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Hello. In the beginning runs one secular account was The Big Bang. All matter in existence today originated around 13,000 million years ago in a phenomenally hot, extraordinarily condensed primordial atom that exploded with incredible force. Hydrogen and helium were shot across the firmament. Gravity caused the gases to condense into clouds, and in these clouds the first stars were formed. Then galaxies came and more galaxies in clusters onwards and outwards, ever expanding. It's still expanding, runs the Orthodox account and may even be speeding up. It's still creating new galaxies, and it continues to colonize more and more infinite space, despite the fact that it's supposedly infinite itself. The word billions dominates all discussions here. So if our universe is expanding, what's it's expanding into, if it's already infinite, how can it be getting any bigger? And is there really only one universe with me? To tackle some of the riddles of the universe is the Astronomer Royal Professor Sir Martin Rees, Royal Society Research professor in astronomy and physics at Cambridge University and author of a new book, Our Cosmic Habitat. Also here is the independent theoretical physicist Julian Barbour, recently published. He recently published a book called The End of Time and John 11 Advanced Fellow in Theoretical Physics at the University of Cambridge, who has also just published a book on cosmology called How the Universe Got Its Spots. Martin Rees as I understand it, one of the main planks of evidence for the support of the Big Bang theory is called redshift. Could you explain what redshift is? 
It's the way in which we infer the universe is expanding. If I take a step backwards, we've known since 1920 that our universe consists basically of galaxies, each one being like our Milky Way, each one containing about 100 billion stars, and these galaxies are moving away from each other. What I mean by that is that if you imagine elastic strings joining all those galaxies, the elastic is stretching all the time. Now, how do we know this? The way we know it is that when we look at the light from these galaxies, then the further away the galaxy is, the more the light is shifted towards the red. And that's analogous to the famous Doppler effect we're familiar with in sound. If the police siren is moving away from you, it's got a slightly lower pitch. So it's that evidence which gives us the view that the universe is expanding. And I think this is one of the sort of bedrocks of modern cosmology. 
If redshift tells us that the universe is expanding, what is there to tell us that it started with the explosion? I mean, going back, I know you've discussed it many times. Many people have, but I wouldn't mind having another crack. What does it tell us about the Big Bang? 
If the galaxies are moving away from us, then the naive assumption is that in the past they were closer together, and you can work out how long ago they would have been all on top of each other, as it were. And that time is about 13,000 million, 13 billion years. So the natural interpretation is that everything was together at that time ago in the past. Now, of course, galaxies couldn't have existed in their present form back then. But the favored view is that everything started off, as you said in your introduction, in a very hot, dense gas, and it expanded and the gas cooled and diluted and at some later stage, maybe after a billion years or so, the galaxies started to form. So the favored view and other lines of evidence to support it is that the universe is an evolving phenomenon. It started off at some finite time in the past very hot, dense and compact. It's been expanding and cooling and all the structures that we see around us, and of which we are a part, are the outcome of forces acting on the universe over this immense span of time. 
Okay, right. Janna Levin, the galaxies, if, uh, if all matter originates from the same ball and the same explosion, why isn't it evenly distributed throughout the universe. 
In a sense, it is evenly distributed throughout the universe in the sense that, um, we are nowhere is identifiably the center. If you have a star that explodes like a supernova explosion, it's clear where the center of the explosion happened. And all matter accumulates on these shells, and it's not evenly distributed in the Big Bang by an enormous contrast. If space itself is created and matter is created in the Big Bang, then matter is, in a sense, roughly evenly distributed everywhere. There is matter in every in every point in space time, and every point could equally well be the center. Um, later on it starts to clump and form galaxies. And and that looks kind of unevenly distributed on small scales. When we look in the scale of an individual galaxy or clusters of galaxies. But on the larger scales, those clusters seem to distribute themselves evenly throughout space. 
Right. Julian Barbour, isn't there another mysterious factor involved in building galaxies? What is dark matter and what does that. Do. 
Well, there's a lot of evidence this is also going back 70 years. Good. To a Swiss astronomer called Zwicky who first saw the evidence for it in the 1930s. Uh, you you can tell that there's matter around in the universe because of the gravitational effects that it has. I mean, there can be a completely invisible object, and you can see that it's there because things are whizzing around it. And essentially, what Zwicky already observed in the 1930s was that the the visible matter is moving around faster than it should if, if, if we can see all the matter that's in the universe. So there is well known to be, uh, I mean, it's virtually certain that that there is more matter in the universe than we can see. And it's got the name dark matter. And one of the really great mysteries is what it's made of and exactly where it is. 
Why? What is the mainstream physics understanding of the force that causes the universe to be expanding, to expand? 
I would say there's a sort of a rather naive idea that, um, that there was a big oomph at the start which sent it all flying. And since then, the rate at which it flies apart is controlled by by gravity. By gravity. Uh, if there was enough oomph, it'll sort of go on forever and never come to rest. If there wasn't enough oomph, it'll fall back on itself. And we'll go from the from the Big bang to the Big Crunch. I have to say, I'm beginning to get skeptical about that view, but that's the that's the standard picture. You have to bear in mind that science comes in two parts. At least it does at the moment. You have the laws of of nature or the laws of dynamics, and then you have initial conditions. So when you say what what drives the universe apart in the standard orthodox picture, it's it's the oomph you give it at the start. 
So that's a kinetic energy. That's the kinetic energy. The matter that is formed causes a gravitational pull. And that matter includes dark matter, which adds to the gravitational attraction and keeps the thing together. 
Yes. And there is this very fascinating thing that, according to the standard picture, we seem to be just about exactly on the dividing line between going on forever and and falling, falling back, I mean, sort of on the dividing line. And that's very mysterious. 
There's a very nice line of skepticism in your remarks. The word use the word naive and mysteriously, that if I can come back to that in a minute or two. But go back to Martin. You write in your new book, Martin quote, we the stars and the visible galaxies are just traces of sediment, almost a seeming afterthought in the cosmos. Something quite different and still unknown controls its large scale structure and eventual fate. Now can you? Those are tantalizing phrases, large scale structure and eventual fate. What are you referring to? Is this the dark energy now from dark matter? It's like Darth Vader. We're on sort of miltonic Manichean Harry Potter terms here, aren't we? 
It is embarrassing that only. 
Darth Vader sorry, got him wrong. Again. 
Only 4 or 5% of all the stuff in the universe is in the ordinary atoms of which we, the stars and the galaxies are composed. All the rest is in some mysterious form. It now seems. 
You use the word mysterious. 
Well, because, as Julian said, the dark matter is something whose nature we don't fully understand. We suspect it's some sort of particles made in the Big Bang, along with the atoms and the radiation. But we don't really know what it is. But we do know that there is more dark stuff in the universe than there is in luminous stars and galaxies, and that is one of the problems. But there's an even more mysterious issue that's come to the fore in the last few years, which is that this dark stuff is in two different kinds. There's the so-called dark matter, which holds the galaxies together. And this goes back to Fritz Zwicky in the 1930s. But we've also discovered that even empty space itself has energy latent in it, and 70% of the mass and energy in the universe, as it were, is in empty space itself, not even in dark matter, still less in actual atoms. And this dark energy is governing the way the universe is expanding. Generally. 
You want to get in on this. 
I just wanted to come in on one point there. There's very few observations which you can interpret without using some sort of theory. And the remark that Martin just made about this extra form of dark matter, which causes the acceleration, that conclusion is drawn from using Einstein's theory as it exists now, and that is possibly dangerous. There is some unification, some deeper understanding is going to come. And I think it's almost certainly going to change the theory that we use to describe the universe. And therefore one must take these claims with a certain pinch of salt, because they're crucially dependent upon the theory that you use to interpret the observations. 
John 11. Can I ask you at one point in your book, you are very emphatic. You write the universe. This cannot be infinite. 
Well, it's probably too emphatic for my own good. 
Yeah, and that's true. I did write that. It's not that I think that there's empirical or logical evidence that suggests that it's impossible for the universe to be infinite. It's just that I think it's an untenable idea. If you think about an infinite universe, it becomes extremely peculiar. If there's an infinite universe, it means that there are infinite possibilities. It means that somewhere else there is a galaxy just like this galaxy. It means that there's an infinite number of these galaxies, and there's an infinite number of planets that look infinitely close to the Earth. And somewhere there's people sitting around a table having a discussion about infinity, and it just becomes this kind of absurdist notion. And that might not be pressing enough, but it does start to hint that something's wrong. 
And as something's wrong, because we can't understand it. 
No, not just because we can't understand it. Because infinity seems to be very dangerous things. We usually mark infinities as the failure of a theory, but then we're. 
Measuring our understanding against the against our own minds. Maybe we're not able to understand. 
Well, it actually kind of works. I think of it more as the theory. It's possible that we might just come to a place where we can't understand something, in which case scientists will give up. I don't I think that the the scientific impulse is to believe that that's never. 
Going to happen. 
And it's brash, but it does it does give us progress. And so one of the things we can do, for instance, is look at where Einstein's theory does begin to break down. It begins to break down at things like the Big Bang, where Martin was discussing the very, very first moment is not something we can talk about precisely because the theory starts to give answers that we no longer trust. They're not as predictive. There's infinite singularities, there's infinite curves, infinite energies. And this is an infinity that's not as bad but is on a similar footing. 
I'm I'm myself too, very attached to the idea that the universe is finite, but that might just be a reflection that my mind is finite. There is something extremely appealing about a finite, self-contained universe, because in a wonderful expression of Einstein, then you have a chance of closing the circle of cause and effect. You can, so to speak, find a total explanation of everything within that. This is hugely attractive and and my life's work really has been devoted to trying to understand the universe in such terms. But every now and then I look at myself with disgust. And it's just because you're a finite little worm and you can't comprehend the infinite. So there may be a danger in that, but I very much feel in sympathy with what you said, Janet. 
If I could just respond to that very quickly. I mean, infinity is a brilliant notion mathematically. So I don't reject the notion mathematically, for instance, and it was rejected for a long time. I just think physically we never see infinity in nature and that the universe was born, as are all of its progeny, and that we are all finite. And everything within this cosmos is finite. And I don't believe that a unified theory which will see gravity, space, energy, and matter all as different reflections of the same thing, will create space infinite, but everything else finite. 
Yeah. Just first of all. 
In praise of the infinite, there's that wonderful saying of David Hilbert. We hopes the mathematicians will be never driven from the Paradise of the infinite. 
That's right. That's beautiful. 
I think it might be interesting to put this in a historical perspective. There have really been only about three great cosmological models in the whole of human thought. The first really clear one is Aristotle's, which is the self-contained spherical universe. And that was sort of the shell of the stars we see. And then it was. 
Then it was. And there was an outermost sphere, the outermost. 
Sphere, the Uranus, the outermost sphere. And beyond that there was literally nothing. No space, no anything. I mean, there might just be gods out there, but nothing else beyond that. Then you get the changeover in the 17th century to the infinite space, the infinite space of Euclidean space and atoms moving in it. And that was then the dominant paradigm until Einstein comes along and in, amazingly, it goes back almost exactly to Aristotle. 


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