The once and future universe

John D Barrow

Today I’d like to tell you about modern scientific ideas about various possible future scenarios for the universe on the larger scale and also more locally on the scale of the earth and the solar system.

This question, although it figures quite largely in lots of the world’s great religions, became a matter of serious scientific interest very early in the 19th century and the reason for that was the Industrial Revolution focused people’s attention upon the workings of machines and the efficiency of machines and the extent to which energy was utilisable. There grew out of that fascination with machines a number of the cornerstones of modern classical physics. In particular, in 1947, Hermann von Helmholtz, German physicist, first wrote down what we now call the ‘Conservation of Energy’.

300 years ago, when Newton developed his Laws of Motion and Gravitation; the concept of the conservation of energy was not used by Newton, it doesn’t figure in his analysis. He was able to sidestep it, but Helmholtz realised that there was a quantity called energy which had different forms: energy of motion, potential energy, electrical energy and so forth, and the total energy budget in any process which took place in the universe could not be changed.

However, the conservation of energy allows, on its own, all sorts of odd things to happen. It allows you to build a perpetual motion machine. If you want to build a perpetual motion machine, you have to introduce the concept that there are different quantities of energy and energy becomes less and less ordered as work is done using that energy. This led Clausius to formulate the Concept of Entropy, a measure of the degree of disorder in energy, and the so called SSecond Law of Thermodynamics which tells us that the quantity of entropy in a closed system never decreases. It can stay constant if the system is in equilibrium, in disequilibrium the amount of entropy will increase. So, the Conservation of Energy is telling you, in some sense, that you can’t win and this is a reinforcement of that stricture.

Lord Kelvin, William Thomson, in Glasgow, produced a more explicit formulation of this idea when he focused upon what he called ‘The Universal Tendency in Nature for the Dissipation of Mechanical Energy’ and he also discussed the consequences of that for the universe as a whole.

Helmholtz returned to the question in 1954 and pointed out that the consequence for the universe as a whole is that really there is only a finite future in the sense that there will come a time when things stop happening in the universe. Time may go on forever but things will only happen, there will only be dissipation of energy, there will only be increasing entropy, he argued, for a finite future of time.

He said, as an even weirder consequence, that if you’re in a closed, finite universe eventually everything must recur with only a finite number of states to be visited and, even if you believed this you might think that on some very, very long time scale everything would come back and recur. Most of these statements are incorrect in the light of what we now know about the universe. Remember, in the 1800’s this was before we discovered that the universe is expanding. You don’t notice any more of the universe than the collection of stars that we locally call our galaxy.

What was behind the thinking of people like Helmholtz and Kelvin was the concept of what I call a ‘steel universe’ as it’s motivated by using industrial applications. But, this measure of the amount of disorder in the universe against time was required to be always increasing. And, so, if you draw a trajectory of the increase of the entropy with time, there are two implications of this picture that philosophers and theologians and physicists used to wonder about.

One was the suggestion to some people that if something was always increasing then maybe it was going to approach some maximum, some high as possible value, or at least a value so high, that nothing could survive, no living system could survive with that degree of disorder in the universe. That became known as ‘The Heat Death of the Universe’.

The other curiosity about this was that if you drew a naive increasing picture like this, it suggested to many people that there must have been some type of beginning of maximal order, a state where there is zero entropy and some people like to call that the beginning of the universe.

But after the discovery that the universe is expanding in the late 1920’s, there was then a rush of very speculative writings about the far, far future of the expanding universe and they are associated with a number of quite famous names. A book was written called The Last Another book which tried to explore the way in which life could reformulate itself. If there were a form of life, not like our own flesh and blood, a way of storing information and being alive, it could survive the strictures of the far, far future when all the stars were dying and the universe was becoming more and more disordered.

The most famous of the authors, Eddington, once wrote an article for the Mathematical Association annual meeting called The End of The World from the Standpoint of Mathematical Physics, Eddington had a wonderful way of characterising the end of the world. Radio waves were of much interest at that time. Radio broadcasts were made; he called them ‘Hertzian Waves’, and he said as the universe expanded forever. One of the features of that ever increasing expansion would be that all radio waves and wavelengths of radiation would be stretched in ever longer wave lengths so they would become, as he said, ‘like a great broadcast’. This was his picture of the end of the universe, everything shifting to longer and longer radio wavelengths, a great broadcast; episodes of ‘Neighbours’ and ‘News at 10’ distributed all around the universe.

Here’s a picture of some measure of the separation between distant clusters of galaxies in the universe and time in billions of years and this shows what we now call the ‘Expansion of the Universe’, the fact that these distant clusters are receding from one and other at a rate that increases with time. We would live around about 13.7 or so billion years after apparent beginning.

The impact of this picture and the discoveries that led to it is that we live in a universe that is constantly changing. In the past it was much denser, it was much hotter; in the future it will be cooler and more rarefied. If we go back to when the universe was about 300,000 years old, it is 1000x more contracted in size, the average temperature was more than 3000 degrees and there are no atoms and no molecules. The universe is not inhabitable by people like ourselves, things that are consequences of chemical complexity.

Then there is a historical sequence of events: the first atoms and molecules form and then galaxies, stars, planets and ultimately, it appears, people. But in the future the long range forecast looks rather bleak. The stars, like the sun, will eventually exhaust their nuclear fuel; they will first of all swell up rather dramatically, vaporise lots of the inner planets in the solar system and then finally contract dramatically to become little larger than the size of the earth. This will be the fate of all stars.

This heat death scenario resonated so much with the pessimistic philosophers of the 1920’s. Dean Inge here in London who was called ‘Gloomy Dean’, was a great theological speculator about the gloom of the end of the world. Even if you didn’t like the heat death of eternal expansion, there is an even worse heat death waiting for you if the universe was of this more claustrophobic form. The alternative to expanding forever was eventually recontracting to a big crunch of apparently infinite temperature and density. So, either way, the long range forecast didn’t look very optimistic.

Now, first of all, we can just clear up some of these options a little. The picture that people like Eddington and Helmholtz were drawing of the consequences of increasing entropy was not really logically correct. First of all it is quite possible for the entropy to be always increasing yet never to have been zero in the past. It would extend all the way back to minus infinity. It’s always increasing, even though it never really had a zero value. We could have a simple scenario, where it did increase and kept on increasing forever. On the other hand, it can keep on increasing forever yet become bonded in the future. It is possible for a graph to be always increasing but not to reach arbitrarily high values. In this universe, the level of disorder is always increasing. In this case it’s increasing but it’s bonded above; there is a maximum possible value.

It has also been speculated periodically that something very strange might go on in our universe, particularly the universe that could collapse to a big crunch in the future. Maybe the direction of entropy would change when the universe started to contract and entropy would start to decrease again. Of course, I don’t believe that’s an idea you should take seriously. This should no more happen when the universe starts to contract than it should happen when a galaxy forms and starts to contract or a star forms and starts to contract. Different places in the universe would have a different turn around time for the expansion and you don’t expect the entropy of the universe locally to suddenly be able to know that the universe has started recontracting a 100 billion light years away, so this is not taken as a serious possibility.

Therefore, as an eschatologist, you have to worry about all sorts of different ways that things could come to a sticky end and it’s always good to organise things so I’ve tried to organise ends into 4 sorts. The first is local, the second is gradual, and the third is sudden and the last is temporary.

What I mean by local ends is, the things that might happen on this planet, which might even happen tomorrow. If the present moment when we’re looking at things and we’re looking at planets and we’re looking at life in the universe is random, there’s a simple theorem of statistics that, with 95% confidence, everything that we observe now if it is, indeed, random, will survive between 1/39th and 39 times its present age. This sounds strange but it’s just the application of the normal distribution to events. It’s a consequence of assuming that the present moment is typical and random.

What we would expect if we are around at a typical time is that the structure at Stonehenge has got a 95% probability of being around for between 102,000 and about 156,000 years.

The reason why this type of analysis became very interesting about 10 or so years ago is that an astronomer used it to make a prediction that the Berlin Wall would fall within a few years and he was completely correct although, of course, every politician, economist and diplomat in the world was taken completely by surprise.

Human beings have been around for about 200,000 years. On this statistical picture we’ve really only got about 5,000 to 7.8 million years to go. So, if you’re looking at life on earth from a cosmic perspective it would not be statistically peculiar if we were annihilated in 5,000 or 6,000 or 10,000 years’ time. It would be entirely understandable. If we look at Microsoft, you can see the lower bounds that there’s only 7 months or so that it might survive. The United States also, about 5 or 6 years and less than about 8,000 years. I put Oxford University at the end, 19 years to 29,000. And, also on the way down, with my calculator, I couldn’t help but put in Gresham College and it was founded in 1597 it says on the front of the lecture brochure so you’ve got between 10.4 years and 15,873 with 95% probability.

Of course it’s all very well thinking that humanity might not be around for more than about 5,000 or 6,000 years but what would be the mechanisms, what could be the causes of their demise? Well, the more you think about it, the more likely our demise seems. There are 2 ways in which life could come to an end on earth. The first is due to internal forces. We know the possibility of catastrophic war or pollution or disease or nuclear accidents or perhaps climatic change induced by human activity. We’re well aware that all these things could produce disastrous accidents. On the other hand, we’ve come to appreciate that external forces, in the long run, are likely to be much more dramatic and disastrous. We’ve come to appreciate that impacts with the earth of asteroids and comets, some of which may not be visible with any of our present telescopes because they are rather dark, outbursts of gamma radiation, super novae nearby. In 1987 there was a super nova in the Large Magellanic Cloud, a dwarf galaxy close to our own. If we had a super nova relatively locally to us here in our galaxy, this could be catastrophic to us.

This problem of impacts has become significant enough for the House of Commons to have a standing committee of astronomers who advise on what measures might be taken to try to minimise or counter threats by approaching comets or asteroids. That’s one of the reasons that missions to try to land space craft on asteroids or cometry cores have become of interest. It’s very important to know what an asteroid is like on the inside, what it’s made of, before you start trying to deflect it or hit it with explosives in the outer regions of the solar system. If it’s a simply big rock and we hit it very hard, it might break in half and the two pieces go their separate ways and miss us. But, suppose it’s like a piece of pumice, a collection of millions of rocks all joined together rather loosely. You hit it rather hard and you now have a million objects coming towards you instead of one and you’ve created an even bigger disaster than you were trying to avert.

Objects which are just a meter across are small enough that they will evaporate and vaporise; they don’t make it to the earth’s surface. They may leave lots of dust around but they won’t make a crater on the ground. Objects which are 10 metres across create impacts that start to affect regions the size of countries, or the size of continents. They then devastate agriculture, cities, and civilisations on the scale of the continent. A crater of 100 kilometres across creates enough dust, enough devastation locally which is thrown up into the atmosphere to create a catastrophe for almost all living things on the earth.

This is no real worry. But if you’re a statistician you can choose a calculation to show that each of you is more likely to die from an asteroidal impact than you are likely to die from a plane crash. That’s because of the enormous consequences of a large impact outweighs the fact that you take many aircraft flights, but not many asteroids hit us.

This problem of asteroidal impacts, impacts as it were, on another aspect of astronomy. Why we don’t see any extraterrestrials writing to us, sending us signals, giving away their presence? Where are they all? Statistically there ought to be many civilisations perhaps in our galaxy and beyond. Why do we not see any signals from them? People have thought of different reasons for this. It could be they don’t want to contact us because we’re too boring, that there are hundreds of thousands of civilisations just like us all over the galaxy. We’re like another species of uninteresting beetle so everything that we do has been seen before and we’re not contacted. On the other hand, it might just be because we’re too special, we’re too interesting and so we’re treated like a game reserve; we’re not interfered with, we’re just observed because we’re so fascinating and what we do is so unique and unusual that it mustn’t be disturbed.

It could be that we’re actually too primitive and that there are conversations going around in the galaxy and beyond all the time using Neutrino Beams or graviton isolations in a sort of galactic clout that you can only join by mastering certain levels of technology responsibly and only when you have done that will you reach the level of technical sophistication that will enable you to join the club and join in the conversation.

It could be that we are too advanced; that we’re statistically in the lead and nobody else within communicating range is able to send signals. Why would that be? Well it could be that, as we have just seen, the effect of impacts is to make long term survival in the galaxy extremely unlikely, that London-like civilisations just don’t exist; they’re wiped out periodically by impacts from comets and asteroids.

And, we’re really rather very lucky we have an unusual solar system. We have a giant planet like Jupiter in the outskirts which intercepts objects that are threatening once they reach the outer parts of the solar system. A few years ago we saw a comet collide into Jupiter. Jupiter’s gravity is so strong it captured that comet; it stopped it getting into the interior of the solar system.

If things do get into the interior of the solar system, they have to get past the moon. The moon is huge by the standard of planets and moons around other planets. It’s almost a double planetary system, the earth and the moon, and you can see from the surface of the moon how many objects have hit it over its history. They would have hit us had the moon not been there. So, we are lucky in this respect.

Another theory, perhaps somewhat unusual, is that it could be that something like consciousness is actually only a passing phase in evolution, that, in fact, eventually it gets left behind. And the most advanced forms of life in the universe are not conscience or intelligent in quite the way that we are.

Stone Age man knew intuitively a lot about Newton ’s mechanics. If you threw a rock or spear at him, he would get out of the way. So his brain, through experience, had developed a way of calculating intuitively rather quickly what the trajectory of the rock would be and there are many things like that which we have an intuitive understanding of, even though we don’t know the mathematical theory of it. As you can imagine in a very advanced civilisation such intuitive appreciation of things might have grown to encompass the whole of the quantum mechanical nature of the world. So, if life was very small and that technological, it might have a full understanding of the workings of the world on a practical, intuitive level without having a scientific understanding of it. So it would have been engendered simply by the evolutionary process.

So much for local sudden disasters, what about the gradual fate of stuff in the universe? Well, there are four things that we need to think about. The first is what happens to stars and galaxies, the next what happens to structures made out of matter, things like atoms, molecules, solids, and then the fate of matter itself, the building blocks of atoms, protons, electrons and finally, it turns out also above all, the fate of black holes, these great sort of cookie monsters in the universe that supposedly just gobble up material and out of which information cannot escape.

In every system, whether it is planets orbiting around stars or collections of stars orbiting around centres of galaxies, there is a gradual winding down process which is a manifestation of the Second Law of Thermodynamics. What happens is that the motions gradually slow down, their energy is radiated away both as ordinary electromagnetic radiation but, over the long term that process makes orbiting systems become more and more tightly bound until eventually they come so close together that they will turn into black holes.

When material falls in to form a black hole, no matter what its properties, what it’s like, whether it’s brass bedsteads, planets, spaceships or whatever, all you can tell about what’s inside is what’s its mass, what’s its angular momentum, how fast it is spinning and what is its electric charge. All other information is effaced. The properties still exist if you’re in the inside but from the outside you can tell nothing else.

But black holes do not just sit there doing nothing, gobbling up more material. Eventually they will gobble up all material in their vicinity; a galaxy will have a giant growing black hole at the centre which will eat up all the stars and material within the galaxy. Those black holes will very, very slowly start to evaporate by quantum mechanical processes. What happens is, the material tunnels out through the horizon of the black hole very, very slowly.

The other way to look at it is that the gravitational field of the black hole is sufficiently strong that it can produce pairs of particles and anti-particles using its own gravitational field energy to do the creating. Those particles radiate away and the mass of the black hole reduces very slowly. This is a process discovered by Stephen Hawking back in 1974. It is usually called ‘Hawking Evaporation’. It’s very, very slow and a black hole the mass of the sun it would take 10^(66) years for the black hole to disappear. A black hole the size of an asteroid would take about the age of the universe, 13 billion years, to evaporate.

What’s happening here is sort of a recycling of the material. Material falls into black holes; it is then evaporated out as radiation, electrons, positrons, and other elementary particles. In this process, energy is conserved and the second Law of Thermodynamics is obeyed. When material comes out, it comes out in a simpler, macerated form; whilst bedsteads fall in, radiation and photons come out.

It used to be thought, long ago, that protons, the building blocks of nuclei that form our bodies, were like diamonds, forever. That is, once you had a proton and you sat it on its own in the universe, you’d always have a proton. But if you want to join together all the different forces of nature into a so called theory of everything, this cannot be true. It must be possible for the quarks that sit inside every proton to be able to transform into electrons and positrons and vice-versa and as a consequence, protons must be able to decay. They could turn into a positron and a Psion that quickly transform into high energy photons. This process would probably take about 10^(36) years or more but it means that any stray protons, atoms that might have escaped this process, will eventually be turned into electrons, positrons and photons.

A collection of stars, for example, if they don’t fall into black holes, will tend to evaporate. Gradually particles will be thrown out from the interior by occasional meeting, going rather too close to another one, receiving another kick and they will get disbursed. Their constituents will evaporate by this process and we’ll end up with a universe that contains lots of radiation and those elementary particles which are stable, so they have nowhere to go, nothing to decay into, and they must be the lightest particles that carry certain conserved quantities. We believe electric charge is conserved in nature so the lightest charged particle has nowhere to go. It can’t get rid of its charge because there isn’t a lighter charge particle to decay into with something else left over. It appears that the electron is the lightest charged particle and its antiparticle, the positron.

So, there’s a big role for electrons and positrons in the future of the universe, together with photons, neutrinos, and other very, very light weakly interacting elementary particles together with black holes steadily evaporating. The big mystery about the black holes is what happens to them when they’ve completely evaporated. What’s left? Is there just some relic object of tiny mass or is there nothing at all? Or is there some singularity, a hole in space time?

The time table involves lots of enormous numbers. In our own galaxy, planets around stars would eventually be certain to be pulled away from their stars by the gravitational affect of another passing star, after about 10^(15), so this a million billion years on the average. A thousand times longer, all the galaxies will have either evaporated into bits and pieces or 10x longer they will have; what’s left will condense to form black holes, giving out gravitational radiation.

If the whole galaxy turns into a black hole, we’ll wait 10^(99) years for them to disappear. Once they start to cluster, on the scale of galaxy clusters, you have black holes that will take 10^(117) years to evaporate. If the protons haven’t disappeared, they will contract and spontaneously form black holes which then evaporate in 10^(122) years. Any ordinary stuff that escaped or missed could eventually jump back and quantum mechanically tunnel to become inert ion but only after some fantastically long period of time.

Things do not look very bright. But, there are groans and moans about these types of extrapolations which makes it much harder to predict the future of the universe than it is to reconstruct the past. There’s much more certainty about what the universe was like when it was 1 second old then what it will be like when it is 10^(50) years old. Why is that?

It can be that there might be very, very weak forces of nature that have no effect on laboratory physics or even on the expansion of the universe today. But, in the long run, they may win out and their effect might start to become significant in the very, very far future. If any of the constants of nature, like the Fine Structure Constant or Newton ’s Gravitational Constant are not really constant but are very, very slowly changing, perhaps imperceptibly slow today, again in the very long run this could turn out to be the dominant effect. If gravity were becoming slightly stronger in the future, this might enable things to hold together for longer. If gravity is becoming very slowly weaker, this might make everything fall to pieces rather sooner.

The reason why everything is so much more difficult to predict in the future is that things are becoming farther and farther away from equilibrium. When we go back to the beginning of the universe, everything becomes a big soup; it’s like looking inside an oven. The only thing you can know about an equilibrium system is its temperature. When you know that, you can work out everything else about it. But, in the future, things are not colliding in equilibrium with everything else and if something explodes in one part of the universe it may not affect things somewhere else and you end up with a difference in temperature between different places. So, the universe becomes much more complicated as time goes on and requires many more things to give it a description.

In the very, very long term, quantum mechanics, which tells you only the probabilities that certain events will happen, might try to persuade us that anything that has a finite probability of happening will eventually happen if you wait long enough. And just as we might believe that if the universe began by suddenly starting to expand out of absolutely nothing, there may be a probability that it will likewise disappear back into nothing by the same quantum mechanically allowed event.

Let me return to of the Heat Death because its still popularly believed that it has a particular form. This was a problem that was solved by Frank Tipler and myself back in the 1980’s and the solution really emerges from Hawking’s discovery that black holes can evaporate. What Hawking was showing was that gravity itself has an entropy so entropy is not just carried by radiation and particles being disordered and working great machines, as the 19th century physicists imagined, but the gravitational field itself has an entropy. In fact, black holes have an entropy, and the entropy of the black hole is given by the its surface area. If you do anything to a black hole, if you collide it with other black holes or you throw things into it, its surface area will always increase. The second Law of Thermodynamics governs it.

This has an interesting implication for the universe. It turns out that in a universe like ours appears to be, if you work out the entropy of everything in stars and galaxies and radiation, then this seems to be steadily increasing. The amount of disorder in the universe is indeed increasing inline with the Second Law. But if you use Hawking’s discovery, you can work out what is the maximum possible entropy that the universe could have and that turns out to be vastly bigger, that our universe is far away from being as disordered as it could be. We could take all the matter in the universe and reorganise it in a way, it would vastly increase its entropy. It turns out that this maximum possible level of disorder in the universe increases much more rapidly than the actual entropy is increasing.

So although you’re heading to higher and higher entropy, we’re actually getting farther and farther away from the heat death, the maximum entropy that you could have. This is rather peculiar and quite different from the old sort of Eddington and Helmholtz picture of the universe. And, just for a reflection, in the future the universe will get farther and farther away from equilibrium because it’s not in a state of complete thermal equilibrium.

In recent years, this picture has been changed again. We’ve discovered that the expansion of the universe seems to be accelerating. This introduces a new ingredient to the story. In an accelerating universe there is maximum possible entropy. What will happen at some time in the far future is that the actual entropy line will in some way gradually approach the maximum possible and at that stage, you can regard the universe as being, as it were, in a ‘life is complete’ thermal equilibrium, that it would have suffered a heat death.

What’s in store for the universe in the future, and is what will happen to the expansion itself? We’ve seen that there’s the possibility that we might head for a big crunch. What would happen to the shape of the universe? We expand fantastically spherically at the moment. One part in 10^(5) is the accuracy in every direction. When we head further and further from the equilibrium we’ll be able to predict what happens. The universes that are heading for big crunchers, the idealised one that we’ve thought about, where everything begins expanding at the same moment, reaches a maximum size and then everything heads back to a big crunch, all in the same way wherever you are in the universe.

The real universe is not like that. There are places where the density is higher than the average. Even if the universe begins simultaneously everywhere, or nearly simultaneously, what will happen is that the denser parts of the universe will hit their big crunch sooner and perhaps a lot sooner than the sparser places. The end of the world is not simultaneous, so people can send a signal at the last moment so you can tell other people about what’s happening. So, nonsimultaneity of the end of the world is something to bear in mind.

The thing that most people worry about is can life, some life in some shape or form, continue into the far future? Now we don’t want to think of life being like us necessarily but we might try to just determine what might be a necessary condition. If you want to have life of any sort, what would have to be ok in the future? We might characterise life like the artificial intelligencia do as some type of processing of information. The processes may be based on DNA molecules as the hardware or it may be something more exotic but if we want to live for ever, perhaps the easiest way of characterising living forever is to ask if you can process an infinite number of bits of information for the future, and you’d regard that as a necessary condition of living forever.

It turns out there’s a simple answer to that question and it just relies on whether the universe has one simple property or not today. It’s whether the expansion of the universe is accelerating or decelerating. So a universe where the size against time trajectory has a rather concave picture is decelerating. The effect of gravity puts the breaks on the expansion and slows it down. But, we discovered back in 1987 that our expansion of our universe has, about 5 or 6 billion years ago, begun to accelerate. So the curvature of the trajectory has started to tip the other way in its convex direction.

We believe at present that we live in an accelerating universe and if the dark energy which drives that acceleration doesn’t decay, we don’t think it would have anywhere to decay to, it will keep on accelerating forever. Unfortunately, it’s best to live in the slow lane because in an accelerating universe, life always dies out. So if you characterise it in that minimalist way, as just the processing of information, that any form of information processing will eventually not be able to keep pace with the expansion of the universe, it will not be able to operate and there will only be a finite number of bits of information processed in the infinite future. But, if the universe decelerates, then life in some abstract form, the form of processing information, can survive for ever. You can have an infinite number of bits of information processed to the future.

How do you do that? Well, here’s a general idea of how that might be possible. You see, as we go into the future of an expanding universe, it doesn’t expand perfectly spherically. Some directions expand a little bit more rapidly than others and what that means is that if the radiation moving in one direction is compared with the radiation moving in another direction, they cool at slightly different rates. You can think of the wavelength as being stretched more rapidly in the directions that expand faster. In the directions which expand fastest, the radiation cools quickest. In the directions that expand more slowly, the radiation cools slowest. And in the in-between modes, it does the in-between thing.

Once you have different temperatures and different directions and different places you have energy flow, thus if you put a piece of ice in a room heat will travel in such a way as to equalise the temperature, going from hot to cold. There will be an energy flow in this direction, in different directions and once you have an energy flow, you have a way of driving a machine or a processor. You are generating entropy. You are able to process information.

Modern physics allows very dramatic things to happen so the world could come to an end in a few seconds’ time, completely without warning. Our understanding of Particle Physics is that there are energy levels for the states of matter in the universe and we believe, naively perhaps, that we are on the bottom floor, that the universe has done all its dropping down stairs, from one level to another, and we are in the ground floor, we are in the basement, and there are no lower levels for the universe to suddenly drop downstairs. But we may be incorrect. We may be on the ground floor, not in the basement.

There certainly could be a change in the energy state of the universe which, if it occurred, would change the properties of all the elementary particles around us. They all might become mass less for example. So, all the protons in our body would turn into radiation and light. We would disappear. We would know nothing about it. That’s the only consolation. So you don’t need to pay off that loan after all.

There was a time when, in the United States, there was great concern from members of the public who’d read a paper about this: that high energy particle collisions might tip the universe over this hill and precipitate one of these disasters. So a committee of inquiry was created to decide that the probability was sufficiently low from evidence of other things. We are being bombarded by cosmic rays all the time with very, very high energy and that hadn’t produced this effect so you could be pretty certain that the particles accelerator wouldn’t be able to do it either, even if you thought it was a possibility.

The underlying theme is of the end of the world coming rather like a thief in the night. Here is us travelling forward in time from the beginning of the universe and there can be an incoming beam of gravitational radiation heading towards us at the speed of light in a minute or two’s time; it will hit us without warning and it’s just part of the initial conditions of the universe. So, sudden, dramatic, and unadvertised things can happen.

You can have an end to the universe, that’s rather temporary, and things can rejuvenate maybe in a rather different form. The classic Fossilating Universe requires the universe to re-collapse. It seems unlikely for our universe at the present. If it does so, it might then re-expand, collapse, re-expand, and so on for ever.

So this was the classical picture of this type of universe. What’s interesting about it is if you include the second law of Thermodynamics, the increase of entropy requires these cycles to get bigger each time. But, unfortunately, if our universe is accelerating, as observation shows, then the dark energy which creates that acceleration will inevitably stop these oscillations eventually and the universe will be left to expand forever after a finite number of these oscillations.

The inflationary universe scenario has been examined for its structure at the present. This was a picture in which we imagined small fluctuations at the beginning of the universe, soon after the expansion begins, and these undergo dramatic acceleration for a short period of time. One of those bubbles that expands rather a lot in this process comes to encompass the whole of our visible universe today and it can predict with enormous detail what should be the fluctuations we would see in the sky in microwave frequencies if this had occurred. The observations indicate with very high precision that this seems to have occurred so we take it seriously.

An extrapolation of this idea, the so called Internal Inflationary Universe, shows that it’s a consequence of this process that if it’s occurred to produce our little bubble, than our bubble has to create within itself the conditions for further inflation of other little sub regions and they in turn will inflate sub regions and so on for ever. So we see our visible universe, being part of an eternal process that has no end and no beginning so this region which we’re in may undergo the fate of which I’ve spoken earlier but beyond it there are other regions which will regenerate and things will start again.

I will end with two sets of rather wild ideas. I can’t say much about them. Ted Harrison once speculated sufficiently advanced civilisations would have the capability of actually controlling that inflationary process. They might be able to control what would be the values of the constants and the expansion that those baby universes emerged with. Others have suggested that perhaps if there is a collapse of part of the universe in that cyclic process, that in that collapse various constants of nature and other properties of the universe might be changed perhaps in a random way so that when it re-emerges and expands we have a very different type of universe. Some of those universes will not be suitable for life, others will be.

Last of all is an amusing suggestion that I made a while ago, about the same time as a certain movie came out, that we do live in a simulated reality so we might think that the thing that we call universe is really just part of somebody else’s simulation. It could come to an end at any time just because they decide to pull the plug or turn off the simulation. It’s been pointed out by philosophers that you should perhaps take this idea more seriously than you might have first thought because if the universe starts to contain advanced intelligent beings who are able to simulate realities with very, very great precision, then simulated realities should become much more common than true realities and, looking at it statistically, we are much more likely to be in a simulated reality than a real one.

You see, today astronomers produce simulations to try to understand how galaxies form. They programme equations of gravity and thermodynamics and they follow the formation of stars, clustering of stars, but imagine that you really had unlimited computer power so that you follow not only the formation of stars but planets and then on the planetary surfaces you followed the biochemistry that evolved what we call life and then you started to follow how those embryonic life forms actually started to communicate with one another. So it’s just a matter of degree rather than qualitative difference that you could follow the simulation, the whole process by which life develops.

And the challenging question, if you think about a train journey, how long is it, how could you tell the difference, what would be a decisive observation that you could propose to tell the difference between living in true reality and living in a simulated reality?

© John D Barrow, 2 November 2004