How many things in life are free? Get outside. Look up, even if it's for five minutes, it will clear your head, I promise you.

Thank you for listening to this episode of Instant Genius. That was Dr Emma Chapman. If you want to know more about the universe's first stars, check out her book, First Light. Or, to hear her tell me about the telescopes of the future, head over to Instant Genius Extra, available only on Apple Podcasts. The May issue of BBC Science Focus magazine is out now.

Pick up a copy in store or visit ScienceFocus.com. This podcast is sponsored by name, audio and phone. focal. The texture and emotional depth of music can be lost through digital sources or poor signal. Name Audio believes you can have digital precision with analogue warmth. Alongside French acoustic specialist focal, name creates high-end audio systems combining innovation with craftsmanship so you can listen to music, just as the artist intended. Discover more at name audio.com.