Can string theory accommodate inflation?
点击量： 时间：2017-12-23 08:02:11
By David Shiga String theory is having trouble producing inflation – the rapid expansion of space thought to have occurred in the early universe – at least in some of the theory’s simplest incarnations, according to a new study. The work suggests squaring string theory with the well-accepted notion of inflation will be challenging at best – and some even say that one or both theories may have to be abandoned. String theory is a leading contender for the “theory of everything”, which would unify all the forces of physics in one framework. Though there are many different versions of string theory, all posit that elementary particles are actually tiny vibrating strings, and that the universe contains extra spatial dimensions beyond the three that we can see. Now, a new study suggests it may be difficult to reconcile string theory with the widely accepted theory of inflation, which explains several key cosmological observations – such as why the universe appears to have the same properties in whichever direction astronomers look. The study was carried out by a team of researchers led by Mark Hertzberg of MIT in Cambridge, US. The team tried to produce inflation in three versions of string theory in which the extra dimensions are shaped like a doughnut – the simplest possibility. But they found that the conditions needed for inflation appear to be impossible to achieve in these simple versions. Many inflation scenarios have been proposed within more complex versions of string theory. But Hertzberg says all of them leave some room for doubt, because not all of the details that underpin them have been verified with complete calculations, though progress has been made in this direction. He stresses that it could still be possible to robustly produce inflation within string theory. “I don’t want to run around saying, ‘Oh no, there’s no inflation in string theory,'” he told New Scientist. “But since we haven’t performed a complete search involving more complicated extra dimensions, we don’t know.” Paul Steinhardt of Princeton University in New Jersey, US, who helped to pioneer the theory of inflation, says the findings are in agreement with work that he and others have done using other versions of string theory. “I think the fact that it is difficult to combine inflation and string theory is very interesting,” he told New Scientist. “It could mean they are completely incompatible, which would force us to abandon at least one of them.” He nevertheless cautions that there is still a chance that someone may find a robust way to achieve inflation in string theory in the future. Another inflation pioneer, Andrei Linde of Stanford University in California, US, is more critical of the work, however. He says the results only apply to a class of string theory versions called type 2a, which are irrelevant to the real universe because they have been shown to be incompatible with dark energy, the mysterious force causing the universe’s expansion to accelerate. “Why would you even try to describe inflation in a theory that cannot describe our universe?” Linde says. Max Tegmark of MIT, a member of Hertzberg’s team, counters that versions of type 2a string theory should not be written off just yet. That is because neither it nor another type of string theory that does account for dark energy gives a perfect description of the real universe, he argues, since they predict that the universe should be filled with exotic “supersymmetric” versions of familiar particles like electrons – which are not actually observed. More sophisticated models of type 2a theories may yet be able to support dark energy, he says. String theorist Gary Shiu of the University of Wisconsin in Madison, US, says the fact that not every version of string theory is compatible with inflation is good news, because it will help narrow down which versions of the theory should be further investigated from the vast number of possibilities. “Constructing a universe consistent with observations is not an easy act,” he told New Scientist. Journal reference: Physical Review D (in press) More on these topics: