Credit: GETTY

It is much touted that string theory — although offering an excess of solutions, and thereby possible universes — has yet to make a testable prediction. The energy scales involved are so huge that no useful laboratory experiment can be envisaged. Even the fact that, at low energies, any such theory would have to reproduce the well-tested standard model of particle physics proves little help in trying to identify a viable string-theory solution in the landscape of myriad possibilities.

But any such solution would also have to reproduce the observed cosmology of our Universe: now that we have some knowledge of the early history of the Universe, particularly through observations of the cosmic background radiation, perhaps here there are better clues to be found. Writing in Physical Review D, Mark P. Hertzberg and colleagues consider the case of inflation and whether string theory can indeed account for this period of exponential expansion, believed to have occurred in the very earliest stages of the Universe's development (Phys. Rev. D 76, 103521; 2007).

First, Hertzberg et al. acknowledge that there is a significant language barrier to communication between string theorists on one side and astrophysicists on the other. To aid astrophysicist readers, they include a short dictionary of 'stringlish', translating string-theory jargon such as 'orientifold p plane' and 'Kähler potential' into phrases that astrophysicists might be a little more comfortable with.

The authors investigate three string-theory models as examples, checking for the necessary conditions to achieve 'slow-roll' inflation. Although a full computation in each case was unfeasible, so they do not claim the results as hard proof, Hertzberg et al. nevertheless find that in none of the models does inflation occur.

All is not lost, however. This study indicates that, as the authors put it, “slow-roll inflation may be a rare and delicate phenomenon in the landscape”, but it also signals that there may be better places to look — in other regions of the landscape where the obstacles to inflation found so far might not exist. String theorists and astrophysicists may proceed together, stringlish dictionary in hand.