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THE CYCLIC UNIVERSE：A Talk With Neil Turok
[NEIL TUROK:] For the last ten years I havemainly been working on the question of how the universe began — or didn'tbegin. What happened at the Big Bang? To me this seems like one of the mostfundamental questions in science, because everything we know of emerged fromthe Big Bang. Whether it's particles or planets or stars or, ultimately, evenlife itself.
In recent years, the searchfor the fundamental laws of nature has forced us to think about the Big Bangmuch more deeply. According to our best theories — string theory and M theory —all of the details of the laws of physics are actually determined by thestructure of the universe; specifically, by the arrangement of tiny, curled-upextra dimensions of space. This is a very beautiful picture: particle physics itselfis now just another aspect of cosmology. But if you want to understand why theextra dimensions are arranged as they are, you have to understand the Big Bangbecause that's where everything came from.
Somehow, until quiterecently, fundamental physics had gotten along without really tackling thatproblem. Even back in the 1920's, Einstein, Friedmann and Lemaitre — thefounders of modern cosmology — realized there was a singularity at the BigBang. That somehow, when you trace the universe back, everything went wrongabout 14 billion years ago. By go wrong, I mean all the laws of physics breakdown: they give infinities and meaningless results. Einstein himself didn'tinterpret this as the beginning of time; he just said, well, my theory fails.Most theories fail in some regime, and then you need a better theory. IsaacNewton's theory fails when particles go very fast; it fails to describe that.You need relativity. Likewise, Einstein said, we need a better theory ofgravity than mine.
But in the 1960's, when theobservational evidence for the Big Bang became very strong, physicists somehowleapt to the conclusion that it must have been the beginning of time. I am notsure why they did so, but perhaps it was due to Fred Hoyle — the main proponentof the rival steady-state theory — who seems to have successfully ridiculed theBig Bang theory by saying it did not make sense because it implied a beginningof time and that sounded nonsensical.
Then the Big Bang wasconfirmed by observation. And I think everyone just bought Hoyle's argument andsaid, oh well, the Big Bang is true, okay, so time must have begun. So weslipped into this way of thinking: that somehow time began and that theprocess, or event, whereby it began is not describable by physics. That's verysad. Everything we see around us rests completely on that event, and yet thatis the event we can't describe. That's basically where things stood incosmology, and people just worried about other questions for the next 20 years.
And then in the 1980s,there was a merging of particle physics and cosmology, when the theory ofinflation was invented. Inflationary theory also didn't deal with the beginningof the universe, but it took us back further towards it. People said, let'sjust assume the universe began, somehow. But, we're going to assume that whenit began, it was full of a weird sort of energy called inflationary energy.This energy is repulsive — its gravitational field is not attractive, likeordinary matter — and the main property of that energy is that it causes theuniverse to expand, hugely fast. Literally like dynamite, it blows up theuniverse.
This inflationary theorybecame very popular. It made some predictions about the universe, and recentobservations are very much in line with them. The type of predictions it madeare rather simple and qualitative descriptions of certain features of theuniverse: it's very smooth and flat on large scales; and it has some densityvariations, of a very simple character. Inflationary theory predicts that thedensity variations are like random noise — something like the ripples on thesurface of the sea — and fractional variation in the density is roughly thesame on all length scales. And these predictions of inflation have been broadlyconfirmed by observation. So people have become very attracted to inflation andmany people think it's correct. But inflationary theory never really dealt withthe beginning of the universe. We just had to assume the universe started outfull of inflationary energy. That was never explained.
My own work in this subject startedabout ten years ago, when I moved to Cambridge from Princeton. There I metStephen Hawking, who, with James Hartle, developed a theory about how theuniverse can begin. So I started to work with Stephen, to do calculations tofigure out what this theory actually predicted. Unfortunately, we quicklyreached the conclusion that the theory predicted an empty universe. Indeed,this is perhaps not so surprising: if you start with nothing, it makes moresense that you'd get an empty universe rather than a full one. I'm beingfacetious, of course, but when you go through the detailed math, Hawking'stheory seems to predict an empty universe, not a full one.
So we tried to think of various waysin which this problem might be cured, but everything we did to improve thatresult — to make the prediction more realistic & mdashspoils the beauty of thetheory. Theoretical physics is really a wonderful subject because it's adiscipline where crime does not pay in the long run. You can fake it forawhile, you can introduce fixes and little gadgets which make your theory work,but in the long run, if it’s no good, it'll fall apart. We know enough aboutthe universe and the laws of nature, and how it all fits together, that it isextremely difficult to make a fully consistent theory. And when you start tocheat, you start to violate special symmetries which are, in fact, the key tothe consistency of the whole structure. If those symmetries fall apart, andthen the whole theory falls apart. Hawking's theory is still an ongoing subjectof research, and people are still working on it and trying to fix it, but Idecided, after four or five years, that the approach wasn't working. It's very,very hard to make a universe begin and be full of inflationary energy. Weneeded to try something radically different.
So, along with Paul Steinhardt, Idecided to organize a workshop at the Isaac Newton Institute in Cambridge,devoted to fundamental challenges in cosmology. And this was the big one: howto sensibly explain the Big Bang. We decided to bring together the mostcreative theorists in string theory, M theory and cosmology to brainstorm andsee if there could be a different approach. The workshop was very stimulating,and our own work emerged from it.
String theory and M theory areprecisely the kinds of theories which Einstein himself had been looking for.His theory of gravity is a wonderful theory and still the most beautiful andsuccessful theory we have, but it doesn't seem to link properly with quantummechanics, which we know is a crucial ingredient for all the other laws ofphysics. If you try to quantize gravity naively, you get infinities whichcannot be removed without spoiling all of the theory's predictive power. Stringtheory succeeds in linking gravity and quantum mechanics within what seems tobe a consistent mathematical framework. Unfortunately, thus far, the only caseswhere we can really calculate well in string theory are not very physicallyrealistic: for example, one can do very precise calculations in static, emptyspace with some gravitational waves. Nevertheless, because of its very tightand consistent mathematical structure, many people feel string theory isprobably on the right track.
String theoryintroduces some weird new concepts. One is that every particle we see isactually a little piece of string. Another is that there are objects calledbranes, short for membranes, which are basically higher-dimensional versions ofstring. At the time of our workshop, a new idea had just emerged: the idea thatthe three dimensions of space we experience could in fact be the dimensionsalong one of these branes. The brane we live on could be a sort of sheet-likeobject floating around in a higher dimension of space. This underlies a modelof the universe which fits particle physics very well and which consists of twoparallel branes separated by a very, very tiny gap. Many people were talkingabout this model in our workshop, including Burt Ovrut, and Paul and I askedthe question of what happens if these two branes collide. Until then, peoplehad generally only considered a static setup. They described the branes sittingthere, with particles on them, and they found that this setup fit a lot of thedata we have about particles and forces very well. But they hadn't consideredthe possibility that branes could move, even though that is perfectly allowedby the theory. And if the branes can move, they can collide. Our initialthought was that, if they collide, that might have been the Big Bang. Thecollision would be a very violent process, in which the clash of the two braneswould generate lots of heat and radiation and particles… just like a Big Bang.
Burt, Paul and Ibegan to study this process of the collision of the branes carefully. Werealized that, if it worked, this idea would imply that the Big Bang was notthe beginning of time but, rather, a perfectly describable physicalevent. We also realized this might have many implications, if it weretrue. For example, not only could we explain the Bang, we could explain theproduction of radiation which fills the universe, because there was a previousexisting universe, within which these two branes were moving. And whatexplained that, you might ask? That's where the cyclic model came in. Thecyclic model emerged from the idea that each Bang was followed by another, andthat this could go on for eternity. The whole universe might have existedforever, and there would have been a series of these Bangs, stretching backinto the infinite past, and into the infinite future.
For the last fiveyears, we've worked on refining this model. The first thing we had to do was tomatch the model to observation, to see if it could reproduce some of theinflationary model's successes. Much to our surprise, we found that it could,and in some cases in a more economical way than inflation. If the two branesattract one another, then as they pull towards one another they acquireripples, like the ripples on the sea I mentioned before. Those ripples turninto density variations as the branes collide and release matter and radiation,and these density variations later lead to the formation of galaxies in theuniverse.
We found that,with some simple assumptions, our model could explain the observations to justthe same accuracy as the inflationary model. That's instructive, because itsays there are these two very different mechanisms which achieve the same end.Both models explain rather broad, simple features of the universe: that it isnearly uniform on large scales. That it is flat, like Euclidean space, and thatit has these simple density variations, with nearly the same strength on everylength scale. These features are explained either by the brane collision modelor by the inflation model. And there might even be another, better model whichno-one has yet thought of. In any case, it is a healthy situation for scienceto have rival theories, which are as different as possible. This helps us toclearly identify which critical tests — be they observational ormathematical/logical — will be the key to distinguishing the theories andproving some of them wrong. Competition between models is good: it helps us seewhat the strengths and weaknesses and our theories are.
In this case, a keybattleground between the more established inflationary model and our new cyclicmodel is theoretical: each model has flaws and puzzles. What happened beforeinflation? Does most of the universe inflate, or only some of it? Or, for thecyclic model, can we calculate all the details of the brane collision, and turnthe rough arguments into precise mathematics? It is our job as theorists topush those problems to the limit to see whether they can be cured, or whetherthey will instead prove fatal for the models.
Equally, if notmore important, is the attempt to test the models observationally, becausescience is nothing without observational test. Even though the cyclic model andinflation have similar predictions, there is at least one way we know oftelling them apart. If there was a period of inflation — a huge burst ofexpansion just after the beginning of the universe — it would have filled spacewith gravitational waves, and those gravitational waves should be measurable inthe universe today. Several experiments are already searching for them and,next year, the European Space Agency's Planck satellite will make the bestattempt yet: it should be capable of detecting the gravitational wavespredicted by the simplest inflation models. Our model with the colliding branespredicts that the Planck satellite and other similar experiments will detectnothing. So we can be proved wrong by experiment.
_____ Something I'mespecially excited about right now is that we have been working on the finermathematical details of what happens at the Bang itself. We've made some verygood progress in understanding the singularity, where, according to Einstein'stheory, everything becomes infinite; where all of space shrinks to a point, sothe density of radiation and matter go to infinity, and Einstein's equationsfall apart.
Our new work isbased on a very beautiful discovery made in string theory about ten years ago,with a very technical name. It's called the Anti-De Sitter Conformal FieldTheory correspondence. I won't attempt to explain that, but basically it's avery beautiful geometrical idea, which says that if I've got a region of spaceand time, which might be very large, then in some situations I can imagine thisuniverse surrounded by what we call a boundary — which is basically a boxenclosing the region we are interested in. About ten years ago, it was shownthat even though the interior of this container is described by gravity, withall of the difficulties that brings&mdashlike the formation of black holesand the various paradoxes they cause — all of that stuff going on inside thebox can be described by a theory that lives on the walls of the box surroundingthe interior. That's the correspondence. A gravitational theory corresponds toanother theory which has no gravity, and which doesn't have any of thosegravitational paradoxes. What we've been doing recently is using this frameworkto study what happens at a cosmic singularity which develops in time, withinthe container. We study the singularity indirectly, by studying what happens onthe surface of the box surrounding the universe. When we do this, we find thatif the universe collapses to make a singularity, it can bounce, and theuniverse can come back out of the bounce. As it passes through the singularity,the universe becomes full of radiation–very much like what happens in thecolliding brane model — and density variations are created.
Thisis very new work, but once it is completed I think it will go a long waytowards convincing people that the Big Bang, or events like it, are actuallydescribable mathematically. The model we're studying is not physicallyrealistic, because it's a universe with four large dimensions of space. Itturns out that's the easiest case to do, for rather technical reasons. Ofcourse, the real universe has only three large dimensions of space, but we'resettling for a four-dimensional model for the moment, because the math iseasier. Qualitatively, what this study is revealing is that you can studysingularities in gravity and make sense of them. I think that's very excitingand I think we're on a very interesting track. I hope we will really understandhow singularities form in gravity, how the universe evolves through them, andhow those singularities go away.
I suspect thatwill be the explanation of the Big Bang — that the Big Bang was the formationof a singularity in the universe. I think by understanding it we'll be betterable to understand how the laws of physics we currently see were actually setin place: why there is electro-magnetism, the strong force, the weak force, andso on. All of these things are a consequence of the structure of the universe, onsmall scales, and that structure was set at the Big Bang. It's a verychallenging field, but I'm very happy we're actually making progress.
_____ The currentproblem which is dominating theoretical physics — wrongly, I believe, because Ithink people ought to be studying the singularity and the Big Bang since that'sclearly where everything came from, but most people are just avoiding thatproblem — is the fact that the laws of physics we see, according to stringtheory, are a result of the specific configuration of the extra dimensions ofspace. So you have three ordinary dimensions, that we're aware of, and thenthere are supposed to be six more dimensions in string theory, which are curledup in a tiny little ball. At every point in our world there would beanother six dimensions, but twisted up in a tiny little knot. And the problemis that there is a huge number of ways of twisting up these extra dimensions.Probably, there are an infinite number of ways. Roughly speaking, you can wrapthem up by wrapping branes and other objects around them, twisting them up likea handkerchief with lots of bits of string and elastic bands wound around.
This caused manypeople to pull their hair out. String theory was supposed to be a unique theoryand to predict one set of laws of physics, but the theory allows for manydifferent types of universes with the extra dimensions twisted up in differentways. Which one do we live in? What some people have been doing, because theyassume the universe simply starts after the Bang at some time, is just throwinga dice. They say, okay, well it could be twisted up in this way, or that way,or the other way, and we have no way of judging which one is more likely thanthe other, so we'll assume it's random. As a result, they can't predictanything. Because they don't have a theory of the Big Bang, they don't have atheory of why those dimensions ended up the way they are. They call this thelandscape; there's a landscape of possible universes, and they accept that theyhave no theory of why we should live at any particular place in the landscape.So what do they do?
Well, they say,maybe we need the anthropic principle. The anthropic principle says, theuniverse is the way it is because if it was any different, we wouldn't be here.The idea is that there's this big landscape with lots of universes in it, butthe only one which can allow us to exist is the one with exactly the laws ofphysics that we see. It sounds like a flaky argument & mdashand it is. It's a very flakyargument. Because it doesn't predict anything. It's a classic example ofpostdiction: its just saying, oh well, it has to be this way, because otherwisewe wouldn't be here talking about it. There are many other logical flaws in theargument which I could point to, but the basic point is that this argumentdoesn't really get you anywhere. It’s not predictive and it isn't testable. Theanthropic principle, as it's currently being used, isn't really leading to anyprogress in the subject. Even worse than that, it is discouraging people fromtackling the important questions, like the fact that string theory, as it iscurrently understood, is incomplete and needs to be extended to deal with theBig Bang. That's just such an obvious point, but at the moment surprisingly fewpeople seem to appreciate it.
I'm not convincedthe landscape is real. There are still some reasonable mathematical doubts,about whether all these twisted up configurations are legitimate. It's not beenproven. But if it is true, then how are you going to decide which one of thoseconfigurations is adopted by the universe? It seems to me that whatever you do,you have to deal with the Big Bang. You need a mathematical theory of how BigBangs works, either one which describes how time began, or one which describeshow the universe passes through an event like the Big Bang and, as it passesthrough, there's going to be some dramatic effect on these twisted-updimensions. To me, the most plausible resolution of a landscape problem wouldbe that the dynamics of the universe will select a certain configuration as themost efficient one for passing through Big Bangs and allowing a Universe whichcycles for a very long time.
For example, justto give a trivial example: if you ask, why is the gas in this room smoothlydistributed, we need a physical theory to explain it. It wouldn't be helpful tosay, well if it wasn't that way, there would be a big vacuum in part of theroom and if I walked into it, I would die. If the distribution of gas wasn'tcompletely uniform, we wouldn't last very long. That's the anthropic principle.But it's not the scientific explanation. The explanation is that moleculesjangle around the room and when you understand their dynamics you understandthat it's vastly more probable for them to settle down in a configuration wherethey're distributed nearly uniformly. It's nothing to do with the existence ofpeople.
In the same way, Ithink the best way to approach the cosmological puzzles, is to begin byunderstanding how the Big Bang works. Then, as we study the dynamics of theBang, we'll hope to discover that the dynamics lead to a universe somethinglike ours. If you can't understand the dynamics, you really can't do much,except give up and resort to the anthropic argument. It's an obvious point, butstrangely enough it's a minority view. In our subject, the majority view at themoment is this rather bizarre landscape picture where somebody, or some randomprocess, and no one knows how it happens, chooses for us to be in one of theseuniverses.
_____ The idea behindthe cyclic universe is that the world we experience, the three dimensions ofspace, are actually an extended object, which you can picture as a membrane aslong as you remember that it is three-dimensional, and we just draw it as two-dimensionalbecause that is easier to visualize. According to this picture, we live on oneof these membranes, and this membrane is not alone, there's another partnermembrane, separated from it by a very tiny gap. There are three dimensions ofspace within a membrane, and a fourth dimension separating the two membranes.It so happens that in this theory there are another six dimensions of space,also curled up in a tiny little ball, but let's forgets about those for themoment.
So you have thisset-up with these two parallel worlds, just literally geometrically parallelworlds, separated by a small gap. We did not dream up this picture. Thispicture emerges from the most sophisticated mathematical models we have of thefundamental particles and forces. When we try to describe reality, quarks,electrons, photons, and all these things, we are led to this picture of the twoparallel worlds separated by a gap, and our starting point was to assume thatthis picture is correct.
These membranesare sometimes called "end of the world branes." Basically becausethey're more like mirrors; they're reflectors. There is nothing outside them.They're literally the end of the world. If you traveled across the gap betweenthe two membranes, you would hit one of them and bounce back from it. There'snothing beyond it. So all you have are these two parallel branes with the gap.But these two membranes can move. So imagine we start from today's universe.We're sitting here, today, and we're living on one of these membranes. There'sthis other membrane, very near to us. We can't see it because light onlytravels along our membrane, but the distance away from us are much tinier thanthe size of an atomic nucleus. It's hardly any distance from us at all. We alsoknow that, in the universe today, there's something called "darkenergy." Dark energy is the energy of empty space. Within the cyclictheory, the energy associated with the force of attraction between these twomembranes is responsible, in part, for the dark energy.
Imagine thatyou've got these two membranes, and they attract each other. When you pull themapart you have to put energy into the system. That's the dark energy. And thedark energy itself causes these two membranes to attract. Right now theuniverse is full of dark energy; we know that from observations. According toour model, the dark energy is actually not stable, and it won't last forever.If you think of a ball rolling on a hill, the stored energy grows as the ballgets higher: likewise the dark energy grows as the gap between membraneswidens. At some point, the ball turns around and falls back downhill. Likewise,after a period of dark energy domination, the two branes start to move towardseach other, and then they collide, and that's the Bang. It is the decay of thedark energy we see today which leads to the next Big Bang, in the cyclic model.
Dark energy wasonly observationally confirmed in 1999 and it was a huge surprise for theinflationary picture. There is no rhyme or reason for its existence in thatpicture: dark energy plays no role in the early universe, according toinflationary theory. Whereas in the cyclic model, dark energy is vital,because it is the decay of dark energy which leads to the next Big Bang.
This picture ofcyclic brane collisions actually resolves one of the longest-standing puzzlesin cyclic models. The idea of a cyclic model isn't new: Friedmann and otherspictured a cyclic model back in the 1930's. They envisaged a finite universewhich collapsed and bounced over and over again. But Richard Tolman soonpointed out that, actually, it wouldn't remove the problem of having abeginning. The reason those cyclic models didn't work is that every bouncemakes more radiation and that means the universe has more stuff in it.According to Einstein's equations, this makes the universe bigger after eachbounce, so that every cycle lasts longer than the one before it. But, tracingback to the past, the duration of each bounce gets shorter and shorter and theduration of the cycles shrinks to zero, meaning that the universe still had tobegin a finite time ago. An eternal cyclic model was impossible, in the oldframework. What is new about our model is that by employing dark energy and byhaving an infinite universe, which dilutes away the radiation and matter afterevery bang, you actually can have an eternal cyclic universe, which could last forever.
NEIL TUROK holds the Chair of Mathematical Physics inthe department of applied mathematics and theoretical physics at Cambridge University. He is coauthor, with Paul Steinhardt, of Endless Universe: Beyond the Big Bang.
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