WHAT happened before the big bang? If some physicists are to be believed, the question is about as meaningless as asking what is north of the North Pole. But others don鈥檛 give up so easily.
According to two cosmologists, before the big bang there was another big bang. And, before that, another. 鈥淚f we鈥檙e right,鈥� says Neil Turok of the University of Cambridge, 鈥渢he big bang is but one in an infinite series of big bangs stretching back into the eternal past.鈥� And into the eternal future.
What Turok and his colleague Paul Steinhardt of Princeton University are advocating is a new version of an idea that dates back to the 1920s. Back then the Russian physicist Aleksandr Friedmann, the father of the big bang idea, realised that if the gravity of all the matter in the Universe is powerful enough, it could stop the expansion of the cosmos and turn it around. The Universe would then carry on contracting down to a 鈥渂ig crunch鈥�. If both expanding and re-collapsing universes are permitted, it鈥檚 a simple step to imagine the one changing seamlessly into the other. From the big crunch the Universe would bounce or rebound in a new big bang and the whole cycle would begin again.
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It was a popular idea until the 1960s, when Roger Penrose and Stephen Hawking scuppered it. Using Einstein鈥檚 general theory of relativity, which explains gravity as a warp in space-time, they showed that the big bang must have started in a singularity. A singularity is a point of infinite density and temperature, and it鈥檚 a big problem for anyone taking a hard look at the physics of the big bang. That鈥檚 because when everything in your equations goes to infinity, the equations are meaningless. Physics breaks down.
That doesn鈥檛 rule out a cyclic Universe. But the singularity is like an opaque curtain, preventing a view through the big bang to earlier times. With the singularity in the way, it makes no sense to talk about any continuous existence. If the Universe passes through a singularity, everything gets scrambled in the breakdown of physics. Nothing in the new universe can be affected by what happened before, so the previous cycle might as well not have existed. This was very discouraging, and people abandoned the idea of a cyclic Universe.
Its rebirth has come about because physicists are now convinced that Einstein鈥檚 theory of gravity breaks down at the big bang. It鈥檚 all because of quantum mechanics, which seems to impose a fundamental fuzziness on things. Quantum theory is usually applied to particles of matter, but many physicists think it must affect space-time too. The implication, they say, is that nothing can collapse to a point. Instead there is a minimum size for anything. The Universe may once have been pretty amazingly small, but it wasn鈥檛 infinitesimal, so its temperature and density weren鈥檛 infinite. 鈥淭he Universe may not, after all, have begun in a singularity,鈥� says Turok.
Over the past decade or so, this idea of space-time fuzziness has encouraged some physicists to think about what happened before the big bang. But on its own, it doesn鈥檛 prove there was anything, or give any hints about what that was.
Then last year, Turok and Steinhardt came up with the first part of their new theory. It builds on what are called brane-world scenarios, an outgrowth of the idea that extra dimensions in space are needed to explain the fundamental forces of nature. To explain why we experience only four of these dimensions, physicists have come up with the peculiar idea that the matter and non-gravitational forces of our Universe are stuck firmly to a four-dimensional island, or 鈥渂rane鈥�, floating within a higher-dimensional space. Whereas most of the extra space dimensions are supposed to be rolled up much smaller than an atom, it may be that one of them is relatively large, and we simply don鈥檛 see it because it is the exclusive realm of gravity (快猫短视频, 29 September 2001, p 26).
鈥淭he brane-world scenario suggests a possible explanation for the big bang,鈥� says Turok. Branes have their own mass, so a moving brane has an enormous amount of kinetic energy. And if our brane collided with another brane, this kinetic energy would be liberated, he thinks. 鈥淭his could have created the fireball of the big bang and ultimately all the matter we see in today鈥檚 galaxies and stars鈥�.
Turok and Steinhardt, who developed this idea with Justin Khoury of Princeton University and Burt Ovrut of the University of Pennsylvania, call it the 鈥渆kpyrotic鈥� universe, from the Greek for 鈥渂orn out of fire鈥�. They have thought through several colliding-brane scenarios, some involving three branes. But what they鈥檝e ended up with is a relatively simple scenario, in which two four-dimensional branes approach each other along a fifth dimension. Turok and his colleagues call them 鈥渂oundary branes鈥� because they form the ultimate boundaries of the Universe.
鈥淲hat we have done is explore what would happen if one brane passes through the other,鈥� he says. They found that the kinetic energy of the colliding branes is converted into heat energy within the branes when they collide with each other, effectively conjuring real particles out of the vacuum. What鈥檚 more, it naturally produces a Universe that is smooth on the largest scales, but has small lumps and bumps in it to turn into galaxies and galaxy clusters.
In this basic model, there鈥檚 still no cycle. Just a phase of approaching, empty branes before the big bang. Then Steinhardt and Turok asked themselves, what could pull the branes together before their collision? That something can only be the vacuum in between them, says Turok-because there鈥檚 nothing else there.
The vacuum, as it turns out, changes everything. 鈥淭he vacuum is like a spring between the plates, or branes,鈥� says Turok. Within our Universe it appears to be generating a repulsive force-the so-called cosmological constant-which is driving apart the galaxies. An attractive force would seem to be incompatible with that. But it turns out that even while there is a repulsion along the space dimensions inside each brane, there can also be an attraction between the branes along the fifth dimension.
Turok鈥檚 team is considering a number of possible mechanisms that might be behind this force. One suggestion is that there is a charge imbalance between the two branes that creates an attractive force between them. 鈥淲e don鈥檛 have a complete theory in which this could be calculated,鈥� Turok says. 鈥淥ur scenario is more of a guide as to how things could work.鈥�
He believes that today, the spring is still being stretched, but in the far future it will reach its maximum extension. Once that happens, the branes will begin to accelerate towards each other until they collide again.
So in the new picture, the oscillation occurs only along the fifth dimension. It happens like this: two branes are pulled together by the vacuum, and collide. Inside both branes a huge amount of energy is released, and the branes expand (if you can imagine an infinite rubber sheet being stretched out, it鈥檚 a little like that). We brane-bound creatures call this event the big bang.
As the branes expand and cool, matter and galaxies form. The galaxies drift apart and age. After a while, the gently repulsive vacuum inside the branes makes this expansion accelerate, so the galaxies fly apart faster still. The end looks bleak.
But meanwhile the two branes have moved apart and then been pulled back together by the attractive vacuum in between them. They rush towards a collision once more, and a new big bang overwhelms both universes.
So from the perspective of someone stuck on the brane, space-time just keeps on expanding, though the expansion is given repeated pushes by successive bangs-that is, brane collisions. In other words, from the off-brane perspective, we have something more like the traditional cyclic universe, yo-yoing back and forth. Meanwhile, from the brane perspective, we have an altogether different kind of cycle in an eternally expanding Universe.
This overcomes another big problem with the old-style cyclic universe. In each cycle, stars radiate heat into space, but these cyclic models involve closed universes, so each bang is hotter than its predecessor. Looking backwards in time, then, the cycles get progressively cooler. The inescapable conclusion is that the cycles must have begun at some time in the past. 鈥淏ut simply pushing the origin of the Universe back before the big bang is not very aesthetically pleasing,鈥� says Turok. 鈥淭his is another reason why the cyclic universe was seen as unsatisfactory.鈥�
The new cyclic universe avoids this problem. After the branes have passed through each other, the spring of the vacuum is in compression and causes the space of the branes to expand for a long time. That dilutes the heat from stars so that the patch of space that experiences each new bang has essentially the same temperature as the previous cycle. Consequently, all cycles are the same and the universe can have oscillated for ever. 鈥淪uch a universe is more aesthetically pleasing than a big bang universe since the question of what happened before is no longer a nagging problem,鈥� says Turok. 鈥淭he Universe has been around for ever. There was no beginning.鈥�
Stars, galaxies and life may therefore have existed in previous cycles of the Universe. But, if the cycles are all identical, wouldn鈥檛 such endless repetition be mind-numbingly dull? Turok and Steinhardt think not, because random events will change the details each time. You won鈥檛 get the same galaxies, planets and people each cycle. 鈥淛ust because the cycles repeat does not mean the events in each cycle are identical,鈥� says Turok.
More speculatively, he points out that the extra rolled-up dimensions might vary their sizes between cycles. The significance of this is that the fundamental forces are suspected to be manifestations of the sizes of these extra dimensions. 鈥淭he laws of physics could change from cycle to cycle,鈥� says Turok.
If the physical laws can change, they might be driven ever closer to some particular set, what physicists call an attractor. 鈥淚f we are lucky, we might find that the sizes of the extra dimensions home in on particular values,鈥� he says. 鈥淲e might then finally have an explanation for, say, the mass of the electron.鈥�
Obviously, both Turok and Steinhardt are excited by all these possibilities. Reactions from their colleagues are more mixed. 鈥淎t the moment I have an open mind on the ekpyrotic universe and its latest oscillating version,鈥� says Tom Kibble of Imperial College in London. 鈥淭here is no doubt an element of hype here, but I think they are right to be excited.鈥�
Their most outspoken opponent is Andrei Linde of Stanford University. 鈥淭his is mostly hype,鈥� he says. He thinks the whole model is unnecessarily complicated, like the epicycles that medieval astronomers used to describe the orbits of the planets in our Solar System.
But if Steinhardt and Turok are right after all, the future is less bleak and more dangerous than we have been told. Some cosmologists suggest that, because the galaxies are now accelerating apart, the future holds nothing but an ever emptier, cooler Universe. Now we have an alternative to look forward to: an almighty surprise, one day, when we and our fellow universe come together and collide once more in a spectacular finale. And who knows what will emerge from the fire?
Smoking gun
In the early 1960s a team led by Robert Dicke and Jim Peebles of Princeton University predicted that if the Universe is oscillating, it should be aglow with leftover heat radiation. That鈥檚 because if each cycle is to start out the same, something must destroy the heavy atoms built up inside stars in the previous cycle. In effect, there must be something wiping the slate clean-and that could only be heat.
Just such a relic, the cosmic microwave background radiation, was discovered soon afterwards. Dicke鈥檚 team was beaten to the discovery by Arno Penzias and Robert Wilson, two astronomers at Bell Labs in New Jersey, who for a while thought they鈥檇 picked up the faint microwave glow of pigeon droppings on their radio telescope.
But it turned out that you don鈥檛 need a cycling universe to explain it, only some kind of big bang. And ironically, Wilson was a supporter of the steady-state theory, in which the Universe had been in existence for ever. So for at least two years after their serendipitous discovery, neither Penzias nor Wilson publicly admitted what they had found was the smoking gun of the big bang.
Baptisms of fire
For 20 years, cosmology has been dominated by one theory that seems to explain all sorts of features of our Universe. The theory, called inflation, postulates a super-fast expansion in the Universe鈥檚 first split second. Now it has a rival: Neil Turok and Paul Steinhardt鈥檚 ekpyrotic model, the idea that the big bang was caused by two colliding universes.
Before inflation came along, nobody could explain why far-flung parts of today鈥檚 Universe are at roughly the same temperature. This was a puzzle, because in the simple big bang model these regions would never have been close enough to exchange light signals, so there would have been no chance for heat to flow between them and even out the temperatures.
The inflation theory is based on the idea that the Universe we see now came from a tiny region of the big bang. This region is so small that all parts of it could once have been in contact. In the ekpyrotic universe, this uniformity is explained in a similar way. It suggests the huge energy liberated in the collision generates a super-fast expansion just like inflation.
Both theories can also produce the seeds of galaxies. In inflation, quantum fluctuations produce some slightly denser regions, which are magnified by the violent expansion of inflation. In the ekpyrotic universe, the quantum fluctuations occur on the brane鈥檚 surface as little hummocks that are tugged on and magnified by the gravity of the approaching brane. 鈥淚t is possible to reproduce inflation鈥檚 greatest successes with a non-inflationary mechanism,鈥� says Steinhardt.
And according to Turok, the ekpyrotic universe does more. For instance, it provides a far more satisfactory framework for the kind of dark energy in the Universe that is pushing the galaxies apart. This vacuum energy is an integral part of ekpyrotic cosmology, and is required to bring the branes together and cause the big bang. In other cosmologies it just has to be conjured out of nothing.
鈥淚f the basic components hold true then I believe that we have finally devised a model that is more compelling than inflation, something I would never have thought to witness, let alone take part in,鈥� says Steinhardt. This is praise indeed since Steinhardt was one of the founding fathers of the theory of inflation.
Crucially, the ekpyrotic universe makes a prediction that inflation does not. And it鈥檚 a prediction that could be checked in the near future. Inflation magnifies all space-time fluctuations, so the Universe today should be awash in a sea of primordial gravity waves. These gravity waves should affect the temperature variations of the cosmic background radiation. The effect is subtle but might be detectable by the European Space Agency鈥檚 Planck space probe, due for launch in 2006.
The ekpyrotic universe predicts no such gravity waves, so if Planck and Planck鈥檚 successors see none, it will be some support for the idea. A sign, perhaps, that the Universe truly is ruled by an endless cycle.
- Further reading: 鈥淭he ekpyrotic universe: colliding branes and the origin of the hot big bang鈥� by Justin Khoury, Burt Ovrut, Paul Steinhardt and Neil Turok, ()
- 鈥淔rom big crunch to big bang鈥� by Justin Khoury, Burt Ovrut, Nathan Seiberg, Paul Steinhardt and Neil Turok ()
- Paul Steinhardt鈥檚 website is at Neil Turok鈥檚 website is at