Reconstruction method in the kinetic gravity braiding theory with shift-symmetric

TL;DR

This paper introduces a reconstruction method for flat FRW spacetime within shift-symmetric kinetic gravity braiding theory, enabling the derivation of model functions and analysis of stability for various cosmological scenarios.

Contribution

The paper develops a novel reconstruction approach for shift-symmetric KGB models, allowing for dynamic solutions and stability checks without initial fluid terms.

Findings

Reconstructed functions $G_2$, $G_3$ for specific cosmological evolutions.

Identified conditions for ghost and Laplacian stability in the models.

Provided examples including dark energy-dark matter unification and post-inflationary transition.

Abstract

We present a reconstruction method for flat Friedman-Robertson-Walker (FRW) spacetime in a subclass of Horndeski theory -- specifically shiftsymmetric, the kinetic gravity braiding (KGB) theory with a non-vanishing conserved current. Choosing the form of the Hubble parameter and kinetic density $X$, we restore the functions $G_2$, $G_3$ of the KGB model. In order to determine whether the model is free of ghosts and Laplacian instabilities and thus cosmologically viable, two conditions related to scalar perturbations are checked. Initially, the Lagrangian does not include the term that describes, for example, the perfect fluid with the EoS parameter $w\neq -1$. This fluid can provide a dynamic solution $H(t)$, $X(t)$. In the presented method, dynamic solutions are provided by a nonzero scalar charge associated with the shift symmetry $\phi\rightarrow\phi+\phi_0$. Reconstruction examples are given for models: an perfect fluid, a unified description dark energy-dark matter, a post-inflationary transition to the radiation-dominated phase.