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Edge: THE CYCLIC UNIVERSE: A Talk With Neil Turok

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  • Roger Bagula
    Posted by: Raven NWRaven@gmail.com mm121elaine Thu May 31, 2007 11:19 pm (PST) http://www.edge.org/3rd_culture/turok07/turok07_index.html In recent years,
    Message 1 of 1 , Jun 1, 2007
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      Posted by: "Raven" NWRaven@... mm121elaine
      Thu May 31, 2007 11:19 pm (PST)
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      "In recent years, the search for the fundamental laws of nature has
      forced us to think about the Big Bang much 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 the structure of the
      universe; specifically, by the arrangement of tiny, curled-up extra
      dimensions of space. This is a very beautiful picture: particle physics
      itself is now just another aspect of cosmology. But if you want to
      understand why the extra dimensions are arranged as they are, you have
      to understand the Big Bang because that's where everything came from."

      THE CYCLIC UNIVERSE [5.17.07]
      A Talk With Neil Turok


      Neil Turok — High | Low

      NEIL TUROK holds the Chair of Mathematical Physics in the department of
      applied mathematics and theoretical physics at Cambridge University. He
      is coauthor, with Paul Steinhardt, of Endless Universe: Beyond the Big Bang.

      NEIL TUROK's Edge Bio Page

      THE CYCLIC UNIVERSE

      [NEIL TUROK:] For the last ten years I have mainly been working on the
      question of how the universe began — or didn't begin. What happened at
      the Big Bang? To me this seems like one of the most fundamental
      questions in science, because everything we know of emerged from the Big
      Bang. Whether it's particles or planets or stars or, ultimately, even
      life itself.

      In recent years, the search for the fundamental laws of nature has
      forced us to think about the Big Bang much 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 the structure of the
      universe; specifically, by the arrangement of tiny, curled-up extra
      dimensions of space. This is a very beautiful picture: particle physics
      itself is now just another aspect of cosmology. But if you want to
      understand why the extra dimensions are arranged as they are, you have
      to understand the Big Bang because that's where everything came from.

      Somehow, until quite recently, fundamental physics had gotten along
      without really tackling that problem. Even back in the 1920's, Einstein,
      Friedmann and Lemaitre — the founders of modern cosmology — realized
      there was a singularity at the Big Bang. That somehow, when you trace
      the universe back, everything went wrong about 14 billion years ago. By
      go wrong, I mean all the laws of physics break down: they give
      infinities and meaningless results. Einstein himself didn't interpret
      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. Isaac
      Newton's theory fails when particles go very fast; it fails to describe
      that. You need relativity. Likewise, Einstein said, we need a better
      theory of gravity than mine.

      But in the 1960's, when the observational evidence for the Big Bang
      became very strong, physicists somehow leapt to the conclusion that it
      must have been the beginning of time. I am not sure why they did so, but
      perhaps it was due to Fred Hoyle — the main proponent of the rival
      steady-state theory — who seems to have successfully ridiculed the Big
      Bang theory by saying it did not make sense because it implied a
      beginning of time and that sounded nonsensical.

      Then the Big Bang was confirmed by observation. And I think everyone
      just bought Hoyle's argument and said, oh well, the Big Bang is true,
      okay, so time must have begun. So we slipped into this way of thinking:
      that somehow time began and that the process, or event, whereby it began
      is not describable by physics. That's very sad. Everything we see around
      us rests completely on that event, and yet that is the event we can't
      describe. That's basically where things stood in cosmology, 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 of inflation was invented. Inflationary
      theory also didn't deal with the beginning of the universe, but it took
      us back further towards it. People said, let's just assume the universe
      began, somehow. But, we're going to assume that when it began, it was
      full of a weird sort of energy called inflationary energy. This energy
      is repulsive — its gravitational field is not attractive, like ordinary
      matter — and the main property of that energy is that it causes the
      universe to expand, hugely fast. Literally like dynamite, it blows up
      the universe.

      This inflationary theory became very popular. It made some predictions
      about the universe, and recent observations are very much in line with
      them. The type of predictions it made are rather simple and qualitative
      descriptions of certain features of the universe: it's very smooth and
      flat on large scales; and it has some density variations, of a very
      simple character. Inflationary theory predicts that the density
      variations are like random noise — something like the ripples on the
      surface of the sea — and fractional variation in the density is roughly
      the same on all length scales. And these predictions of inflation have
      been broadly confirmed by observation. So people have become very
      attracted to inflation and many people think it's correct. But
      inflationary theory never really dealt with the beginning of the
      universe. We just had to assume the universe started out full of
      inflationary energy. That was never explained.

      My own work in this subject started about ten years ago, when I moved to
      Cambridge from Princeton. There I met Stephen Hawking, who, with James
      Hartle, developed a theory about how the universe can begin. So I
      started to work with Stephen, to do calculations to figure out what this
      theory actually predicted. Unfortunately, we quickly reached the
      conclusion that the theory predicted an empty universe. Indeed, this is
      perhaps not so surprising: if you start with nothing, it makes more
      sense that you'd get an empty universe rather than a full one. I'm being
      facetious, of course, but when you go through the detailed math,
      Hawking's theory seems to predict an empty universe, not a full one.

      So we tried to think of various ways in which this problem might be
      cured, but everything we did to improve that result — to make the
      prediction more realistic&mdashspoils the beauty of the theory.
      Theoretical physics is really a wonderful subject because it's a
      discipline where crime does not pay in the long run. You can fake it for
      awhile, you can introduce fixes and little gadgets which make your
      theory work, but in the long run, if its no good, it'll fall apart. We
      know enough about the universe and the laws of nature, and how it all
      fits together, that it is extremely difficult to make a fully consistent
      theory. And when you start to cheat, you start to violate special
      symmetries which are, in fact, the key to the consistency of the whole
      structure. If those symmetries fall apart, and then the whole theory
      falls apart. Hawking's theory is still an ongoing subject of research,
      and people are still working on it and trying to fix it, but I decided,
      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.
      We needed to try something radically different.

      So, along with Paul Steinhardt, I decided to organize a workshop at the
      Isaac Newton Institute in Cambridge, devoted to fundamental challenges
      in cosmology. And this was the big one: how to sensibly explain the Big
      Bang. We decided to bring together the most creative theorists in string
      theory, M theory and cosmology to brainstorm and see if there could be a
      different approach. The workshop was very stimulating, and our own work
      emerged from it.

      String theory and M theory are precisely the kinds of theories which
      Einstein himself had been looking for. His theory of gravity is a
      wonderful theory and still the most beautiful and successful theory we
      have, but it doesn't seem to link properly with quantum mechanics, which
      we know is a crucial ingredient for all the other laws of physics. If
      you try to quantize gravity naively, you get infinities which cannot be
      removed without spoiling all of the theory's predictive power. String
      theory succeeds in linking gravity and quantum mechanics within what
      seems to be a consistent mathematical framework. Unfortunately, thus
      far, the only cases where we can really calculate well in string theory
      are not very physically realistic: for example, one can do very precise
      calculations in static, empty space with some gravitational waves.
      Nevertheless, because of its very tight and consistent mathematical
      structure, many people feel string theory is probably on the right track.

      String theory introduces some weird new concepts. One is that every
      particle we see is actually a little piece of string. Another is that
      there are objects called branes, short for membranes, which are
      basically higher-dimensional versions of string. At the time of our
      workshop, a new idea had just emerged: the idea that the three
      dimensions of space we experience could in fact be the dimensions along
      one of these branes. The brane we live on could be a sort of sheet-like
      object floating around in a higher dimension of space. This underlies a
      model of the universe which fits particle physics very well and which
      consists of two parallel branes separated by a very, very tiny gap. Many
      people were talking about this model in our workshop, including Burt
      Ovrut, and Paul and I asked the question of what happens if these two
      branes collide. Until then, people had generally only considered a
      static setup. They described the branes sitting there, with particles on
      them, and they found that this setup fit a lot of the data we have about
      particles and forces very well. But they hadn't considered the
      possibility that branes could move, even though that is perfectly
      allowed by the theory. And if the branes can move, they can collide. Our
      initial thought was that, if they collide, that might have been the Big
      Bang. The collision would be a very violent process, in which the clash
      of the two branes would generate lots of heat and radiation and
      particles… just like a Big Bang.

      Burt, Paul and I began to study this process of the collision of the
      branes carefully. We realized that, if it worked, this idea would imply
      that the Big Bang was not the beginning of time but, rather, a perfectly
      describable physical event. We also realized this might have many
      implications, if it were true. For example, not only could we explain
      the Bang, we could explain the production of radiation which fills the
      universe, because there was a previous existing universe, within which
      these two branes were moving. And what explained that, you might ask?
      That's where the cyclic model came in. The cyclic model emerged from the
      idea that each Bang was followed by another, and that this could go on
      for eternity. The whole universe might have existed forever, and there
      would have been a series of these Bangs, stretching back into the
      infinite past, and into the infinite future.

      For the last five years, we've worked on refining this model. The first
      thing we had to do was to match the model to observation, to see if it
      could reproduce some of the inflationary 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 branes attract one another, then as they
      pull towards one another they acquire ripples, like the ripples on the
      sea I mentioned before. Those ripples turn into density variations as
      the branes collide and release matter and radiation, and these density
      variations later lead to the formation of galaxies in the universe.

      We found that, with some simple assumptions, our model could explain the
      observations to just the same accuracy as the inflationary model. That's
      instructive, because it says there are these two very different
      mechanisms which achieve the same end. Both models explain rather broad,
      simple features of the universe: that it is nearly uniform on large
      scales. That it is flat, like Euclidean space, and that it has these
      simple density variations, with nearly the same strength on every length
      scale. These features are explained either by the brane collision model
      or by the inflation model. And there might even be another, better model
      which no-one has yet thought of. In any case, it is a healthy situation
      for science to have rival theories, which are as different as possible.
      This helps us to clearly identify which critical tests — be they
      observational or mathematical/logical — will be the key to
      distinguishing the theories and proving some of them wrong. Competition
      between models is good: it helps us see what the strengths and
      weaknesses and our theories are.

      In this case, a key battleground between the more established
      inflationary model and our new cyclic model is theoretical: each model
      has flaws and puzzles. What happened before inflation? Does most of the
      universe inflate, or only some of it? Or, for the cyclic model, can we
      calculate all the details of the brane collision, and turn the rough
      arguments into precise mathematics? It is our job as theorists to push
      those problems to the limit to see whether they can be cured, or whether
      they will instead prove fatal for the models.

      Equally, if not more important, is the attempt to test the models
      observationally, because science is nothing without observational test.
      Even though the cyclic model and inflation have similar predictions,
      there is at least one way we know of telling them apart. If there was a
      period of inflation — a huge burst of expansion just after the beginning
      of the universe — it would have filled space with gravitational waves,
      and those gravitational waves should be measurable in the universe
      today. Several experiments are already searching for them and, next
      year, the European Space Agency's Planck satellite will make the best
      attempt yet: it should be capable of detecting the gravitational waves
      predicted by the simplest inflation models. Our model with the colliding
      branes predicts that the Planck satellite and other similar experiments
      will detect nothing. So we can be proved wrong by experiment.

      _____

      Something I'm especially excited about right now is that we have been
      working on the finer mathematical details of what happens at the Bang
      itself. We've made some very good progress in understanding the
      singularity, where, according to Einstein's theory, everything becomes
      infinite; where all of space shrinks to a point, so the density of
      radiation and matter go to infinity, and Einstein's equations fall apart.

      Our new work is based 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 Field Theory correspondence. I won't attempt to
      explain that, but basically it's a very beautiful geometrical idea,
      which says that if I've got a region of space and time, which might be
      very large, then in some situations I can imagine this universe
      surrounded by what we call a boundary — which is basically a box
      enclosing the region we are interested in. About ten years ago, it was
      shown that even though the interior of this container is described by
      gravity, with all of the difficulties that brings&mdashlike the
      formation of black holes and the various paradoxes they cause — all of
      that stuff going on inside the box can be described by a theory that
      lives on the walls of the box surrounding the interior. That's the
      correspondence. A gravitational theory corresponds to another theory
      which has no gravity, and which doesn't have any of those gravitational
      paradoxes. What we've been doing recently is using this framework to
      study what happens at a cosmic singularity which develops in time,
      within the container. We study the singularity indirectly, by studying
      what happens on the surface of the box surrounding the universe. When we
      do this, we find that if the universe collapses to make a singularity,
      it can bounce, and the universe 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 the colliding brane model — and
      density variations are created.

      This is very new work, but once it is completed I think it will go a
      long way towards convincing people that the Big Bang, or events like it,
      are actually describable mathematically. The model we're studying is not
      physically realistic, because it's a universe with four large dimensions
      of space. It turns out that's the easiest case to do, for rather
      technical reasons. Of course, the real universe has only three large
      dimensions of space, but we're settling for a four-dimensional model for
      the moment, because the math is easier. Qualitatively, what this study
      is revealing is that you can study singularities in gravity and make
      sense of them. I think that's very exciting and I think we're on a very
      interesting track. I hope we will really understand how singularities
      form in gravity, how the universe evolves through them, and how those
      singularities go away.

      I suspect that will be the explanation of the Big Bang — that the Big
      Bang was the formation of a singularity in the universe. I think by
      understanding it we'll be better able to understand how the laws of
      physics we currently see were actually set in place: why there is
      electro-magnetism, the strong force, the weak force, and so on. All of
      these things are a consequence of the structure of the universe, on
      small scales, and that structure was set at the Big Bang. It's a very
      challenging field, but I'm very happy we're actually making progress.

      _____

      The current problem which is dominating theoretical physics — wrongly, I
      believe, because I think people ought to be studying the singularity and
      the Big Bang since that's clearly where everything came from, but most
      people are just avoiding that problem — is the fact that the laws of
      physics we see, according to string theory, are a result of the specific
      configuration of the extra dimensions of space. So you have three
      ordinary dimensions, that we're aware of, and then there are supposed to
      be six more dimensions in string theory, which are curled up in a tiny
      little ball. At every point in our world there would be another six
      dimensions, but twisted up in a tiny little knot. And the problem is
      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 wrap them up by wrapping branes and other objects
      around them, twisting them up like a handkerchief with lots of bits of
      string and elastic bands wound around.

      This caused many people to pull their hair out. String theory was
      supposed to be a unique theory and to predict one set of laws of
      physics, but the theory allows for many different types of universes
      with the extra dimensions twisted up in different ways. Which one do we
      live in? What some people have been doing, because they assume the
      universe simply starts after the Bang at some time, is just throwing a
      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 than the other, so we'll assume it's random. As a result, they
      can't predict anything. Because they don't have a theory of the Big
      Bang, they don't have a theory of why those dimensions ended up the way
      they are. They call this the landscape; there's a landscape of possible
      universes, and they accept that they have 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, the universe 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, but the only one which can allow
      us to exist is the one with exactly the laws of physics that we see. It
      sounds like a flaky argument&mdashand it is. It's a very flaky argument.
      Because it doesn't predict anything. It's a classic example of
      postdiction: its just saying, oh well, it has to be this way, because
      otherwise we wouldn't be here talking about it. There are many other
      logical flaws in the argument which I could point to, but the basic
      point is that this argument doesn't really get you anywhere. Its not
      predictive and it isn't testable. The anthropic principle, as it's
      currently being used, isn't really leading to any progress in the
      subject. Even worse than that, it is discouraging people from tackling
      the important questions, like the fact that string theory, as it is
      currently understood, is incomplete and needs to be extended to deal
      with the Big Bang. That's just such an obvious point, but at the moment
      surprisingly few people seem to appreciate it.

      I'm not convinced the landscape is real. There are still some reasonable
      mathematical doubts, about whether all these twisted up configurations
      are legitimate. It's not been proven. But if it is true, then how are
      you going to decide which one of those configurations 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 Big Bangs works, either
      one which describes how time began, or one which describes how the
      universe passes through an event like the Big Bang and, as it passes
      through, there's going to be some dramatic effect on these twisted-up
      dimensions. To me, the most plausible resolution of a landscape problem
      would be that the dynamics of the universe will select a certain
      configuration as the most efficient one for passing through Big Bangs
      and allowing a Universe which cycles for a very long time.

      For example, just to give a trivial example: if you ask, why is the gas
      in this room smoothly distributed, we need a physical theory to explain
      it. It wouldn't be helpful to say, well if it wasn't that way, there
      would be a big vacuum in part of the room and if I walked into it, I
      would die. If the distribution of gas wasn't completely uniform, we
      wouldn't last very long. That's the anthropic principle. But it's not
      the scientific explanation. The explanation is that molecules jangle
      around the room and when you understand their dynamics you understand
      that it's vastly more probable for them to settle down in a
      configuration where they're distributed nearly uniformly. It's nothing
      to do with the existence of people.

      In the same way, I think the best way to approach the cosmological
      puzzles, is to begin by understanding how the Big Bang works. Then, as
      we study the dynamics of the Bang, we'll hope to discover that the
      dynamics lead to a universe something like 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, but strangely enough it's a
      minority view. In our subject, the majority view at the moment is this
      rather bizarre landscape picture where somebody, or some random process,
      and no one knows how it happens, chooses for us to be in one of these
      universes.

      _____

      The idea behind the cyclic universe is that the world we experience, the
      three dimensions of space, are actually an extended object, which you
      can picture as a membrane as long as you remember that it is
      three-dimensional, and we just draw it as two-dimensional because that
      is easier to visualize. According to this picture, we live on one of
      these membranes, and this membrane is not alone, there's another partner
      membrane, separated from it by a very tiny gap. There are three
      dimensions of space 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
      forget about those for the moment.

      So you have this set-up with these two parallel worlds, just literally
      geometrically parallel worlds, separated by a small gap. We did not
      dream up this picture. This picture emerges from the most sophisticated
      mathematical models we have of the fundamental particles and forces.
      When we try to describe reality, quarks, electrons, photons, and all
      these things, we are led to this picture of the two parallel worlds
      separated by a gap, and our starting point was to assume that this
      picture is correct.

      These membranes are sometimes called "end of the world branes."
      Basically because they'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 between the two membranes, you would hit one of
      them and bounce back from it. There's nothing 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's this other
      membrane, very near to us. We can't see it because light only travels
      along our membrane, but the distance away from us is much tinier than
      the size of an atomic nucleus. It's hardly any distance from us at all.
      We also know that, in the universe today, there's something called "dark
      energy." Dark energy is the energy of empty space. Within the cyclic
      theory, the energy associated with the force of attraction between these
      two membranes is responsible, in part, for the dark energy.

      Imagine that you've got these two membranes, and they attract each
      other. When you pull them apart you have to put energy into the system.
      That's the dark energy. And the dark energy itself causes these two
      membranes to attract. Right now the universe is full of dark energy; we
      know that from observations. According to our 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 ball gets higher:
      likewise the dark energy grows as the gap between membranes widens. 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
      towards each other, and then they collide, and that's the Bang. It is
      the decay of the dark energy we see today which leads to the next Big
      Bang, in the cyclic model.

      Dark energy was only observationally confirmed in 1999 and it was a huge
      surprise for the inflationary picture. There is no rhyme or reason for
      its existence in that picture: dark energy plays no role in the early
      universe, according to inflationary 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 of cyclic brane collisions actually resolves one of the
      longest-standing puzzles in cyclic models. The idea of a cyclic model
      isn't new: Friedmann and others pictured a cyclic model back in the
      1930's. They envisaged a finite universe which collapsed and bounced
      over and over again. But Richard Tolman soon pointed out that, actually,
      it wouldn't remove the problem of having to have a beginning. The reason
      those cyclic models didn't work is that every bounce makes more
      radiation and that means the universe has more stuff in it. According to
      Einstein's equations, this makes the universe bigger after each bounce,
      so that every cycle lasts longer than the one before it. But, tracing
      back to the past, the duration of each bounce gets shorter and shorter
      and the duration of the cycles shrinks to zero, meaning that the
      universe still had to begin a finite time ago. An eternal cyclic model
      was impossible, in the old framework. What is new about our model is
      that by employing dark energy and by having an infinite universe, which
      dilutes away the radiation and matter after every bang, you actually can
      have an eternal cyclic universe, which could last forever.

      John Brockman, Editor and Publisher
      Russell Weinberger, Associate Publisher
      contact: editor@...
      Copyright © 2007 By Edge Foundation, Inc
      All Rights Reserved.

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