In Our Time - The Age of the Universe

Episode Date: March 3, 2011

Melvyn Bragg and his guests discuss the age of the Universe.Since the 18th century, when scientists first realised that the Universe had existed for more than a few thousand years, cosmologists have debated its likely age. The discovery that the Universe was expanding allowed the first informed estimates of its age to be made by the great astronomer Edwin Hubble in the early decades of the twentieth century. Hubble's estimate of the rate at which the Universe is expanding, the so-called Hubble Constant, has been progressively improved. Today cosmologists have a variety of other methods for ageing the Universe, most recently the detailed measurements of cosmic microwave background radiation - the afterglow of the Big Bang - made in the last decade. And all these methods seem to agree on one thing: the Universe has existed for around 13.75 billion years.With:Martin ReesAstronomer Royal and Emeritus Professor of Cosmology and Astrophysics at the University of CambridgeCarolin CrawfordMember of the Institute of Astronomy and Fellow of Emmanuel College at the University of CambridgeCarlos FrenkDirector of the Institute for Computational Cosmology at the University of Durham.Producer: Thomas Morris.

This BBC podcast is supported by ads outside the UK. 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 programme. Hello, in 1654, the Anglican Archbishop of Armour, James Usher, published a research, published research, in which he proved that the universe had been created at 6 o'clock

on the evening of the 22nd of October 400B.C. No wonder I stumbled. The evidence for this immacutably precise date was mainly biblical and relied on the bishop's formidable knowledge of world history. By the 19th century, many scientists realized that the Earth had to be much more than a few thousand years old, and by looking deep into space, astronomers such as Edwin Hubble proved them right. Today, scientists tell us that the universe began around 13.7 billion years ago, but how did they arrive at this figure? and could the estimate yet be proved wrong.

With me to discuss the age of the universe are Martin Rees, astronomer Royal, an emeritus professor of cosmology and astrophysics at the University of Cambridge, Carolyn Crawford, a member of the Institute of Astronomy and Fellow of Emmanuel College at the University of Cambridge, and Carlos Frank, director of the Institute for Computational Cosmology at the University of Durham. Martin Rees, before we go into deep space, let's start on Earth.

When did people begin to think about the age of the universe? earth? Well, of course, in the 19th century, they realised that Bishop Aschia couldn't be right, and were really two lines of evidence that led people to think that the earth must have been much older. The first was geological evidence. People realised that it would have taken tens or even hundreds of millions of years to lay down a geological strata and to produce all the valleys, etc.

But the second line of evidence at the same time was Darwin's theory of natural selection, because he inferred that it would have taken at least 100 million years for species to evolve by that method. So there were strong arguments in the mid-19th century that the earth must have been around 100 million years old at least. Can you tell us how the geologists were arriving at that and just refreshed people about Darwin's, how Darwin's theories needed much more time?

Well, I think in both cases the actual timescales were rather uncertain. Certainly it took a long time to lay down the strata, and in fact it's rather interesting that Darwin in his book, the origin of species, made an estimate of how long it would take to evolve the wield of Kent near where he lived, and he estimated rather roughly it would take 300 million years, that's just by estimating how fast the erosion would have occurred, and that's the sort of estimate other people are making. The similar timescale he thought was needed to produce natural selection all the way from simple species up to humans. And it was rather interesting that he was really upset when just a couple of years after

he produced the original species, William Thompson, later Lord Kelvin, the greatest visit of his time, made an estimate of how old the sun was. And this, of course, is a separate problem because clearly the Earth can't be older than the sun. And of course they didn't anything about nuclear energy in those days. And Kelvin thought that the only way the sun could derive energy was from gravity. He thought the sun was gradually shrinking under gravity and being squeezed, and that provided extra heat that leaked out. And that process he could calculate would keep the sun shining for a few million years.

And this was a very good argument, and Darwin actually took it very seriously to the extent that in the third edition of his origin, he cut out the reference to the wheel of Kent. but he was very puzzled indeed, and that discrepancy remained until the 20th century when people realised that stars were fuelled by nuclear power, which of course allowed them to keep shining for billions of years. But towards the end of the 19th century, just to sort of sum up then, we're talking about some physicists thinking about 75 million years using the sun, Darwin thinking, well, the wield of Kent must have been at least 300 million years. This is a small dispute in the corner of knowledge, is it?

It's not sort of inflaming the population. People are not beset by the desire to know the age of the earth. Well, of course, the colossal age of the earth was culturally important compared to what had been believed by many people in the 17th and 18th century. But, of course, we're still talking now about the age of the Earth and the Sun. We're not talking about the universe, because remember that the universe, in the sense we understand it today, of billions of galaxies, each with billions of stars in them,

was only appreciated in the 1930s. So when people talked about the age of the cosmos, they were really thinking about the age of the Earth and the age of the sun. Colin Crawford, Martin Rees mentioned radioactivity. Can you tell us when that came into play and how it altered things? Well, this really started at the beginning of last century when radioactivity was discovered. And just to very briefly divert about what we're talking about here,

If you have certain elements, they're radioactive, they're unstable, they've got too many subatomic particles, too many protons, neutrons in the nucleus, and they spit these out until they evolve to become a stable element. In doing so, they release energy, they release heat. And there was a brief idea by Ernest Rutherford in 1904 that this could account for the heat of the sun. But again, it was found not to be an efficient way of reducing the heat of the sun. It was also of interest because this process is going on in the rocks in the earth. And suddenly it provided a means to date rocks in the earth. So the time that it takes an unstable element to transmute to a stable element happens and that it's spitting out these particles at a characteristic rate that you can measure in the lab.

And so if you have a rock sample, you can compare the proportions of the radioactive element, to the stable element. You know how long it takes to change. We call it a half-life, half of a mass of the material to change to the stable element. So if you measure the proportions you see in the rock, you can see how long it will have taken you to produce the stable element from the radioactive element. The key thing is that there are some elements and some what we call isotopes, versions of uranium, for example, that decay down to lead over a period of four and a half billion years. They start sampling enormous times. timescales. And so, radioactivity started to provide a way to date the rocks on Earth. Where were these experiments taking place? And were they, had they arrived? By what date did they arrive at that figure, four and a half billion, which is a big change? Oh, no, we haven't got to four. I'm just saying this particular element decays on these enormous time scales. In practice, using this on the rocks on earth, it was a lot more complicated because even

the radioactivity was discovered at the start of the century. Who's discovering it, though? Where is it happening? In this country, in America, all over the, but simultaneously? It's European discoveries, but the application to geology is happening in Britain. And one of the pioneers of this was Arthur Holmes. Because even though it seemed like a very simple way to date rock samples, in practice it's very complicated, because it's not just one element turns into another. There's a whole decay chain in each of these processes have their own half-lives. and you've also got to understand what the original proportion of those elements were in the rock. And that discouraged a lot of geologists from using this technique. But the British geologist Arthur Holmes, by the 1920s, he had sampled rocks from all over the world,

and he was coming down to timescales of perhaps one and a half to three billion, so that's 100,000 million years for the rocks on Earth. We've talked about the Earth, but so far a little on that, but move to the sun, how did this affect the ageing of the sun? Well, the radioactivity doesn't immediately affect the age of the sun, though it doesn't provide an important clue later. In terms of the age of the sun, this dilemma, this contradiction between, you know, the time scales necessary both the natural selection on Earth

and also with the radioactive dating of the rocks. And the age of the sun were only resolved really, again, in the 1920s. and here the crucial point was the Einstein's equation, which we use, we're all familiar with, of E equals MC squared. So the sun can't just be burning, chemical burning, as we understand it, because there isn't enough mass there. It doesn't, you don't get enough energy for mass that you burn

and it would only last on really short time scales of tens of thousands of years. You need another very similar process where you get a lot more energy out for a little, bit of mass. And it was in the 1920s that Arthur Eddington was realizing that the temperatures inside the sun, right in the core of the sun, were hot enough that the atoms there were what we call ionized. You've got the protons, the electrons, are all independent of each other. And in those conditions, they can collide with each other's at high energy, and you can start changing hydrogen protons into helium. And as you do this, there's a sort of net change in

mass. Using Einstein's equal MC squared, that tiny change in mass, the C here is the speed of light, which is an enormous number. A tiny change in mass transmutes to an enormous change in energy. So we have this process of nuclear fusion, and it's much more efficient at churning the mass of the sun that we see there into the energy that it radiates. And so at this point, it was realized the sun could exist for billions of years and could be compatible with the air. of the Earth that was being obtained from the radioactive dating. So that was nuclear fusion in the sun. I just very briefly want to go back to radioactive dating

because that's how we still get the four and a half billion-year age for the solar system. And the rocks on the Earth turn out not to be good samples. There have been geological processes like volcanism, tectonic plate movements. The samples are all mixed up. If you apply the same techniques to meteorites, These are lumps of space, rock, debris left over from the formation of the solar system.

These are much more pristine samples of the early universe. When we apply radioactive dating to those meteorites, again, this is the 1950s and work done by the American Claire Patterson, that's when these meteorites start to come to very consistent values of 4.5 billion years old. And that puts a very good handle on the age, just of the Earth, all the rest of the planets and the Sun itself. Carlos, Frank, we talked about the Earth, we talked about the Sun, we talked about fusion, radioactivity and fusion.

Now to the universe as a whole, what implications, when did this begin to have an application to the universe as a whole? Well, the question of how old is the universe really goes back to the beginning of physics, of modern physics, as we understand it. So, for example, that's a question that already preoccupied Newton. Now, Newton had a very neat and clear and unfortunately very incorrect answer. So Newton's universe was a lot simpler than the universe in which we now know we live because Newton's universe was infinite, it was static, and it was eternal. It'd been there forever.

Now, but it's actually quite interesting to understand how Newton arrived at this conclusion. I mean, in part, it was a legacy really going all the way back to Aristotle, but Newton actually reasoned as a scientist should rather than a philosopher. Now, Newton had in the 17th century discovered the law of gravitation, whereby matter attracts other matter, and he used this to conclude that the universe had to be infinite. And the way he did that is really an example of the impeccable logic of science. So Newton argued like this.

He said, let's imagine that the universe is finite. Then if it's finite, it would contain a finite number of stars, and each of these stars would add as a center of attraction. for matter around it. So matter would flow to these stars. Eventually the stars themselves would begin to attract each other and so all the matter in the universe

would pile up in some great big lump of matter. Newton said this is patently not the way the universe is and therefore he concluded that the universe could not be finite. The universe had to be infinite. Now then in an infinite universe he reasoned stars would move

because the gravitational forces from all other stars would cancel out. And so the universe, if it was infinite, had to be static, and an infinite static universe, naturally, the most natural interpretation is that it's been there forever on changing. It's eternal.

So that was Newton's solution to the problem, and it was a solution to which he arrived on the basis of empirical and theoretical evidence that he had at the time. As I understand it, more or less, that held until the early 20th century,

until 1915, Einstein, his general theory of relativity. Exactly, yes. Which changed it in what way? Well, so Einstein in 1915, like Newton, still thought the universe was static because there was no evidence against that. In 1915, the galaxies had yet to be discovered, let alone measured to be moving away from each other in what cosmology is now called the expansion of the universe.

So as far as Einstein was concerned, the universe was static. Now, this is again a very interesting development because when Einstein looked at the equations of general relativity, when he had a first goal at these equations, he had gravity in them, and he recognized that gravity doesn't like things to be static because gravity attracts material and attracts matter. And so in order to have a static,

universe, he changed his equations, introduced a repulsive force that would balance gravity in order to have this very finely tuned, finely adjusted universe where gravity was compensated for by this repulsive force, which he called the cosmological constant. And this fine balance would then enable the universe to be static. But very soon after that, a... Dutch theoretical astronomer, Wilhelm de Sitter, immediately realized that this really couldn't hold, that that universe was not stable and that it will soon start contracting or expanding.

And soon after, two mathematicians, one of them actually also a clergyman, George Lemitre, a Belgian mathematician clergyman, and a Russian mathematician Alexander Friedman, actually sold Einstein's equations. and found solutions in which the universe would be expanding or contracting, and of course they didn't know. They had no evidence one way or the other,

but they informed Einstein about this. And Einstein reacted with great eagerness when he recognized that there had to be something more to the universe. Of course, the universe is not static than the question of how old is the universe becomes a very fundamental central question. It's only when the universe is static that perhaps you don't need to worry about the age of the universe. You can naturally assume it's been there forever. but when you know the universe is evolving,

then the question of how old is the universe becomes a very central question. And Martin Rees, we learned a lot more about this when in the 1920s with the American astronomer Edwin Hubble and his increasingly magnificent telescopes. Yes, but he used the 100-inch telescope on Mount Wilson and he found that the galaxies seem to be moving away from each other.

You can infer this because when you take the spectrum of the lights, you find it shifted towards the red, and this is analogous to the way a sound appears deeper in pitch if the source is moving away. And so by this method, he was able to show that galaxies are systematically moving away from us. The further away they are from us, the faster they're moving. And this was the first evidence of the expanding universe, and as Carlos Frank indicated, this is consistent with the ideas

which to sita and Friedman had shown them the possible consequences of Einstein's theory. So the expanding universe really stemmed from the work of Hubble. He was the first person who in the 1930s claimed evidence for this systematic expansion. Now, he can measure the speeds of galaxies away from us. What's more difficult is to estimate how far away they are, because roughly speaking, the age of the universe, the time since the Big Bang or since everything was crowded together, is got by inferring how far aware galaxy is

and dividing that by its speed. We know the speed, but what is much harder is to measure the distance. And Hubble's first estimate of the distance of these galaxies led him to infer that the age of the universe was about 2 billion years. That was a time since the Big Bang, if the galaxies had moved apart at their present speed. And that, of course, was already a rather low, speed because the age of the earth, as Carolyn has said, was already at that time believed to be at least two billion years.

As it turned out, Hubble had underestimated the distance of galaxies and a succession of corrections eventually increased the distance by about a factor of four or five. and for the last 50 years or so the crude estimate of the age of the universe got by dividing the distance of galaxies by their speed has been in the range of between 10 and 20 billion years. It's now firmed up as we'll hear to about 13.7 but for 50 years or so we've known that the characterously time scale for the universe to evolve

is about 10 billion years or more. Karen Crawford, can you tell us about Hubble's constant in all this, something called Hubble's Constant? Well, Hubble undertook these observations of the distance and the speed with which galaxies were receding from us over a sample of galaxies. And if you plot the speed against the distance to the galaxy, you find that you get what's called a linear relationship. It fits a straight line. Now, the way that, I mean, this means that the velocity is related to the distance just by a constant multiplier, and this is known as the Hubble constant. And this plot of velocity against distance, this is Hubble's law.

And as Martin says, it's the first clear evidence that the universe is expanding. Space between the galaxies is expanding. So you have this fundamental characteristic of this expansion called the Hubble constant. And that's really what all through the 20th century people were trying to refine the exact number of the rate of change of velocity with distance. and again just picking up Martin's point if you see a galaxy moving away from you at a velocity V you see its distance is D you can work at how long it took to get there so just from this plot this sample

you can use this Hubble constant to infer the age since all the galaxies were compressed back at some inconceivably high moment that is the big bang the point of the unit the starting point of the universe so this Hubble's law this whole way that the galaxies move apart, their distances, gives you immediately a measure of the age of the universe. Carlos, Frank, can you take that on a bit?

Talk about the way the galaxies were brought into these equations because for a long time they weren't thought, they weren't seen? They were thought to be part of the one thing, weren't they? Oh, yes. So originally, the astronomers had known for quite a while that when they looked out in the sky, they could see not just stars, which are essentially point sources,

but diffuse material, which they call the nebulae. And for a long time, there was a debate as to whether these nebulae would use gas clouds inside our own Milky Way or whether there were actually external objects that were essentially worlds like our own, all the galaxies, external galaxies, similar to our own.

And it all hinged on the question of how far these clouds were because there were already some ideas as to how big, the Milky Way was, and the question then became, if these clouds are sufficiently close, this nebula, they will just be perhaps a component of our Milky Way, gas clouds where new stars are born, whereas if they're very distant,

then they could be worlds in their own right, galaxies, like the Milky Way. So the debate, actually, was just a question of how distant are these gas clouds? And there was arguments both ways, and eventually the question was settled when distances were obtained for these clouds in the 1920s. And Hobble eventually recognized that they were, in fact, nebulae were in fact all the galaxies. And then he then proceeded,

he wrote a wonderful book called The Realm of the Nebula where it was clearly stated that these were other worlds with maybe billions of stars like our own Milky Way. and once that had been established, then the doors, the gateway was open to understanding the universe because one could now go and perform measurements on these galaxies, and that is precisely how the expansion of the universe was discovered. So, we first required establishing the existence of these other universes and then, or these other galaxies, sorry, and then measuring their properties

and in particular their recession velocities and their distances. But before it went on in a straight line, there was a halt, wasn't there Martin Rees, with the theory being proffered by Fred Hoyle, a very eminent insist indeed, of the steady state theory. Well, that's right, and it's partly motivated by the contradiction, apparently, between the time since a big bang,

if there was a big bang, and the age of the oldest things known in the universe, which was a real concern in the 1940s when the scale of universe was not correctly determined by Hubble's work. And Hoyle and his collaboration, as Bondi and gold had a rather ingenious idea. They said perhaps the universe has existed as it were from everlasting to everlasting, and it always looked the same,

because even though the galaxy were moving away, new ones formed, as it were, in the gaps between them. So their idea was that the universe had always been the same, it was indeed expanding, but new materials had been created, new galaxies created, so it had always looked the same. And that got round the problem which, they believed to be serious regarding the ages.

Now, the big development in 1960s was to be able to test whether the steady state theory was correct or whether there was a big bang. Did they bring, they must have done, what sort of evidence did they bring to bear for this theory, Fred Hoyle and his colleagues? Well, I mean, it was no evidence. They just claimed it got round a problem

which the other theory had at the time because of the underestimate of the time since the Big Bang. But it was only in the 1960s, really, that it became possible to test cosmological theories. And that's because if you look at very distant objects, you see them as they were a long time ago. And therefore, in principle, if you can look far enough away into space, you can see if the universe was the same,

five billion years ago, say, as it is today. Until the 1950s, no one could look that far away. But in the 1960s, they were able to do this, and then they found two things. They found first that there was evidence that the expansion speed was different in the past for what it is now. And they also found that when you look at galaxies a long way away,

they don't look the same on average as galaxies now. In a state-state theory, of course, the population would look the same at any time. But in the Big Bang theory, we'd expect that if we look a long way away, we see galaxies as they were when they were younger, and they would look different. They would have different kinds of stuff. in them, they might be more explosive, etc.

And evidence came in the 1960s that the universe was indeed different in the past. Galaxies looked different, they were more likely to have explosive activity in their centres, and they had more gas in them which hadn't yet turned into stars. Exactly what you would expect if we were seeing an evolving system. Carlos Frank, he died quite young, Edmund Hubble, but his work was taken up by many people,

especially Alan Sandidge, and he went on to prove the validity of Hubble's approach. Can you tell the listener something about how he did that? Yes. So in fact, Hobel died, as you say, suddenly. He died of a heart attack, and it's a great loss to science, but in a perverse way it was great benefit to Sandwich, because Sandidge, who was Hubble's observing assistant, inherited not only all of Hubble's data, but he also inherited Hubble's observing time at the 200-meter.

Hale Telescope in California. So you rented your observing California. Well, he just inherited from Hobo, but he actually Sandich really took Hobble's work to

expanded it in a big way. It was the same program that Hobel had initiated and the program was really to try and measure distances as accurately as possible for as large a number of galaxies as possible. So as Mark

in Rees indicated earlier, measuring distances is possibly the hardest problem in astronomy, because when you see a light in the dark, you have no way of telling whether this is a very bright light that's at a large distance or a much dimmer light that is close to you. You have no way to tell. They could look the same. And the same is true in astronomy. When you see a galaxy, you have no idea whether it is a very bright galaxy at a large distance or a dimmer galaxy close. a buy. So finding distances, measuring distances is really the hardest problem in astronomy

and it is one that Hubble began to tackle and then Sandwich took much further. The key really is if somebody divulges to you the vital information of how intrinsically bright this light is, then you're

in business. So this is for example why lighthouses are so useful for sailors. A lighthouse would be useless unless the captain of the ship knows how bright it is and then he can tell from how dim or bright it looks how far away the lighthouse is and of course if it's foggy then this doesn't work as we know but so so the point was finding in the universe some object whose luminosity could be established from all the grounds intrinsic brightness could be known and then you could proceed from there to measure distances

and what hobble began and sandwich took further was to find what we call stander that candles in astronomy, objects of non-luminoosity. It turns out that there are a particular kind of stars called seafid stars that pulsate, and they change in brightness. And physicists who understand the structure of stars can relate these pulsations to the intrinsic brightness of these so-called seafid variables.

Hobel began to use those. Sandwich understood them better, and then use this as a way to bootstrap his way to further and to measuring distances for further and further galaxies. So essentially Sandwich built up what we now referred to as the distance ladder, which is a procedure

for bootstrapping, beginning with the C-Feed variables, calibrating other standard candles so that in the end one is enabled to measure distances to very distant galaxies. And that's what Sandich did. He did it meticulous, it's really fantastic work

in detail and the understanding of all the systematics. It's very detailed very profound and very impressive work. Now Sandwich, through this method, measured the hoboconstant, and found the hoboconstant in the units that astronomers used of 50, which implied an age for the universe of somewhere between 15 and 20 billion years. So that was Sandidge's life program, a very successful program. Carolyn Crawford, Galaxy is receding, the only evidence.

What does star clusters tell us about dating? Well, yeah, this is interesting because the age of the universe wasn't just a problem for cosmologists. There are people trying to find the edge of the universe by much more lateral techniques, and the principle that the universe has to be as old as the oldest things you find in it. And so one very old kind of system

is something called a globular star cluster. So these are large balls of stars, maybe millions, hundreds of thousands, of stars. And they're thought to predate a lot of other stars. If we look at their composition, they don't have many chemicals, we think these are some of the earliest stars. Now, if you can work at how old those stars are, it gives you a lower limit to the age of the universe. And this, again, was an intense subject of observation for much of the latter part of the 20th century. And how you get the age from a star cluster is quite ingenious, because

you can only observe the light from the star and the nature of that light so we might observe its brightness and its colour which are related to physical properties like its luminosity in temperature. Now from our theory of how stars behave, how they change, they evolve, we know that the luminosity and the temperature of a star depends on its age, its composition and its mass. Now if you have a large star cluster, you can assume that all those stars formed at the same time

from the same cloud. So there's no variation in composition or age. The stars only differ from each other in terms of their mass. So all you do is you look at this star cluster, you look at the luminosity and temperature of the stars, and you find which is the most massive star present. And the idea being that any more massive stars have evolved away from this main pattern of nuclear fusion, hydrogen, helium. And from that mass, the most massive star present, you can work out the lifetime and therefore get a lower limit to the age of the cluster. So if you look at a star cluster, and you can say the most massive star there is like 10 million times the mass of our sun, you know, well, the age of those stars is perhaps a couple of tens of millions of years,

and that could be an age for the cluster. However, many of these star clusters, the most massive stars are much less massive than the sun, and you would get ages of tens, if not 20 billion years. Martin, we have to sort of get to where I'd like to get, which is the future. Can we just briefly talk about the cosmic microwave background radiation? I'm reading that very carefully, in the mid-60s, came upon you.

Yes. Well, I think Everton's, Hubble's work, then there was an inference that there was some epoch in the past when everything was squeezed into a hot, dense state at the beginning. But it was in 1965 that radio storm was discovered

that even into galactic space isn't completely cold. It's warmed by weak microwaves. And those weak microwaves are, believed, on very good grounds, to be a relic of the hot, dense radiation in the early universe, which has expanded,

cool and diluted, but it's still around. So since the mid-60s, it's been fairly good evidence. We have in the universe a very good relic of a time when the entire universe was squeezed hotter than the centre of a star. And that epoch, by carefully studying the radiation, can be dated.

And so we now not only know a good deal about what the universe was like early on, but by rather complicated methods involving studying how the radiation varies across the sky, we can have a quite independent way of working out the time since the Big Bang. And it's about, by this method, 13.7 billion years. And this actually agrees with the time that we infer from the Hubble relation. There's a slight gloss which you haven't yet mentioned, which is that the Hubble law obviously tells us the time since the Big Bang

if the speeds of the galaxy have been constant. We now know that's not the case. We know rather oddly that the expansion of the universe was slowing down for the first five billion years, and then it started speeding up. But the net result is that the average speed is about the same as the present speed, so the Hubble argument gives us about the same age for the universe. And going back to what Carolyn Crawford was saying, the oldest stars are about 12 billion years.

And so we now really have a sort of time chart for what happened in the universe. It started about 3.7 billion years ago. It took 2 or 300 million years before the first stars formed. Those first stars formed in systems rather smaller than present-day galaxies, and it took a few billion years before galaxies like our own had built up. And then, of course, it took a few billion years further for our solar system to form because there's 9 billion years between when the Big Bang occurred, and when a solar system formed.

And we need that time to explain everything because all the materials that we are made of, carbon, oxygen, etc., which make up us and the Earth, were synthesized inside stars, which lived and died before the solar system formed. So there must be many generations of stars, which formed in the few billion years after our galaxy, formed and before the sun did.

Now, you might think it's hard to have so many generations of stars, but you're back to what Carolyn was saying, big stars burn their energy up much more rapidly. It takes 10 billion years for the sun to use up its fuel, but a star 10 times as heavy as the sun is a thousand times brighter, and therefore it lasts much less long. So there are lots of action before our sun formed, and all the atoms that we're made of owe their origin

to a star that lived and died before. that. So that's the way in which we understand the present as this chain of events starting with atoms in the early universe, the first stars, the first galaxies, then building up all the period table and then our sun. But as to what's going to happen in the future, well, of course, long-range forecasts are never very reliable, but the best bet is that the universe will go and expanding forever, and that it will become ever colder and ever emptier. And certainly the future is much longer than the past. I think this is a very important thing we learn, Can I wait for that question for a couple of minutes, Martin?

I just would like to... We've got a bit of time, enough time, to talk about cosmic inflation theory in the 1980s, which Karlas is going to give us in a few, in a short paragraph or two. We're not accused of missing it by all the budding astrophysicists listening in 10. 10 to minus 36 seconds. I cannot describe inflation in a short period as the process took, which... So inflation is a process that we think now. and we have evidence for it,

occurred when the universe was very, very young indeed, about 10 to the minus 35 seconds, and it's a decimal point, 34 seconds, and then it won that fraction of a second. And the universe, we believe, underwent a very traumatic episode, where it expanded very, very rapidly in a very brief period of time.

It increases in size from something much smaller than the size of a proton to an object that you could hold in your hand, a marble or something. You wouldn't want to hold the universe in your hand because it's rather heavy, but the universe went through this brief period of inflation. And the reason it did that, it has to do with particle physics. We know from particle physics theories that the very high energies that prevailed close to the Big Bang, gravity, the forms of matter appeared that turned gravity on its head. Instead of being attractive, they were repulsive. And it was this force,

is this energy, repulsive energy, that caused the universe to expand. And in this very rapid way, eventually this repulsive energy, this sort of anti-gravity-type force, decayed away, and the result of that was the production of the particles that make our universe today. I think we have to go back to the future, Martin, because we... Can we bring in the idea of the Big Bang

a phrase first uttered in this building by Fred Hoyle as a pejorative term? and the dark energy which is lurking around. Let's concentrate now on what you think is happening and will therefore happen. We'll go around the table with that. I interrupted you. I wanted to get that in from Carlos and then away we go.

Yes. Well, it does now seem that the expansion of the universe is actually speeding up. So if we were to stay around for the far future, then, of course, the first thing would happen is that after five or six billion years, the sun would die. It reminds me of a nice anecdote for military to all astronomy lecturers. One was asked by a student,

did you mean to say the sun would flare up and burn us to a crisp in six billion years? The lecturer said yes, and the student replies, that's a relief. I thought you said six million years. So there's plenty of time, even in the sun's lifetime,

for evolution to go as far from human beings as has come already. We know about Darwin and what's happened on this Earth over the last four billion years. No astronomers could believe that we're the end of evolution because the time ahead even for the Earth is going to be several billion years. So post-human evolution will be as prolonged as the evolution has led to us. But going to the further away, the stars will eventually all die and the galaxies will all recede from us

and the universe in the far future will be very cold, very dark, and very empty. The remnants of our galaxy and its few neighbours will be all that you could in principle see if you were around at that time. That's a long range forecast, but let me put in the health warning that long range forecast should not be taken too seriously. Well, there's a beautiful bleak ending to the programme, but we have more time. So, Carolyn, can you say what you would like to say on this?

Well, again, just returning to the age of the universe, taking from what Martin said, we're trying to work out what could be the eventual age of the universe, and all of That pins down the nature of this dark energy. And I would say a lot of the cosmological research at the minute is, we now know that the expansion of the universe is accelerating. We now need to see what the dark energy is causing that. And one way is to look at the rate of change of the acceleration of the expansion of the universe. And so the next step for cosmologists is to perhaps tie in the observations of the rate of change of the acceleration in the universe

with the possible causes for dark energy. and that has large implications for the eventual fate of our universe. And finally, Carlos. Well, let me just try to be precise at what is it that we've been talking about this program. 13.7 billion years refers to the age of the universe as we know it. But what we don't know, it was there before the Big Bang. And for all we know, there is a much bigger universe, which as far as we know, could have existed for a lot longer.

So today we know very precisely the age of our local parts of the universe, but we must be aware of the fact that there's still many open questions in cosmology and one of the biggest one is what was there before the Big Bang and how long did that face last if it existed at all? Well, thank you very much for a very challenging ending there. Thank you very much to Carlos Frank, Martin Rees and Carolyn Crawford. Next week it's our 500th program and we'll be talking about free will. If you've enjoyed this Radio 4 podcast, why not try others such as Thinking Aloud,

where Laurie Taylor discusses the latest social science research. To find out more, visit BBC.com.com.uk forward slash radio four.