What if our universe didn’t emerge from nothing, but is a recycled version of one that went before? Anil Ananthaswamy investigates

went on to improve on the idea. Singh and Pawlowski developed computer simulations of the universe according to LQC, and that’s From big bang when they saw the universe bounce. When they ran time backwards, instead of becoming infinitely dense at the big bang, the universe stopped collapsing and reversed direction. The big bang singularity had truly disappeared to big bounce (Physical Review Letters, vol 96, p 141301). But the celebration was short-lived. When the team used LQC to look at the behaviour of our universe long after expansion began, ABHAY ASHTEKAR remembers his Ashtekar rewrote the equations of general they were in for a shock – it started to collapse, reaction the first time he saw the relativity in a quantum-mechanical challenging everything we know about the ● universe bounce. “I was taken aback,” framework. Together with theoretical cosmos. “This was a complete departure from he says. He was watching a simulation of physicists Lee Smolin and Carlo Rovelli, general relativity,” says Singh, who is now at the universe rewind towards the big bang. Ashtekar later used this framework to show the Perimeter Institute for Theoretical Physics Mostly the universe behaved as expected, that the fabric of space-time is woven from in Waterloo, Canada. “It was blatantly wrong.” becoming smaller and denser as the galaxies loops of gravitational field lines. Zoom out Ashtekar took it hard. “I was pretty converged. But then, instead of reaching far enough and space appears smooth and depressed,” he says. “It didn’t bode well the big bang “singularity”, the universe unbroken, but a closer look reveals that space for LQC.” However, after more feverish bounced and started expanding again. comes in indivisible chunks, or quanta, mathematics, Ashtekar, Singh and Pawlowski What on earth was happening? 10-35 square metres in size. solved the problem. Early versions of the Ashtekar wanted to be sure of what he was In 2000, Martin Bojowald, then a postdoc theory described the evolution of the universe seeing, so he asked his colleagues to sit on the with Ashtekar at the Pennsylvania State in terms of quanta of area, but a closer look result for six months before publishing it in University in University Park, used loop revealed a subtle error. Ashtekar, Singh 2006. And no wonder. The theory that the quantum gravity to create a simple model and Pawlowski corrected this and found recycled universe was based on, called loop of the universe. LQC was born. that the calculations now involved tiny quantum cosmology (LQC), had managed to Bojowald’s major realisation was that volumes of space. illuminate the very birth of the universe – unlike general relativity, the physics of LQC It made a crucial difference. Now the something even Einstein’s general theory did not break down at the big bang. universe according to LQC agreed brilliantly of relativity fails to do. with general relativity when expansion was LQC has been tantalising physicists since well advanced, while still eliminating the 2003 with the idea that our universe could “Einstein’s relativity fails singularity at the big bang. Rovelli, based at conceivably have emerged from the collapse the University of the Mediterranean in of a previous universe. Now the theory is poised to explain the very birth Marseille, France, was impressed. “This to make predictions we can actually test. If they was a very big deal,” he says. “Everyone are verified, the big bang will give way to a big of the universe” had hoped that once we learned to treat the bounce and we will finally know the quantum quantum universe correctly, the big bang structure of space-time. Instead of a universe Cosmologists dread the singularity because singularity would disappear. But it had that emerged from a point of infinite density, at this point gravity becomes infinite, along never happened before.” we will have one that recycles, possibly with the temperature and density of the Physicist Claus Kiefer at the University through an eternal series of expansions and universe. As its equations cannot cope with of Cologne in Germany, who has written contractions, with no beginning and no end. such infinities, general relativity fails to extensively about the subject, agrees. “It is LQC is in fact the first tangible application describe what happens at the big bang. really a new perspective on how we can view of another theory called loop quantum gravity, Bojowald’s work showed how to avoid the hated the early universe,” he says. “Now, you have a which cunningly combines Einstein’s theory singularity, albeit mathematically. “I was very theory that can give you a natural explanation of gravity with quantum mechanics. We need impressed by it,” says Ashtekar, “and still am.” for a singularity-free universe.” He adds that theories like this to work out what happens Jerzy Lewandowski of the University of while competing theories of quantum gravity, when microscopic volumes experience an Warsaw in Poland, along with Bojowald, such as string theory, have their own insights extreme gravitational force, as happened near Ashtekar and two more of his postdocs, to offer cosmology, none of these theories has the big bang, for example. In the mid 1980s, Parampreet Singh and Tomasz Pawlowski, fully embraced quantum mechanics.

32 | NewScientist | 13 December 2008 www.newscientist.com 33rpm for dutch uncle www.newscientist.com

13 December 2008 | NewScientist | Cover story | 33 If LQC turns out to be right, our universe emerged from a pre-existing universe that had been expanding before contracting due to gravity. As all the matter squeezed into a microscopic volume, this universe approached the so-called Planck density, 5.1 × 1096 kilograms per cubic metre. At this stage, it stopped contracting and rebounded, giving us our universe. “You cannot reach the Planck density. It is Will our universe bounce? forbidden by theory,” says Singh. According to Bojowald, that is because an extraordinary repulsive force develops in the fabric of space- According to the big bounce picture that view. They show not just that together. A question mark also time at densities equivalent to compressing formulated by theoretical physicist the universe is expanding, but also hangs over the universe’s matter a trillion solar masses down to the size of Abhay Ashtekar and others, the that the expansion is speeding up and energy density, which we a proton. At this point, the quanta of space- cosmos grew from the collapse of due to a mysterious repulsive force have not measured with sufficient time cannot be squeezed any further. The a pre-existing universe. Will the that cosmologists call “dark energy”. accuracy to be sure that the universe compressed space-time reacts by exerting same fate await us? So if the universe fails to contract, has will not eventually stop expanding. an outward force strong enough to repulse It depends. We used to think it already bounced its last bounce? If it turns out to be a smidgen gravity. This momentary act of repulsion that the universe was dominated Perhaps not. Cosmologists are greater than current observations, causes the universe to rebound. From then on, by the gravity of its stars and other still very much in the dark about then it is a recipe for cosmic collapse. the universe keeps expanding because of the matter: either the universe is dense dark energy. Some theoretical According to the big bounce, inertia of the big bounce. Nothing can slow it enough for gravity to halt the models speculate that the nature in both scenarios the universe will down – except gravity. expansion from the big bang of dark energy could change over eventually collapse until it reaches LQC also illuminates another mysterious and pull everything back, or else it time, switching from a repulsive the highest density allowed by phase of our universe. In classical cosmology, isn’t, in which case the expansion to an attractive force that behaves the theory. At this point, the a phenomenon called inflation caused the would carry on forever. However, much like gravity. If that happens, universe will rebound and begin universe to expand at incredible speed in the observations of distant supernovae the universe will stop expanding expanding again – the ultimate first fractions of a second after the big bang. in the past 10 years have challenged and the galaxies will begin to rush in cosmic recycling. This inflationary phase is needed to explain why the temperature of faraway regions of the universe is almost identical, even though heat should not have had time to spread that far – the so-called horizon problem. It also explains why the universe is so finely balanced between expanding forever and contracting eventually under gravity – the flatness problem. Cosmologists invoke a particle called the inflaton to make inflation happen, but precious little is known about it.

Cosmic recall More importantly, even less is known about the pre-inflationary universe. Cosmologists have always assumed that they could ignore quantum effects and regard space-time as smooth at the onset of inflation, as general relativity requires. This had always been an educated guess – until now. LQC shows that at the time inflation begins, space-time can be treated as smooth. “This is not an assumption any more,” says Singh. “It’s actually a prediction from loop quantum cosmology.” The models developed by Ashtekar, Singh, Bojowald and Pawlowski represent an enormous step forward. This is the first time that a theory is able to make predictions about what was happening prior to inflation, while correctly predicting what happens post- inflation. “To do both of these things at the utch u n cle d utch same time has been difficult,” says Ashtekar.

33rpm for 33rpm If the universe we inhabit emerged from a

34 | NewScientist | 13 December 2008 www.newscientist.com previous cosmos, can we know something though the CMB originated 370,000 years about the universe that preceded ours? LQC after the big bang, its seeds were laid out simulations show that it too would have had much earlier, says Bojowald. “That could stars and galaxies. But opinions differ when it be a period when quantum gravity effects comes to the quantum phase just before and might play a role.” after the big bounce, when it is impossible to Bojowald has discovered that such effects pin down the volume of the universe due to would have dominated when, according to quantum fluctuations. Bojowald’s calculations LQC, the universe went through a short phase show that some of the information about the of accelerated expansion before the onset of earlier universe is wiped out as it goes through inflation. Dubbed superinflation, it occurred the big bounce. In other words, there is no due to the immense repulsive forces of the “cosmic recall” (Nature Physics, vol 3, p 523). high-density quantum universe rather than in the fabric of space-time would have In contrast, another detailed analysis done the presence of inflatons. Exactly how this polarised the CMB, though the effect is too by Singh and Alejandro Corichi, of the phase might affect the CMB is unclear, but faint for today’s instruments to detect. Things already there are hints that LQC might predict might change next year, however, when the something different from classical cosmology. European Space Agency launches the Planck “The pre-existing universe “This is what we are going to work on in the satellite, promising the most detailed view of next two years. We are going to find robust the microwave background to date. Copeland’s was squeezed into a predictions,” says Singh. work suggests that superinflation would Meanwhile, Ed Copeland of the University suppress the production of gravitational microscopic volume” of Nottingham, UK, and his colleagues have waves at cosmological scales, and that there shown that superinflation can produce the would be no such imprint in the CMB. “If you Autonomous National University of Mexico kind of quantum fluctuations in the fabric of do detect them, it would probably count in Michoacán, suggests otherwise (Physical space-time that eventually became seeds for against LQC,” he says. Review Letters, vol 100, p 161302). the formation of galaxies and clusters of Kiefer cautions that all the predictions Ashtekar likens the spirited spat among galaxies. This suggests that superinflation of LQC are subject to one big caveat. The his former postdocs and students to watching might make inflation unnecessary, thus predictions of classical cosmology come his children squabble. “It’s much ado about removing what has essentially always been an from solving the equations of general nothing,” he says. Though arguments about add-on to standard cosmological theory. It is relativity, albeit with certain simplifying the universe possibly having a cosmic recall early days for superinflation, though, because assumptions about the universe. Ideally, LQC may be of philosophical interest, they are it cannot yet solve the horizon and flatness should be put on the same footing – all its premature. “We should be worrying about problems that inflation so elegantly resolves. equations should be derived from loop making contact with experiments today.” Copeland says that future experiments quantum gravity. Instead, Bojowald and That day may be near. The researchers’ might reveal whether our universe underwent others obtained LQC by starting with an first target is the cosmic microwave inflation or superinflation by looking for idealised universe derived from general background (CMB), a radiation released long a pattern of gravitational waves that only relativity and then using techniques from after the universe’s quantum phase. Even inflation could have created. These ripples loop quantum gravity to quantise gravity in the model. “From a physicist’s point of view, it J>;
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www.newscientist.com 13 December 2008 | NewScientist | 35