Thu May 31, 2007 11:19 pm (PST)

http://www.edge.org/3rd_culture/turok07/turok07_index.html

"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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