Improved DHOST Genesis

Abstract

We improve the DHOST Genesis proposed in [1], such that the near scale invariant scalar power spectrum can be generated from the model itself, without invoking extra mechanism like a string gas. Besides, the superluminality problem of scalar perturbation plagued in [1] can be rescued by choosing proper DHOST action.

Perturbative unitarity and NEC violation in genesis cosmology

“…The "no-go" theorem proved in [56,57] indicates that pathological instabilities of perturbations appear in either the NEC-violating period or sooner or later in spatial flat nonsingular cosmology constructed by Horndeski theory [58][59][60], see also [61][62][63][64][65][66][67][68][69]. It is then demonstrated explicitly with the EFT method [70][71][72][73][74] that fully stable NEC-violating nonsingular cosmological models can be constructed in "beyond Horndeski" theories [75,76], see [77][78][79][80][81][82][83][84][85][86][87][88][89][90] for later developments. Notably, the physics represented by higher derivative "beyond Horndeski" operators plays an essential role in realizing fully stable NEC-violating nonsingular cosmology.…”

Perturbative unitarity and NEC violation in genesis cosmology

“…see [85][86][87][88] for recent studies. It can be checked that the action (3.4) still belongs to the "beyond Horndeski" (GLPV) theory [74,75], 2 which is free from the Ostrogradsky instabilities.…”

“…If we assume that the universe eventually enters the standard hot Big Bang expansion, during which γ = H > 0, we will find γ cross 0 at some time t γ after the end of the genesis phase. 3 In order to avoid the gradient instability induced by the γ−crossing around t γ , we should carefully design the behavior of Q m4 , see e.g., the condition (13) of [71], see also [88] for an example of the numerical simulation. In this paper, instead of handling the explicit constructions of γ and Q m4 , we will focus our discussion on the behavior of genesis in the limit |t| M −1 for our purpose.…”

“…The "no-go" theorem proved in [55,56] indicates that pathological instabilities of perturbations appear in either the NEC-violating period or sooner or later in spatial flat nonsingular cosmology constructed by Horndeski theory [57][58][59], see also [60][61][62][63][64][65][66][67][68]. It is then demonstrated explicitly with the EFT method [69][70][71][72][73] that fully stable NEC-violating nonsingular cosmological models can be constructed in "beyond Horndeski" theories [74,75], see [76][77][78][79][80][81][82][83][84][85][86][87][88][89] for later developments. Notably, the physics represented by higher derivative "beyond Horndeski" operators plays an essential role in realizing fully stable NEC-violating nonsingular cosmology.…”

Generating enhanced primordial GWs during inflation with intermittent violation of NEC and diminishment of GW propagating speed

“…Primordial null energy condition (NEC) violation (i.e., T µν k µ k ν < 0, where T µν is the energy-momentum tensor and k µ is any null vector) could occur during the inflationary era [25][26][27][28][29] (see also [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] for the Genesis scenario) and generate GW background with rich features in the power spectrum, see e.g. [22] for a Great Wall-like spectrum.…”