Instant Genius - The Big Bang, with Prof Jim Al-Khalili

Episode Date: June 16, 2021

Jim Al-Khalili, a theoretical physicist and Professor of Public Engagement in Science, tells us about the origins of the Universe. 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: https://www.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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Hello and welcome to Instant Genius, a bite-sized masterclass in podcast for. Each week, you'll hear world-leading scientists and experts talking about the most fascinating ideas in science and technology today. I'm Sarah Rigby, online assistant at BBC Science Focus magazine. In this episode, I talked to Jim Alki Lili. He's a theoretical physicist and professor of public engagement in science at the University of Surrey. He tells me everything I need to know.

know about the Big Bang. So first of all, we know that the universe started with the Big Bang, but can you just in very simple terms explain what exactly is the Big Bang and how do we know that it happened? Okay, well, the term Big Bang was coined by British astrophysicist Fred Hoyle. In fact, he meant it as a derogatory term because he didn't believe that the Big Bang is true that could have happened. It's basically what we refer to as the beginning of our universe, our space, and time as we look around and see everything, all the stars and galaxies, it must have started, I mean, there's, when we talk about this in a moment, must have started at a particular moment in time. What most people find very difficult to appreciate is that it didn't happen in a

particular place. Just because if it happened some time ago that we can measure, doesn't mean it happened somewhere, because it was the birth of everything, all of the space, everything we see around us, all started at what we refer to as the Big Bang. And how do we know that it happened? What evidence is there for the Big Bang? Oh, lots. I mean, you know, scientists are obviously very careful to say we are certain that something is true because we can never be certain.

Our theories can be replaced by better ones. New experimental evidence can highlight something we haven't understood. But we're pretty confident that the Big Bang happened. It was predicted by Einstein's theory of relativity, even though Einstein himself, at the time, it's going back just over 100 years, didn't believe that the universe could have started, you know, at a beginning. He believed the universe was static as it is today.

So other cosmologists were suggesting maybe the universe had started from a particular point, a moment in time. The first experimental evidence was Fred Hoyle, the American astronomer who in the late 1920s detected that distant galaxies were, in all directions, were moving away from us and from each other. So the idea that space is expanding comes from this very clear observational evidence

that we can see through our telescopes. Distant objects are moving away from each other, and that fitted in with Einstein's theory of relativity. And that's when people thought, oh, okay, well, that is. The space is expanding, so if it's bigger tomorrow, it'll have been smaller yesterday.

And if you go back long enough in time, everything would have been in the same point. So that was one bit of evidence. there's still people saying, well, you know, maybe there are other ways of explaining things. But then nuclear physicists figured out that all the elements, all the different elements and different atoms in the universe, how they would have been created. In particular, one nuclear physicist, George Gamovs came up with this idea, and this became known as the hot Big Bang model. But in the earliest moments of the universe after the Big Bang, when atoms first formed, hydrogen would have formed, and then helium, these are the lightest, the first two elements in the periodic table. And then the universe expanded and cooled down and other elements couldn't form.

They had to wait to be cooked inside stars. So that story of how the atoms were made is exactly what you see when you look at what's called the abundances of the elements in outer space. Most of stuff in the universe is hydrogen because that was made after Big Bang. Then a third of that is helium and then the tiny sort of sprinkling of everything else. So that all fitted. But the real clincher was the discovery. of what's called the cosmic microwave background.

So this was in the 1960s. Two astronomers, Penzias and Wilson, detected, basically they measured the temperature of deep space, just under three degrees Kelvin, three degrees above absolute zero. And that's exactly what, again, is predicted by the model, by the Big Bang theory.

If the Big Bang happened, the temperature of deep space should be this. And that's exactly what they measured. So that was basically the clincher then. No one was really in any doubt that the Big Bang happened. Then it was simply a case of dotting the eyes and crossing the T's. Okay. So how long ago exactly did the Big Bang happen? This is a measurement that's getting more and more refined, more and more accurate. The moment, I think the current value is 13.82 billion years.

That's very precise. Considering we don't know a lot about the actual moment or the Big Bang itself, maybe we say something about that in a moment, it's quite remarkable that we know exactly when happen. We can go back in time. As we get closer and closer to the Big Bang down to fractions of a second, things start to get a bit fuzzy and hazy. But, you know, fractions of a second compared with billions of years, we're pretty confident. So 13.82 billion years is the age of our universe. Wow, that's a lot. So astronomers like to say that when you look into the sky with the telescope, you're looking back in time because of the speed of light and the time it has taken that light to arrive at Earth. Does that mean if we could look far enough out into space,

we could see all the way back to the Big Bang? Almost, not quite. We can see back to sort of a few hundred thousand years after the Big Bang. But the reason for that is because of the property of matter just after the Big Bang. The universe was so hot and energetic as it first expanded, it exploded from the Big Bang, that atoms hadn't formed and light couldn't sort of travel. So you're absolutely right. You know, the further out into space we look, the further back in time we're seeing things

because we're seeing things as they were when that light left them and it's taken all that time to reach us. But the earliest moment we can see is the moment when light was able to spread throughout the universe. And that is what we call the cosmic microwave background. So that's the moment a few hundred thousand years after Big Bang, when atoms formed, the universe became transparent because light could then travel through. So we can actually see almost

back to those moments. I mean, when we look at the furthest galaxies, obviously we're looking at a time when galaxies had started to form. So that's not quite as early as we'd like. We'd like to go back further. What were the stages that happened in the moments after the Big Bang? Because I get the impression it all happened very quickly, didn't it? Yes, and there are different, what I call epochs, different eras. They start off as being sort of tiny fractions of a second long, and then they gradually get longer as the universe cools down and becomes less exciting. The very earliest fractions of a second, the universe is just pure energy.

And out of that pure energy, particles condense. But those particles are bumping into each other, moving around very, very quickly. you can't form stable matter. You can't form stable atoms. So if an atom is made of a nucleus with electrons buzzing around, well, those electrons in the early universe after they were made, they were moving too quickly

to actually be calmed down and caught within an atom. The universe had to expand and cool, as it's expanded, enough for atoms to form. So the earliest moments of the universe it was really just a soup of energy and high energy particles. Gradually it calmed down, you form atoms. Once atoms form, gravity can start to play a role

and it sort of pulls atoms of gas together and you start to get the first stars and the first galaxies. And then gradually things calm down. Then you can start talking about changes over the course of millions or billions of years. Now, I'd like to address what is maybe a common misconception about the Big Bang. Was it literally a bang? No, unfortunately, it's a misnomer. I don't know. I'm sure there are astronomers and cosmologists who have better names for it, but it's certainly wasn't big and it certainly wasn't a bang. So I guess in that sense, it's a really bad name. By definition, it wasn't big because by definition it was, you know, everything was squashed together in one place. But of course, things get confusing when we ask the question,

is the universe infinite? Does it go on forever? And if the universe goes on forever, then how could something that was tiny and finite become something infinite? So the, you know, the big bang must also have been infinite in extent. But the bang side of things certainly is wrong. I mean, when we talk about something going bang, we mean sound. And we humans detect sound because it's vibrations of air molecules, which cause vibrations inside our ears. There was no air as such at the Big Bang. So there was no what we would understand as sound. It was simply a moment in which matter and energy came into existence and started spreading outwards. What we don't understand yet completely is how it came into existence and the details of why and why did it happen then

and that sort of thing. So do we have any idea what triggered the Big Bang? No. I mean, if you just look, think purely in terms of Einstein's general theory of relativity, then the glib answer is there was nothing that caused. the Big Bang because for something to cause it must have happened before it. And there is no before the Big Bang because the Big Bang marked the birth of time as well as space. And so there was no time before it to embed the word before in. It just makes no sense. And the analogy I always use is that, you know, if I tell you, walk to the South Pole and when you get to the South Pole, keep heading south, it makes no sense because every step you take from the South Pole takes you back north again.

So the South Pole is the furthest south you can go. In a similar sort of way, according to Einstein's theory of relativity, the Big Bang is the earliest moment in time. There is no time before the Big Bang. Having said all that, cosmologists today are starting to ask the serious question was whether there was something before the Big Bang, because it may be that what we call the Big Bang, the birth of everything, is only the birth of our universe.

There may be much more than just our space and time, our universe maybe just be a bubble floating in some higher dimensional multiverse, which could have existed forever for what we know. So the Big Bang marked the beginning of our universe, because then you can ask the question, well, what caused our bubble universe to suddenly pop into existence? The difficulty is you can come up with lots of very clever maths and interesting ideas. How do you test it experimentally?

It's difficult. We can't go back to before the Big Bang and check it out. And you said briefly about whether our universe, universe is infinite. Is our universe infinite? Do we know? Or does it have an edge? Well, what we refer to as the visible universe has an edge. That's as far as we can look. Because beyond that, the light from that part of the universe hasn't reached us yet. Nor will it ever reach us because the universe is expanding. So if the universe was static, we'd still only be able to see. And let's say it's not expanding, but it was born at a moment in time, right?

We'd still only be able to see part of it because we only see the light coming from those objects at a distance where the light has just had time to reach us now since the Big Bang. Objects further away than those, their light is still traveling towards us. But the situation is complicated by the fact that the universe is also expanding. And at the edge of the visible universe, it's expanding faster than light can even escape it. Although we talk about nothing going faster than light, what we mean is nothing can go faster than light traveling through space. But space itself is stretching more and more quickly the further away you look. So it may be beyond the edge of the visible universe, our universe goes on forever.

And there are different mathematical scenarios to explain that. But we don't know it. Maybe the universe is finite. Maybe it's enclosed. it's closed in as like a finite space, but with no edge in the same way that the surface of the earth is finite surface area, but you don't fall off the edge of the earth. So how is it that everything can be moving apart from everything else in the whole universe? This is one of the most frequently asked questions when I or many physicists and astronomers and cosmologists give public talks, because it is, difficult, you know, the logical assumption is that if you look through a telescope in every

direction and see everything receding from us, is that therefore we must be in the middle, you know, and we're stuck in the middle, everything's moving away from us. But in fact, everything's moving away from everything else. The usual analogy is blowing up a balloon. So imagine if you put dots on a balloon before you blow it up, so the dots are all close together. As you blow up the balloon and it gets bigger and bigger, all the dots become separated from each other. So it's not that the dots are moving themselves across the rubber, the surface of the balloon. They are staying where they are. It's just that rubber itself is stretching. That's what we mean by space stretching. Now, with the balloon analogy, it's a bit dangerous

because you think, oh, yeah, but there's a center to the balloon. It's stuck in the middle. With the universe, it's the surface that is everything. There is no centre. We only have that visualization in our minds because it's a simple analogy that our 3D brains can cope with. But imagine that the surface of the balloon is everything. There is no inside. Every dot is a galaxy,

and we are sitting on one of those dots in one of those galaxies. If we look around us, we'll see as the balloon gets bigger, every other dot around us is moving away from us. Now, it's not actually, as I say, it's not actually moving. It's staying where it is. We're staying where we are, but the space in between us are stretching. And if you were to go and sit on one of those dots, you will again see all the dots around you moving away from you.

So you'd think, oh, I must be in the centre now. So there is no place, one place, where the Big Bang happened. Because if you run the film backwards and that balloon shrinks down to a point, all those dots will be in the same place. That same place is the Big Bang. So the Big Bang happened everywhere. The Big Bang happened exactly where I'm sitting now, but it also happened on the other side of the visible universe

because that other side of the invisible universe was the same place as me at the Big Bang. Out of everything we've talked about so far today, what three things do I need to know to become an instant genius on the Big Bang? Okay, well, I would say they're the three things that people find most counterintuitive. One is that the Big Bang has, happened everywhere because everywhere was in the same place.

The second is that distant galaxies are moving away from us, but they're not moving through space. It's the space between us and them that is stretching. And the third thing is the Big Bang theory isn't just a theory. It's backed up by tons of experimental evidence and data, which means we are pretty confident that the Big Bang theory. bang actually happened. Thank you for listening to this episode of Instant Genius.

That was Professor Jim El Kilili. If you want to know more about how the universe works, check out his book, The World According to Physics. Or, to hear him tell me more about the Big Bang, head over to the Instant Genius Extra podcast. The June issue of BBC Science Focus magazine is out now. Pick up a coffee in store, or visit sciencefocus.com. This podcast is sponsored by,

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