Emergent universe and Genesis from the DHOST cosmology

Abstract

In this article, we present an emergent universe scenario that can be derived from DHOST cosmology. The universe starts asymptotically Minkowski in the far past just like the regular Galileon Genesis, but evolves to a radiation dominated period at the late stage, and therefore, the universe has a graceful exit which is absent in the regular Galileon Genesis. We analyze the behavior of cosmological perturbations and show that both the scalar and tensor modes are free from the gradient instability problem. We further analyze the primordial scalar spectrum generated in various situations and discuss whether a scale invariance can be achieved.

Improved DHOST Genesis

“…In Galileon Genesis, the behavior of Hubble parameter H ∝ (−t) −2α−1 determines the scalar spectrum index as n s = 5 − 2α [37,47,48,64,65]. We improve the action of our previous model [1] accordingly, and show that the improved Genesis model can produce a scale invariant power spectrum. Hence our improved model can serve as a possible candidate for the early universe, without invoking additional mechanisms.…”

“…To evade the possible puzzles, we recently propose an emergent universe model from the DHOST cosmology [1]. We modified the original action of Galileon Genesis such that the universe can gracefully exit from the quasi-Minkowskian state to a radiation dominated phase.…”

“…Throughout the paper, we take the sign of the metric as (+,-,-,-). The canonical kinetic term of the scalar field π is defined as X ≡ 1 2 ∇ µ π∇ µ π. The d'Alembert operator is ≡ ∇ µ ∇ µ .…”

“…Firstly, there are no-go theorems stating that no classically stable Genesis solution exists within the framework of Horndeski/Galileon theory [53][54][55][56]. 1 One possible ways out is to consider models which are strongly coupled in the asymptotic past/future [48,60], at the cost of potentially triggering the strong-coupling problem [61,62]. Besides, it is not very clear how such a Genesis scenario can be connected to the standard cosmology.…”

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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.

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