Large gravitational waves and the Lyth bound in multi-brane inflation

Abstract

Abstract. It is shown that multi M5-brane inflation in heterotic M-theory gives rise to a detectable gravitational wave power spectrum with tensor fraction r typically larger than the projected experimental sensitivity, r exp = 0.01. A measurable gravitational wave power spectrum entails a large inflationary energy scale and a superPlanckian inflaton variation. They present serious problems for particle theory model building resp. a reliable effective field theory description. These problems are eased or even absent in multi-brane inflation models and multi M5-brane inflation, in particular.

Axion inflation and gravity waves in string theory

“…However, to find the effective theory around such vacua is difficult [62]. 11 For example, a model in [66] based on heterotic M-theory predicts observable r. However, the problem Given both the results discussed in this paper and other work in this area, we must conclude that finding measurable IGW in stringy models remains problematic. Accordingly, should the next generation of CMB experiments measure appreciable B-mode polarization and thus infer a substantial scalar-tensor ratio, this would pose a significant challenge for string theory.…”

Inflation and String Theory

“…27 It was argued in [701] that the tensor-to-scalar ratio r can be large in multi-M5-brane inflation. 28 Vector bundle moduli are not necessarily stabilized, but are sometimes assumed to be absent.…”

Staggered multi-field inflation

“…This may seem artificial at first glance, but it is actually quite generic in multi-field models of inflation within string theory 3 . For instance, in the model of [8] (see also [20,27]) the inflatons are related to the distances between adjacent M5-branes. These branes are located along an orbifold S 1 /Z 2 and slowly separate from each other in the orbifold direction, corresponding to the inflationary phase in the effective four dimensional description.…”