On the ghost-induced instability on de Sitter background

摘要

It is known that the perturbative instability of tensor excitations in higherderivative gravity may not take place if the initial frequency of thegravitational waves are below the Planck threshold. One can assume that this isa natural requirement if the cosmological background is sufficiently mild,since in this case the situation is qualitatively close to the freegravitational wave in flat space. Here, we explore the opposite situation andconsider the effect of a very far from Minkowski radiation-dominated or deSitter cosmological background with a large Hubble rate, e.g., typical of aninflationary period. It turns out that, then, for initial Planckian or eventrans-Planckian frequencies, the instability is rapidly suppressed by the veryfast expansion of the universe.