What kind of bang was the big bang?

• New Scientist 02 July 2012 
• by Amanda Gefter
• Magazine issue 2871.
• http://www.newscientist.com/article/mg21428710.100-what-kind-of-bang-was-the-big-bang.html?full=true

There's trouble at the start of time: the theory of cosmic inflation has got way out of control. Can quantum theory and holograms tame it?

"You know how sometimes you meet somebody and they're really nice, so you invite them over to your house and you keep talking with them and they keep telling you more and more cool stuff? But then at some point you're like, maybe we should we call it a day, but they just won't leave and they keep talking and as more stuff comes up it becomes more and more disturbing and you're like, just stop already? That's kind of what happened with inflation."

Max Tegmark's voice is animated as he talks about the idea underpinning our story of the universe's origins. The theory of cosmic inflation states that in the first fraction of a second after the big bang, the universe's fabric expanded faster than light. Without a trick like that, we have difficulty squaring some crucial features of today's cosmos with a universe that began as a hot, dense soup and has been growing and cooling since.

Yet Tegmark, a cosmologist at the Massachusetts Institute of Technology, is not the only person asking whether inflation has outstayed its welcome. For all its attractions, the theory has unpalatable consequences - so unpalatable that they threaten to undermine our entire understanding of the cosmos. Debate has been reopened on a question many thought had been settled: what kind of a bang was the big bang?

When inflation was proposed in 1980 by Alan Guth, then a young postdoc at Cornell University in Ithaca, New York, it was a godsend. Studies of the cosmic microwave background radiation were causing serious headaches among proponents of the big bang.

This radiation seems in itself to be a spectacular confirmation of the big bang theory. Some 380,000 years after the universe's origin in a "singularity" of unimaginable density and temperature, the cosmos had expanded and cooled enough for the first atoms to form. Photons of light pinged off these atoms and have been travelling in all directions through the cosmos ever since. Sitting in our particular spot some 14 billion years later, we see them as a background radiation that suffuses the sky at an almost uniform temperature of 2.7 kelvin.

But that success hides some problematic details. One is that you can measure the cosmic background 10 billion light years away in one direction and 10 billion light years in the other and still observe that pleasing uniformity. Run a simple story of a steadily expanding cosmos backwards, however, and it would take 20 billion years for these patches of space to meet - more than the age of the universe. In a simple big-bang universe, they were never close enough to equalise their temperatures, and that uniformity is a highly improbable coincidence.

Then there is the universe's geometry. Information encoded in the cosmic background shows space is extremely "flat": Euclidean geometry reigns supreme and parallel lines never meet. This flatness is also highly unlikely, given what we know about gravity and its compulsion to warp space. Again, a simple big bang cannot explain it.

Inflationary pressure

An inflationary big bang can. At the beginning of time, so the idea goes, all that existed was a quantum field called the inflaton. It found itself in a "false vacuum" - a state that is temporarily stable, but not the lowest, true-vacuum energy state. It is as if the inflaton were poised on a small plateau on a steep mountainside. All things being equal, it could rest there undisturbed, but the slightest jiggle would send it careering down towards the true vacuum below.

In the universe's first fraction of a second, all things were not equal. Random quantum fluctuations in energy provided just the jiggle to set the inflaton on its way. As it fell towards the true vacuum, it generated a kind of repulsive gravity that pushed the space out around it. The further it fell, the more it pushed until space was ballooning outward at a speed far greater than that of light.

This is physically all above board. Einstein's relativity forbids objects from travelling faster than light through space, but places no constraints on what space itself can do. And when the inflaton hit rock bottom, all the kinetic energy it acquired in its headlong descent poured into the universe, creating the matter and radiation that went on to form stars, planets and, eventually, us. All this happened in considerably less than the blink of an eye: in just 10^-33 seconds, the observable universe ballooned over 20 orders of magnitude in size, from a diameter about a billionth that of an atomic nucleus to a mini-cosmos about a centimetre across (see diagram).

Unwanted universes

In one fell swoop, inflation solved the big bang's problems. Those patches of sky no longer need a 20-billion-year rewind to have met and mingled: inflation gave them the shove-off they needed to ensure they arrived far faster at the far reaches of the cosmos. And that absurdly unlikely flatness is nothing of the sort: inflation makes the universe so large that any measurable region must look flat, the same way the ground at your feet looks flat even though Earth's surface is curved.

Inflation's munificence didn't stop there. By inflating tiny quantum fluctuations in the density of the cosmos to astronomical proportions, it produced a blueprint for how stuff clumped into ever-larger agglomerations of matter such as the galaxies we see today.

"That would have been the perfect point for inflation to bow, wait for applause and exit stage left," says Tegmark. But that didn't happen. Instead, inflation kept on predicting still more things - things that nobody wanted.

Things like other universes.

The problem is that once inflation starts, it is nearly impossible to stop. Even in the tiny pre-inflation cosmos, quantum fluctuations ensured that the inflaton field had different energies in different places - a bit like a mountain having many balls balanced precariously at different heights. As each one starts rolling, it kicks off the inflation of a different region of space, which races away from the others at speeds above that of light. Because no influence may travel faster than light, these mini-universes become completely detached from one another. As the inflaton continues its headlong descent in each one, more and more bits of space begin to bud off to independent existences: an infinite "multiverse" of universes is formed (see diagram).

This is not good news for our hopes for cosmic enlightenment. In a single universe, an underlying theory of physics might offer a prediction for how flat the universe should be, say, or for the value of dark energy, the mysterious entity that seems to be driving an accelerated expansion of the universe. Astronomers could then go out and test that prediction against observations.

That's not possible in an infinite multiverse: there are no definite predictions, only probabilities. Every conceivable value of dark energy or anything else will exist an infinite number of times among the infinite number of universes, and any universal theory of physics valid throughout the multiverse must reproduce all those values. That makes the odds of observing any particular value infinity divided by infinity: a nonsense that mathematicians call "undefined".

At first, cosmologists hoped to make sense of these infinities by taking a finite snapshot of the multiverse at some particular time, and then extrapolating the relative probabilities of various observations out to later and later times and an ever larger number of universes. Einstein stymied that approach. His relativity means there is no single clock ticking away the seconds of the multiverse, and there is an infinite number of ways to take snapshots of it, each giving a different set of probabilities. This "measure problem" destroys inflation's ability to make predictions about anything at all, including the smoothness of the cosmic background, the curvature of space, or anything else that made us believe in the theory in the first place.

"We thought that inflation predicted a smooth, flat universe," says Paul Steinhardt of Princeton University, a pioneer of inflation who has become a vocal detractor. "Instead, it predicts every possibility an infinite number of times. We're back to square one." Tegmark agrees: "Inflation has destroyed itself. It logically self-destructed."

Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena, is more circumspect. "Inflation is still the dominant paradigm," he says. "But we've become a lot less convinced that it's obviously true." That's not just because of the measure problem, he says. More basically, we don't know what an inflaton field is, why it was in a false vacuum and where it and its energy came from. Having an inflaton field perched so perfectly and precariously atop that mountainside seems no more likely than the flukes the idea was intended to explain. "If you pick a universe out of a hat, it's not going to be one that starts with inflation," says Carroll.

So if it isn't inflation that made the universe as we see it today, what did? Are there other sorts of big bang whose predictions square with our observations, but don't have inflation's unwanted side effects?

Perhaps. In 2001, Steinhardt was one of the first to suggest an alternative, together with his colleagues Justin Khoury, Burt Ovrut and Neil Turok. Their idea was to revisit our interpretation of the big bang. Rather than marking a singularity at the absolute beginning of space and time, it was just a recent event in a much longer history. The inspiration for this idea came from string theory, the most widespread approach to get Einstein's general theory of relativity, which best describes space and time, to play nicely with quantum mechanics, which best describes everything else.

String theory proposes that the various particles that make up matter and transmit forces are vibrations of tiny quantum-mechanical strings, including one that produces a "graviton", an as-yet-undetected particle that transmits gravity. It also predicts the existence of extra dimensions beyond the four of space and time we see.

Brane drain

This leads to the possibility that our 4D cosmos is situated on a "brane", a lower-dimensional object floating in a higher-dimensional space. In this picture, ours is not necessarily the only brane: two branes floating a microscopic distance away from each other can form the boundaries to a five-dimensional space in between, like two slices of bread bounding a sandwich. Steinhardt and his colleagues' idea was that every few trillion years or so neighbouring branes float together and touch, resulting in an explosive exchange of energy as the fifth dimension briefly disappears into a singularity, reappearing as the branes move apart again (see diagram). Our 4D brane receives a huge injection of energy in this collision, something we interpret as a big bang. It doesn't represent the absolute beginning of our space and time, however, but a reinvigoration of a cosmos with an eternal existence (Physical Review D, vol 64, p 123522).

This "cyclic model" of the big bang does much of what the inflationary big bang was invented to do. "You can solve problems that would have been intractable without inflation given only 14 billion years," says Steinhardt. The branes are essentially flat to begin with, so the flatness problem disappears. When the branes clash, they hit at almost the same time everywhere, so the energy that will form matter and radiation pours in almost uniformly, creating a nearly homogeneous cosmos. Small quantum energy fluctuations are all that are needed to give enough density variation to eventually seed galaxies. And because the idea needs no multiverse, the measure problem disappears.

If nothing else, the cyclic model introduced some competition into the big bang market. "It shows that you're not stuck with inflation - other ideas are possible," says Steinhardt. "But whether or not you like this particular alternative is a matter of taste."

Not everyone did. Models of the big bang that involve a singularity in our space-time, including the inflationary big bang, neatly excuse us from explaining what happened at the universe's beginning: the singularity is a place where the universe falls off the cliff of existence and the laws of physics break down. But in the cyclic model, we must explain how the fifth dimension survives its momentary lapse into a singularity. "To me, it doesn't seem to work," saysThomas Hertog of the Catholic University of Leuven (KUL) in Belgium, who worked on the idea for a couple of years. "The calculations suggest that the transition through the singularity is very unlikely."

The many clashes between branes that the model implies just compound the problem, says Carroll. "If you follow the cyclic universe backward in time, the conditions that you need become more and more special, or unlikely." The way to solve that problem is to postulate some kind of beginning that provided a special set of conditions - but that seems to defeat the object of the theory.

Many histories

So if neither inflation nor the cyclic universe can deliver a plausible beginning to the cosmos, what can? Carroll suggests a middle way. Steinhardt might have had the right idea in seeking an answer to the big-bang conundrum in the unification of general relativity and quantum mechanics, he says. But the answer could lie in using those ideas not to replace inflation, but to make it better.

"We use a half-assed version of quantum mechanics when we do cosmology," Carroll says. Inflation is a theory about space-time and gravity, so it is anchored in general relativity. It incorporates a few aspects of quantum physics, such as the uncertainty fluctuations that push the inflaton off the mountain ledge, but as we lack a sure-fire way of connecting relativity and quantum theory it remains a "semiclassical" theory. "Maybe that's not good enough," says Carroll.

But how can we put the quantum into cosmology? Three decades ago, just after inflation had burst on to the scene, physicists Stephen Hawking of the University of Cambridge and James Hartle of the University of California, Santa Barbara, made a stab at it. In quantum physics, when a particle travels from A to B it doesn't take a single path but can pass along two or more paths simultaneously, interfering with itself at the other end as if it were a wave. To find out which path we are most likely to observe, we must add together the quantum-mechanical "wave functions" encoding each possible path, working out how their individual peaks and troughs cancel and amplify each other. Encoded within this total wave function is everything we need to know about the quantum particle at B, including the probabilities for the outcomes of any measurement we choose to make.

Hawking and Hartle argued that a similar approach could be applied to the universe as a whole. Point B is the universe we see today. Looking back towards its origin, we can trace many valid histories of its expansion back towards a point - point A - where semiclassical physics breaks down and quantum space and time become so gnarled that the two cease to be clearly distinguished. This point is no longer a beginning of time in a singularity as in the standard inflationary cosmology, but a timeless point where the universe - or rather a superposition of all possible historical universes - pops into existence from nothing with all its laws of physics intact. Because of this lack of a clear beginning, Hawking and Hartle called it the no-boundary proposal.

Following the rules of quantum mechanics, they added up all the possible histories that began in a universe with no boundary and ended in the universe we see today. The idea is a kind of multiverse in reverse: a single universe with multiple histories (see diagram). The resulting wave function gets rid of the measure problem, as it encodes a unique set of probabilities for anything we might observe. And because the admissibility of the histories is determined by what we see in the cosmos today, problems such as the flatness of space-time or the homogeneity of the cosmic microwave background cease to be problems: instead, they are the inputs to the theory.

The no-boundary universe had its attractions for many physicists. "If it turns out the universe has to have an origin, I find this initial state to be a very plausible one," says Guth. And as far as Guth's theory of inflation is concerned, the no-boundary proposal turned up a pleasant surprise. As Hartle, Hawking and Hertog showed in 2008, although the theory was not dreamed up with inflation in mind, it crops up naturally along many paths the universe could have taken to get here (Physical Review Letters, vol 100, p 201301). "You can calculate the probability that inflation occurred, and it turns out that probability is very high," says Hertog.

That all sounds very neat, but there was still no reason to believe the no-boundary proposal was true. It was difficult to see where it fitted in to the sort of unifying theoretical constructs, such as string theory, which are needed to explain events in the early, high-energy days of the universe.

That might just have changed, thanks to one of the most profound ideas to come out of string theory in recent years: the holographic principle. This states that the physics of a 4D universe such as ours, including gravity, is mathematically equivalent to the physics on its 3D boundary without gravity. The implication is that the world we see around us is nothing but a holographic projection of information from the edge of reality. It sounds implausible, but the principle pops up not just in string theory, but in almost any approach to unifying relativity and quantum theory dreamed up so far.

Although the no-boundary proposal says that the universe has no boundary in the far past, it does give a boundary in the infinitely far future. By calculating the physics on this boundary, Hertog extracted the probabilities of all the possible universes that can emerge as its holographic projections. Remarkably, the probabilities for things like the homogeneity of the cosmic background or the amount of dark energy are the same as those that you get from the no-boundary wave function. This supplies a direct connection between string theory, the most popular route towards a theory of everything, and the no-boundary proposal, which produces inflation naturally.

"Originally the no-boundary wave function was sort of picked out of thin air," says Hertog. "But now we see that it lies at the heart of the holographic principle. This is very encouraging for inflation." Together with Hawking and Hartle, he set out the thinking in a paper posted online last month (arxiv.org/abs/1205.3807).

Cosmologists are still digesting the new proposal, with some questioning whether the assumptions it makes are justified. Guth says that as yet no one is sure about the validity of the specific holographic correspondence Hertog, Hawking and Hartle have employed. "It's certainly a worthwhile line of research, but what they are trying to establish is a very difficult thing," he says.

We are not yet there, at the true story of the beginning of the universe. But it seems undeniable now that working out whether inflation is the imposter doubters such as Tegmark believe it is will depend on finding some way to consistently apply quantum theory to the fabric of the universe. Only then will we truly know what kind of a bang the big bang was.

Testing, testing

Theorising about the big bang can only take us so far: we need observations and experiments to determine the true story of the universe's beginnings. Fortunately, a number are in progress that might help us discriminate between rival theories (see main story).

One is the European Space Agency's Planck satellite. This telescope blasted off in May 2009 to make the most detailed measurements yet of the cosmic microwave background from an orbit 1.5 million kilometres above Earth, and its first cosmological dataset is scheduled to be released next year. If the extraordinarily powerful burst of early expansion implied by the theory of cosmic inflation really did occur after the big bang, it would have sent ripples through the fabric of space-time. These gravity waves would have had subtle effects on the polarisation of the microwave radiation that makes up the background - and Planck should be capable of spotting them. "That would be very exciting," says Alan Guth, the physicist who originally proposed the theory of inflation.

Meanwhile, Celine Boehm of the University of Durham, UK, and her colleagues proposed last month that the high-energy collisions at theLarge Hadron Collider at CERN near Geneva, Switzerland, could be used to test whether the inflaton field and its associated particle, the inflaton, existed and if so what it looked like. The particle itself is far too massive to be made at the LHC, but its existence might influence the decay of other, less weighty, particles - and that could be tested (arxiv.org/abs/1205.2815).

Amanda Gefter is a consultant for New Scientist based in Boston