Waterfall "kination" can generate observable primordial gravitational waves

Abstract

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 α-attractors. Consequently, after the phase transition which terminates hybrid inflation, the waterfall field never rolls to its VEV. Instead, it drives a period of "kination", where the stiff barotropic parameter of the Universe w ≈ 1/2 results in a mild spike in the spectrum of primordial gravitational waves, which will be observable by the forthcoming LISA mission.

E- and T-model hybrid inflation

“…Moreover, the R symmetry preserved by the superpotential [10] restricts further the possible forms of K , if it is imposed also to it. Modification of the present model in order to generate gravitational waves compatible with the NANOGrav reported signal [19,20], via a late production of cosmic strings [17,18] or via a subcritical stage of fast-roll inflation [22] could be another interesting target for future analysis.…”

“…For both models, K W parameterizes flat manifold -contrary to the setting in Refs. [22,40] -whereas the geometry induced by K I is hyperbolic, i.e., the curvature of the moduli space is negative. In particular, the scalar curvatures associated with K W and K I respectively are R W = 0 and…”

“…Between them, the metastable cosmic strings are currently of special interest since they may decay generating a stochastic background of gravitational waves [17,18] and interpreting, thereby, recent results [19,20] -for other sources of gravitational waves' production after the end of HI see Refs. [21,22].…”