Waterfall stiff period can generate observable primordial gravitational waves

A toy-model is studied, which considers two flat directions meeting at an enhanced symmetry point such that they realise the usual hybrid inflation mechanism. The kinetic term of the waterfall field features a pole at its Planckian vacuum expectation value (VEV), as with alpha-attractors. Consequently, after the phase transition which terminates hybrid inflation, the waterfall field never rolls to its VEV. Instead, it drives a stiff period, where the barotropic parameter of the Universe w approximate to 1/2 results in a peak in the spectrum of primordial gravitational waves, which will be observable by the forthcoming LISA mission as well as by Advanced LIGO.

Automated summary

In this study, a toy-model is used to investigate the situation where two flat directions meet at an enhanced symmetry point and realize the usual hybrid inflation mechanism. The kinetic term of the waterfall field has a pole at its vacuum expectation value (VEV), similar to alpha-attractors. Therefore, after the phase transition that terminates hybrid inflation, the waterfall field does not roll to its VEV but instead drives a stiff period, resulting in a peak in the spectrum of primordial gravitational waves with a barotropic parameter w approximately equal to 1/2. This peak can be observed by the upcoming LISA mission and Advanced LIGO.