In Our Time - Dark Energy

Episode Date: March 17, 2005

Melvyn Bragg and guests discuss 'dark energy'. Only 5% of our universe is composed of visible matter, stars, planets and people; something called 'dark matter' makes up about 25% and an enormous 70% of the universe is pervaded with the mysteriously named 'dark energy'. It is a recent discovery and may be only a conjecture, but it has been invoked to explain an abiding riddle of the cosmos: if the expansion of the universe is powered by the energy of the Big Bang, then why isn't the expansion slowing down over time as the initial energy runs down and the attractive force of gravity asserts itself? Scientists had predicted a Big Crunch as the logical opposite of the Big Bang, but far from retracting, the expansion of the universe is actually accelerating...it's running away with itself.How do we know that the universe is behaving like this and what's causing it? If dark energy is the culprit, then what is this elusive, though omnipresent entity?With Sir Martin Rees, Astronomer Royal and Professor of Cosmology and Astrophysics, Cambridge University; Carolin Crawford, Royal Society University Research Fellow at the Institute of Astronomy, University of Cambridge; Sir Roger Penrose, Emeritus Rouse Ball Professor of Maths at Oxford University.

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or wherever you get your pods. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio 4. I hope you enjoy the program. Hello, only 5% of our universe is composed of visible matter, stars, planets and people. Something called dark matter makes up about 25%, and an enormous 70% of the universe is pervaded with the mysteriously named Dark Energy.

It's a recent discovery and maybe only a conjecture, but it has been invoked to explain an abiding riddle of the cosmos. If the expansion of the universe is powered by the energy of the Big Bang, then why isn't the expansion slowing down over time as the initial energy runs down, and the attractive force of gravity asserts itself? Scientists had predicted a big crunch, as the logical opposite of the Big Bang,

but far from retracting the expansion of the universe, is actually accelerating. It's running away with itself. How do we know that the universe is behaving like this? and what's causing it. If dark energy is the cause, then what is this elusive, though apparently omnipresent entity?

With me to discuss dark energy is the astronomer royal, Professor Sir Martin Rees, Professor of Astrophysics at Cambridge University. Carolyn Crawford, Royal Society Research Fellow at the Institute of Astronomy, University of Cambridge,

and Sir Roger Penrose, Emeritus Rouse Ball Professor of Maths at Oxford. Martin Rees, Einstein, when he wrote his equations for relativity, didn't think the universe was expanding. So what was the single most important observation made after him that showed us that it was still expanding and accelerating? In the 1920s people realised that the universe was far bigger than that had been thought before.

They realised that our galaxy, the Milky Way, which contains all the stars we see with the naked eye, is just one island, as it were, in a universe which contains, as we now realize, zillions of other galaxies like ours. and in the 1920s, astronomers studied how these galaxies were moving, and they found that the light from these distant galaxies was reddened, implying a sort of Doppler shift, implying that rather as a siren appears to have a lower pitch moving away from you, the light is reddened if a galaxy is moving away from you. And they found that these galaxies were spread uniformly through space,

but the further away they were, the more the light was reddened. And this suggested that contrary to what Einstein had thought, the universe wasn't just static, it was dynamic. It was expanding in a sense that if you imagined rods joining all the galaxies, then those rods were all getting longer at the same sort of rate. Everything's getting further away. There's no centre, but everything is expanding. And this came from the Big Bang, about which at the very moment of the Big Bang, we still are in the dark. I'm sorry to keep above that word, but that seems to be the case. But we know he came from what you call the Big Bang, not say the Big Whisperer, I think it's got to be a Big Bang. Well, in the 1920s that wasn't clear, but we now believe that this expansion can be run in reverse, as it were,

and we can trace things back to an era about 13 or 14 billion years ago when everything was squeezed together in a very dense state, what we call the aftermath of the Big Bang. And the universe has expanded from that time. At some stage, the galaxies formed, and after 13 or 40 billion years, years, we see the kind of cosmic panorama that astronomers can study. But just to nail this one before we move on, you say, the trouble with the big bang is that we don't know what banged or why it banged. Indeed, we don't, because we can extrapolate back to very close to the initial incident.

But as we imagine extrapolating back, everything gets hotter, denser, and more energetic. And we can go back a long way. We can go back to within, I would say, about a microsecond, a millionth of a second. of the Big Bang. But a lot can have happened even before then. And what happened before then was something which is mysterious because it depends on physical laws

which we don't really understand because we can't test them in the lab. It's beyond the range of conditions we can ever simulate here in our lab. So it's very speculative. But indeed, until we can actually probe the mystery of that first initial incident, we won't know what banged

and why it bangs, why it's expanding the way it is, and why our universe has the ingredients which we observe. But evidence for that comes with the cosmic background radiation, which was discovered in the 1960s. Can you tell us a bit about that? Yes, that's right. The first evidence that the universe did come for Big Bang

came from discovering that even empty space is not completely cold. It's warmed to about three degrees above absolute zero of temperature by radiation, microwave radiation, pervading all of space. believe this is the cooled afterglow of the initial hot dense state. This radiation pervades all of space. It's a little bit of warmth left over from the hot

beginnings of the universe. And in the 1960s, when that was discovered, that really caused a shift towards a near consensus that our universe started with a big bag. And you regard this evidence as sound as evidence, the early geological evidence, for

what happened on Earth? I do. I think... There's that level of is that quality? I think the extrapolation back to a fraction of a second is as firmly established as much of what a geologist would tell you about the early earth. But of course, as always in science,

each advance brings into focus a new mystery and the big mystery we have is the very beginning and how we can understand that better. Carolyn Crawford, until the 1990s, the assumption was that the expansion of the universe was slowing down. Can you develop that a little and then explain why the way there was, was that assumption?

Well, very simply, the universe is expanding, and from this expansion it's got kinetic energy. But at some point, this expansion was expected to slow down due to the gravitational traction between all the matter within the universe. It's like if the universe has sufficient kinetic energy that's far more than the gravitational potential energy of the universe, then it's going to carry on expanding forever. However, if there's a lot of matter and energy within the universe that's going to cause gravitational attraction that will eventually pull the universe back in in itself.

And we have what's called a critical density which divides these two options. If you have more mass in the universe than this, or matter density than this critical density, then the gravitational attraction of all the matter in the density is eventually going to stop this expansion and pull the universe back into the big crunch. if you've got much less than this critical density

then there's never going to be enough gravitation to pull the universe back in together and it's going to carry on expanding forever now in the 1990s we thought the universe was expanding but at some point it was going to start decelerating because of the power so we've got these two forces kinetic energy it's expanding it's going out

and gravity is pulling it breaking it down and almost pulling it together giving it matter making a mass there and the idea was it would gradually slow down as the energy began slowly to peter out? Not quite. It depends on how much stuff there is in the universe as to which is going to win. It's not going to create the matter. It's just if there's enough matter there, then it's going to cause this, the gravitational attraction is going to be strong enough to slow down this expansion.

Now, you've mentioned the cosmic microwave background, and from studies of the cosmic microwave background, we think our universe is poised on the threshold between these two eventualities. the amount of matter in the universe is at this critical density. Now, this would mean the universe expands, but at some point it starts to slow down. It doesn't go expanding on forever, and it doesn't necessarily collapse down at a big crunch. And people are interested in actually tracking this deceleration

of the expansion of the universe. And to do this, they were looking at supernovae, as if you like, these are markers for the expansion of the universe. Why are they useful to look at, Gala? Well, supernovae are very powerful markers, especially this particular type. supernova. Now this is a thermonuclear explosion of a star and it happens under a very sort of

very specific set of circumstances. First of all, this is from a white dwarf star. So this is a star that's more massive than our sun but it's contained within a radius up to like a hundred times smaller. Now if this is in a binary system with another star it's got this intense gravitational field and it pulls matter from the other star and it accretes matter. And this is okay, up to a point where it's like 50 times the mass of our sun, it can no longer support all this extra weight. And it starts to collapse under its own weight

and through this sort of very rapid sequence events, which then transforms this into this explosion. Now the key things about this is this explosion is incredibly bright. It can at that point outshine the host galaxy. So you can see these halfway across the universe. So this makes some very good traces for seeing across the universe. The other things, because it happens at this exact point, it's triggered with the same mass of the star,

and we understand well that you've got the same kind of physics involved in the explosion, you expect them to attain the same luminosity, the same brightness within the explosion. So we know how bright the explosion should be. We can compare that to the brightness we see. These are related through the inverse square law to the distance to the object. So if you like, we're tracking out this expansion of the universe. We're looking at the distance of the object.

We're mapping that to the velocity of reset, of the object, we can get this from looking at the light of the underlying host galaxy. And so we're mapping up the expansion of the universe on much greater length scales than the astronomers were doing in the 1920s. Now, astronomers were doing this to

to map the deceleration in the universe, but instead they found the universe was actually accelerating. Now, this obviously was, was this, thank you very much, Roger, was this obviously a big, for Judge Panos, was this a big shock to find that the universe was accelerating more quickly? not only not slowing down, but accelerating more quickly than anybody had anticipated.

It was expressed as a shock. I've always been rather puzzled by this because, well, in 1915 Einstein produced his general theory of relativity, and in 1917 he introduced what's called the cosmological constant. I mean, his reason for doing this had to do with trying to make the universe static, which, as Martin has explained, universe isn't static, so that turns out not to have been. been a good reason. But on the other hand, his equations had this freedom, and I don't know how long he knew that. He didn't take advantage of it before. The equations basically involved two constants. One of these constants is the gravitational constant, which we have to fix from Newton's theory,

and the other constant is called the cosmological constant. Now, Einstein, after he realized that the universe was expanding, he retracted, having mentioned this idea, but nevertheless, it is a freedom in his equations. And this freedom is just a number, if you like, which could be there. It's the only number which could be there which doesn't affect the general nature of his equations.

Can we just stick to the accelerating universe at the moment? That's exactly what this is. Yeah, but can we just come back to the idea, before we go into the cosmological context, can we go into the other evidence for the accelerating universe that was discovered in the 90s?

I mean, you've talked about barium density and that sort of thing. Well, there's different factors here. I mean, there's more matter in the universe than one sees in galaxies. But this is different from that. And that's called dark matter. I think there's a slight confusion because the term dark

has used both for dark matter which is material out there, which seems to be present, but which is not what we see stars made of and so on. So it seems to be matter out there which contributes to the general overall density of the universe. But there's also... But dark matter is to do with gravity and to do with the material causes of the universe.

If you like, it's just like ordinary matter. It's just ordinary matter of a kind that you don't directly see. And it seems to be the majority, most of galaxies and so on. The matter out there in the universe seems to be mainly what's called dark matter. but then there's also this thing that Carolyn was referring to which seems to produce an expansion of the universe now ordinary matter wouldn't do that

because it would pull the universe back if there was more ordinary matter out there it would cause the universe not to accelerate outwards but to collapse back inwards before we leave dark matter away has it ever been seen how do you know there's supposed to be is it 10 times more dark matter than

scene matter in the galaxy. As far as I'm aware the main reason for believing in it has to do with the rotation of galaxies. If you had if the dark matter wasn't there, the galaxy seemed to rotate, the further you go out they seem to be rotating too fast, and so what holds them together? Why don't they just sort of fling themselves apart? And so there

apparently is this extra material which is not directly observed, which holds the galaxy together in a sense, in which cool. Nevertheless, in broad terms, before we leave this part of the argument, discovering in the nighties that universe was expanding in even more rapidly than had been thought must have been a bit of a shock to people like yourself. It wasn't a shock to, I mean, it was a shock to me because I wasn't expecting

it. But it wasn't a shock to me in the sense that it was waiting there in the equations. It was, I mean, I've been to conferences many times before this, but people would say, well, it would be nice to be able to measure the acceleration parameter that would tell us what the value of the cosmological constant is. And what we find now is that the value of this cosmological constant is something. It has a certain positive value which causes the universe to accelerate. Now, I'm not as surprised as many people seem to me because it was all really there in the equations, but sorry, go ahead.

No, I was just going to say, I believe it was a surprise because it wasn't behaving as we expected. I mean, for example, imagine you throw a tennis ball up in the air. What you expect to see is it rises up and then it begins to decelerate and break and then fall back towards you under gravity. Now, the equivalent of what the astronomers were seeing is they saw the tennis ball rise up through the air, begin to decelerate, and then it accelerates away from you. Now, if you saw that happen, you would say,

we really don't understand something about gravity, or there's some other force that's occurring here. So it's that kind of analogy. There's something we didn't understand about the way... There's something wrong with the analogy, but it doesn't mean... I mean, it was there in the equations. Yes. And it was in every cosmology book.

It's not as though people should have been surprised because it was there in every respectable cosmology book. Healthy disagreement here, this is leaving me away behind, but I'm enjoying it. I was somewhat surprised. Most of us, of course, start off making the simplest guesses. The very simplest guess is that the universe is full of just atoms like we see. We had to jettison that idea about 30 years ago when we found, as you've mentioned,

that there was also this so-called dark material which helps the whole galaxies together by its gravity. and this dark material is probably some kind of particles made in the big bang along with the atoms and along with the heat. And so that was a surprise about 30 years ago, and that's firmed up. I suppose most of us then thought, well, maybe that's all there is. Maybe we have a simple theory of the universe where there's atoms and dark matter. And although I agree with Roger that the equations of Einstein did allow this extra quantity,

people had the, I wouldn't say the hope, but they had the expectation that perhaps there was nothing else and perhaps there would be enough dark matter to provide what Kowling called the critical density. This density is actually like about five atoms per cubic meter, an incredibly near vacuum, but that's all you would need on average in the universe to make the expansion eventually halt. Now what they found was that the amount of dark matter, although more than the amount of ordinary atoms in the universe was only 25 or 30% of what would be needed to make the universe have this very simple behavior

of just having a balance between gravity and expansion energy. And so that showed that we weren't in the simplest possible universe and meant there was something else left over and that something else was probably this expression which Einstein first conjectured. And so does that, cannot bring us to dark energy? Are we allowed to move to the territory of dark energy? Can you lead us on that then?

What, Martin, briefly, how did, how was dark energy as it were noticed, discovered? And just as an overall point, what does it mean? Well, the simplest interpretation of the acceleration is that Einstein's mistake was not a mistake. and that the... You would have to tell people what Einstein... He said he'd made a mistake, but I don't think everybody is... Einstein introduced this extra number in his equations

because he thought back in 1917 that the universe was static. He put in an extra term which gave a repulsion which balanced the attraction of gravity and allowed the universe to be static. We then discovered that the universe was actually expanding and he then rather lost interest. in this extra number which he had introduced,

and he thought it was probably zero. But what we now realize is that probably there is this extra force in the universe, which is actually overwhelming gravity on the cosmic scale. If there was just the atom and the dark matter, the universe would be slowing down, although not quite enough to come to a halt. But the surprise in the last five years is that the universe is actually speeding up. So there's something which is quite...

If one regards it as a sort of battle of the forces, I'm sorry to be so terribly simple about this. I really apologize. But if you do, then you've got gravity with dark matter coming to its age, far more dark matter than you'd ever thought. But the expansion, the kinetic energy, has got dark energy coming to it. I can see your brow crinkling, Caroline, but could you just tell us more about dark energy? And the dark energy plus expansion is outgunning gravity plus dark matter. That's right. It's causing the acceleration of the universe. The expansion is speeding up. The key thing is that obviously in the history of the universe, at one time, the galaxies were much closer together, and gravitational attraction was important.

But as the universe has expanded, the galaxies have diluted. And at some point, we think this happened about six billion years ago, that the gravitational attraction became overridden, as it were, by this other force, this dark energy, this repulsive gravity. if you like, that then started pushing the galaxies further and further apart and causing this acceleration. Roger, can you come in on this? So, may I just introduce a slight worry about the name?

I think this is one of my troubles with it. Being called dark energy, it suggests because energy by Einstein's E equals MC squared is equivalent to mass. So therefore this energy ought to behave like mass. It ought, therefore, to attract like mass does ordinarily.

but this stuff, which I prefer to call the cosmological constant, is actually repels rather than the tracks. So it's not like energy in the ordinary sense. It's quite different from energy. Although at the moment we're talking about it as dark energy, but you dispute the name. It's just the name, I think, is very confusing. I don't quite know why it was introduced.

Can you bring the cosmological... Well, it wasn't my... No, I'm not blaming you with... You guys... Yes, okay. You're just glaring at me, Larry. Let's go back to the cosmological constant here Because Einstein had that in, as Martin explained,

to square his view that the universe was static. Right, now then you say, even though he thought he got it wrong, you think actually the cosmological concert still works, even given these new discoveries of dark matter. Can I put it in another way, you see? I think in a certain sense, Einstein's theory was greater than he was. You see, I mean, he produced this theory, tremendous insights, no question about it.

But then he guessed about what the universe was. was like in one way or another. And his guesses about the universe as a whole turned out to be wrong, usually. But the theory allows for this extra constant. And Einstein recognized this. And therefore, one has to take the theory seriously. This constant could be there.

Okay, like most people, I probably would have guessed it was zero, like Martin was saying. Nevertheless, the fact that it seems to be out there is a feature of these equations. So we must take the equation seriously. The equations say, okay, there could be a cosmological constant.

You can call it dark energy if you like. I'd prefer not to myself. But this stuff out there, which is the cosmological term, is a feature of the equations. And the equations allow for it, and it's the only thing you can put in the equations without completely shattering them.

But you're preserving the equations. You're preserving the equations at the expense of the observations. No, no, no, no. The equations say this number, this thing we call lambda, the cosmological constant, could be there. It's the only thing which could be there.

there without destroying the equations. And observationally, we find it is there. Now, it's a little surprising that the value is some small number, which happens to be of the same sort of general scale as other matter we find out in the universe. But nevertheless, the equations are allowed for it. The equations tell us how it should behave. The equations tell us that it should be on a cosmical scale and expansion, a repulsion. Repulsion overall, but something which contributes in another sense to the overall.

density of the universe. Ordinary matter won't do that. Ordinary dark energy in the sense of energy won't do that. But this term in Einstein's equations will do it, and it seems to do exactly what we find, as far as I can tell. Martin. Another way to say this is that we found that empty space is not absolute nothingness. You might first have guessed that if you had a region of space and you took away all

the atoms, all the dark matter, all the stars and galaxies, then you just be left with nothing. but what it seems is that we're left with something which exerts a force if you put two little particles in empty space with nothing else then they will move apart at an accelerating rate so there is this energy latent even in the emptier space we can imagine and that's the modern way we think of Einstein's extra term and in a way it's less surprising to us with our modern perspective

because one thing we have learned is that if you were to look at empty space space on a tiny, tiny scale, it would actually not be simply, but very complicated. We're used to the idea that if we take a lump of material like the table I'm sitting by and break it up, we get down to the scale of atoms. It's grainy and complicated. We now believe that that may be true of space itself. Space itself on a very tiny scale indeed may have all kinds of structure latent in it. So empty space isn't just philosophical nothingness.

It's got all kinds of structure. So in our modern perspective, I think it's not surprising that there is a force, even when there's nothing there but space, because space itself isn't as simple as we thought 50 years ago. But, Caroline, can I come to you? You think, Roger is very passionate about Einstein's cosmological constant. Do you think that's sufficient to explain the observations that have been made recently? I think it's one of the possibilities.

The trouble with Einstein's things, of general relativity and our ideas of gravitation is that we've put them through lots of observational tests. We've looked at pulsars and black holes, things where there's a very intense gravitational field. And it's passed all these tests with flying colours, that the theory seems robust. The difficulty is actually testing the gravitation equations

on cosmological scales. We're coming to a new challenge where gravity has not really been tested before. We understand how it works on the scale of solar systems, of planets, of galaxies, of galaxies, of galaxies, but actually we will have to be clever and devise further tests for gravity on

the most immense scales possible in the universe before we can say whether this cosmological constant is truly the solution. Roger, can I get back to this dark energy which you don't think merits the should be called dark energy? It's fascinating, right. So what is it? If you don't think it's dark energy,

what is it? You tell us what you think it is. Because the word force has been used now. Do you think it's a force? The trouble is these are ways of comparing it with things we're familiar with. And as Carolyn points out, one's familiar with the idea of things, you know, ball, you throw them up in the air and it falls back to the ground. And if you fall it, throw it fast in a certain speed, it keeps on going. They're familiar with that kind of notion.

What we're not familiar with is some repulsion term, which gradually takes over. If you throw it hard enough, then it keeps on accelerating away from us. and that's what the Einstein equations, with the extra term, the cosmological term, that's what they tell us to expect. It's difficult to understand that in ordinary terms. I don't think calling it dark energy helps at all, because it tells us that it's like other kinds of energy, which is just not. But if it's pushing towards, I mean, let me blunder in here, if it's, if it's pushing towards expanding the universe, shouldn't it, doesn't it marry some name that suggests energy, force, power or something? Well, I suppose there's a reason people use. that term and a cosmological constant is a bit too dry term. It also contains the word constant

which some people don't like. They'd rather have this number something which could be changing for one reason or another. I think that's dangerous because once you allow it to change, you have to explain why it's almost exactly constant and you have to explain why the equations... Yes, I mean you could have other explanations if you call them explanations for what this stuff is, if you like, other than the cosmological constant. And people, I think, call it dark energy because they prefer the idea of something which might be changing. It doesn't have to be a constant. I don't think it helps, but that's what people, I think, one of the reasons they do that. Because Martin Reis, Martin Reyes, another complicating factor is that it seems,

black energy seems to have had a stronger influence at sometimes than had others, and this brings in the notion of inflation. Right. Well, away you go. I mean, I'm... Let me first add a footnote to what Roger Penrose just said, and that is that the evidence we have is consistent with this acceleration in the universe being produced by the same effect that Einstein conjectured in 1917. But we can't be quite sure because the evidence for the acceleration is fairly convincing but not very detailed. If we understood the fine details of exactly how the universe is speeding up, we might be able to test that and also see if some further tweaking or refinement.

of the equations is needed. And I think it's important to bear in mind that the equations of Einstein are, of course, a great advance on those of Newton. They apply when gravity is strong. They apply when speed are... Those of speed of light, they apply to an entire universe. But they may not be the last word.

They may, as Carolyn Corford was implying, need some modification on very large scales. It could be that Einstein's equations need modification in some other way, which we're just starting to discern. But also, you mention that there's a link between these ideas about the far future of the universe and the other deep mystery of the universe, which is what happened at the very beginning. As I said, we have no idea why the universe banged and what banged,

and we certainly don't know why the universe is as big and as smooth as it is, because all of us talk cavalierly about the universe. And, of course, we can only do that because the cosmos is. rather simply than we had any right to expect. Let me give an analogy. If you're in the middle of an ocean, you can talk about the average properties of the waves you see. You look out to a horizon and you see lots and lots of waves and you can talk about the average properties. Whereas if you're in an alpine landscape, you look to the horizon, it's dominated by a whole big peak and you can't talk about the average properties, the average expansion speed, etc. And cosmology is only possible

We can only talk about the universe because our universe has this combination of being grainy on small scales but being smooth on large scales, well, like an ocean surface. And how it came that way is one of the things that must have been imprinted in this first microsecond. Caroline, do you think that this... I'm rather embarrassed to keep coming in dark energy, the presence of Roger Penrose, but there we go.

Do you think that this has increased in power and decreased? This idea of inflation, that dark energy was more powerful at some time and then became less powerful and is now more powerful again? Well, that's just one idea. I mean, relating the idea of dark energy to the inflation of the universe, it could have the same cause and it could have a completely different cause. The idea of inflation ties in, we've talked about modifying our equations of relativity, Martin's talked about the energy of the vacuum.

The idea of inflation is tying into a third major idea about dark energy, which goes under this sort of blanket name of quintessence, and just this name itself tells you that we don't really know what it is, because it's just only but fifth essence after the idea of earth, fire, air and water. But this is the invocation of an energy density for like an energy field throughout the universe that is slowly changing with time, and when fields change with time, they accompany this with a release of energy that could take the form of a repulsive gravity

now so it could be that today we're in a very much milder form of this inflation that we invoke to explain a lot of the properties of our current universe and that happened just in this micro-second after the Big Bang or it could be a completely new field but at the minute this is an area

that's very much in its infancy and it's one that covers many different ideas Roger Penner well as Martin was saying the idea of inflation was to try, and one of the attempts to try and explain why the universe has this very smooth,

on the overall scale, very smooth nature. Personally, I don't think it works at all. I mean, the idea is very popular for reasons which I don't fully understand. It might even be true of the universe, but I think the initial motivations that would put forward in favour of inflation

just don't hang together. So I'm not at all... Is that because they're breaking, what do you think, are laws of physics? It's just, the motivations don't work. I mean, they try to explain why, well, we've had the microwave background mentioned before. This is this flash of the Big Bang, it cooled down to about three degrees, absolute now. And you can look in different directions, and you see this temperature in different directions is almost exactly the same. And people found that very surprising, and so they said, well, perhaps we need a model of the universe in which those two regions of the universe were actually at one stage together. And the reason the temperature, is, is, is, that agrees so well, is because

of formalization that says they were different temperatures before and then they became equal, you know, as you have a hot object and a cold object to bring them together, they become both lukewarm, you see. But this is a manifestation of what's called the second law of thermodynamics. Now the second law of thermodynamics

is, to my way, thinking, one of the great, huge mysteries of the universe. Because what it says is that the universe, in a sense, is getting more and more random as time goes, on. Now, that's not so surprising, but if you take this backwards in time, it says the universe was getting less and less random as time goes back in time. So that means that the very start of the universe must have been very, very, very special. Now, this is a fact of the second law

of thermodynamics, that it has to have been very special. And inflation is one of the attempts that people put forward to try to explain why the universe looks as it is, starting from some view that it was in some sense random at the early stage. stages, which seems to be just quite wrong, because it can't have been random. It has to have been very, very specially organized. Now... What is... What the distinction you're making between this time that we do not know about, the pre-first microsecond, being random or being organized?

What distinction you're actually making there? Well, it has to be in the geometry of the universe. The background radiation, the microwave background, seems to be what's called a thermal state, which means you might say it's very random. in the early stages. But what was very special about the universe, and there has to have been something very special,

or we wouldn't have had a second law. What was very special was that universe was very, very uniform. And this is a great surprise when you think about it. Why was the universe so uniform? And inflation, if you like, is one attempt to try and explain why it was so uniform. It doesn't work because it's based on an assumption that the universe was in some sense more random before that.

but the universe has to have been very special in the early stages so we have to come to terms with that we have to have a theory which explains why the universe as Martin was pointing out why was it in the way that we find it was in this very early stage there is a great surprise about that Martin and I agree with Roddy Peno is we don't have such a theory there are lots of ideas and the reason we don't have a convincing theory

is essentially because we have no foothold in experiment in discussing these very extreme conditions. The early universe was so dense and so hot that we can't conceivably simulate any conditions like it here on Earth, and so we have just conjectures, and so that's why our theories are in a rather fragile state. But I think they're fascinating, and one generic feature of many ideas that people talk about now

is that perhaps we have to make a further leap in the scale that we think of when we talk about the cosmos. I mentioned at the beginning that in the 1920s, people realize that our own galaxy, our Milky Way, wasn't all there is. It's one of zillions of other galaxies, which we can now see with our telescopes. We may have to go one stage further.

We may have to envisage that what we have traditionally called our universe, the aftermath of our Big Bang, is not the whole of physical reality. There may be a lot more to it than that. and this is certainly something which is a consequence of some other ideas for explaining the present universe and the Big Bang, the idea that it's not unique.

And so just as we've got the idea that our solar system isn't unique, our galaxy isn't unique, then maybe our Big Bang isn't either, and this takes us to a still grander concept of the cosmos, but even more speculative. And I should emphasize a health warning in this part of what I'm saying, because back before the first microsecond, really have no confidence in any specific laws. We have lots of beautiful

ideas which we hope to test one day by observations, but we don't have any firm views yet. But I think it's important to say that we only make progress when we have observations. The whole of science is driven by new observations and new technologies. We're no wiser than

Aristotle was. We've made progress because astronomers have been able to use more and more sensitive instruments to detect faint light to look far back in the past and to detect other kinds of radiation. That's how we've made progress. And I think it's only going to be by probing the early universe in other ways

that would be able to settle these questions. Do you want to talk about the state of observation at the moment, Carolyn? I mean, how far is it getting, as it were? Well, I would say it's a very exciting stage for observers. You might think, listen to some of this program, that dark energy is this wonderful theoretical construct that's been invented to amused cosmologists. I want to stress that it's firmly rooted in observations.

We've got at least three separate independent lines of evidence that support this accelerating universe. Now, in terms of the observations, if we're going to try and understand the characteristics of dark energy, we're going to have to map out the acceleration of the universe when it kicked in. Is it consistent with a changing value for the dark energy, or is it what you might expect from quintessence,

or is it consistent with some constants, which you might expect from the vacuum energy or the cosmological constant. And to do this, there are currently many experiments being either planned or underway to maybe look at more supernovae or greater length scales to the universe and refine and confirm these observations. But it's an exciting time not only for astronomers, but there's an intense overlap with particle physics. When we're talking about the vacuum energy and ideas of vacuum energy,

a lot of that relies on our understanding of particle physics. and here again we can expect great changes over the next few years and things like the Large Hadron Collider come online that our understanding of particle physics will also increase in depth and we're really looking at where experiments in both these realms can come together and provide constraints for the theorists to work towards. That was very, very clear. Roger, do you think that what you're saying about the very beginning

and what Martin is calling speculation is calling for other equations, developments of, Einstein, not replacements, obviously he didn't replace Newton, he developed, but developments are nice, as it were, searching for equations, as much as searching for evidence. Well, it's undoubtedly true. I mean, I think the observational part of it is extremely important,

and as Carolyn points out, it is an exciting time. We're actually making observations, I say we, I'm not, but cosmologists are making observations of very, very detailed nature, something quite unknown in previous eras. So it's entering a phase where one can actually make very, very detailed measurements and test theory against these, and I think that's tremendously exciting.

But it doesn't, as you were suggesting, move us forward on the theoretical side, which as Martin was pointing out, we really need a theory which tells us how to deal with the beginning of the universe. And I think, yes, we absolutely need something which replaces, is Einstein's view of space time. But more importantly, as I think Einstein would very much agree with, we need something which replaces present-day quantum mechanics.

And I think that all these things have to come together in a way which will be vitally important in the Big Bang. And when such a theory is at hand, which it isn't at the moment, I think there will be experimental tests, which will tell us whether these theories are any good or not, and which will have some relationship to the, beginning of the universe. At the moment direct observations

on the very, very early stage of the universe are not available, but I hope this will change in the future. And it will be, the theories will be fed by the sort of observations that Karin was talking about. It will certainly be an important input, I think there's no doubt about that. The two great

triumph of 20th century physics were on the one hand Einstein's theory of gravity, generativity, and on the other hand, the quantum theory which tells us about the micro-world. The for this century is to link them together. In practice, we get on fairly well without that linkage, because we think about quantum theory when we talk about atoms, we think about gravity, when we talk about stars and galaxies. But when we extrapolate back right to the very beginning of the universe,

the whole universe was, as it were, the size of an atom. So plainly, we have to have a theory which consistently covers both quantum ideas and Einstein's ideas, and that's the challenge to link together the very large the very small in some new unified theory. And there are lots of variance on this. Roger Penrose has a very exciting idea. A theory called superstrings is another one. But these are all aimed at going beyond what 20th century physics did and unifying the very small and the very large.

That's the main unfitious business for physics in the 21st century. Well, thank you very much. It was, for non-physicists like myself, it was quite a ride. but you were, I thought it went well and I thought you explained it extremely well. Carolyn Crawford, Martin, Rees and Roger Pendos, thank you very much indeed. I don't know I'm going to say this,

but next week we're going to talk about angels, archangels, cherubines, seraphims, and the heavenly hosts. We really are. Thank you for listening. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programmes about history, science and philosophy at BBC.com.com.com.

Thank you.