Generalized Galileon cosmology

TL;DR

<3-5 sentence high-level summary>

Abstract

We study the cosmology of a generalized Galileon field $φ$ with five covariant Lagrangians in which $φ$ is replaced by general scalar functions $f_{i}(φ)$ (i=1,...,5). For these theories, the equations of motion remain at second-order in time derivatives. We restrict the functional forms of $f_{i}(φ)$ from the demand to obtain de Sitter solutions responsible for dark energy. There are two possible choices for power-law functions $f_{i}(φ)$, depending on whether the coupling $F(φ)$ with the Ricci scalar $R$ is independent of $φ$ or depends on $φ$. The former corresponds to the covariant Galileon theory that respects the Galilean symmetry in the Minkowski space-time. For generalized Galileon theories we derive the conditions for the avoidance of ghosts and Laplacian instabilities associated with scalar and tensor perturbations as well as the condition for the stability of de Sitter solutions. We also carry out detailed analytic and numerical study for the cosmological dynamics in those theories.

Figures (10)

• Figure 1: The viable parameter space in the $(\alpha,\beta)$ plane determined by the conditions (\ref{['eq:qcs']})-(\ref{['eq:qcs4']}), (\ref{['at3']}), (\ref{['at1d']}), and (\ref{['ctpo']}) along the tracker solution $r_{1}=1$DT2.
• Figure 2: Evolution of $\Omega_{{\rm DE}}$, $\Omega_{m}$, $\Omega_{r}$, and $w_{{\rm eff}}$ versus the redshift $z=1/a-1$ for $\alpha=0.3$, $\beta=0.14$, $\epsilon_{2}=1$, $\epsilon_{4}=1$, and $x_{{\rm dS}}=1$. We choose the initial conditions $r_{1}=1.500\times10^{-10}$, $r_{2}=2.667\times10^{-12}$, and $\Omega_{r}=0.999992$ at $z=3.63\times10^{8}$.
• Figure 3: Variation of $w_{{\rm DE}}$ versus $z$ for $\alpha=0.3$, $\beta=0.14$, $\epsilon_{2}=1$, $\epsilon_{4}=1$, and $x_{{\rm dS}}=1$ [cases (a)-(d)]. We choose four different initial conditions: (a) $r_{1}=5.000\times10^{-11}$, $r_{2}=8.000\times10^{-12}$, and $\Omega_{r}=0.999995$ at $z=5.89\times10^{8}$, (b) $r_{1}=1.500\times10^{-10}$, $r_{2}=2.667\times10^{-12}$, and $\Omega_{r}=0.999992$ at $z=3.63\times10^{8}$, (c) $r_{1}=5.000\times10^{-9}$, $r_{2}=8.000\times10^{-14}$, and $\Omega_{r}=0.99995$ at $z=6.72\times10^{7}$, (d) $r_{1}=5.000\times10^{-6}$, $r_{2}=8.000\times10^{-17}$, and $\Omega_{r}=0.9986$ at $z=2.04\times10^{6}$. The case (e) corresponds to $\alpha=-1.5$, $\beta=-0.9$, $\epsilon_{2}=1$, $\epsilon_{4}=-1$, and $x_{{\rm dS}}=1$ with initial conditions $r_{1}=1$, $r_{2}=10^{-60}$, and $\Omega_{r}=0.99999$ at $z=3.12\times10^{8}$.
• Figure 4: Evolution of $c_{S}^{2}$ versus $z$ for the same model parameters and initial conditions as given in Fig. \ref{['fig3']}.
• Figure 5: Evolution of $c_{T}^{2}$ versus $z$ for two cases: (a) $\alpha=0.3$, $\beta=0.14$, $\epsilon_{2}=1$, $\epsilon_{4}=1$, and $x_{{\rm dS}}=1$ with initial conditions $r_{1}=1.500\times10^{-10}$, $r_{2}=2.667\times10^{-12}$, and $\Omega_{r}=0.999992$ at $z=3.63\times10^{8}$, (b) $\alpha=-1.5$, $\beta=-0.9$, $\epsilon_{2}=1$, $\epsilon_{4}=-1$, and $x_{{\rm dS}}=1$ with initial conditions $r_{1}=1$, $r_{2}=10^{-60}$, and $\Omega_{r}=0.99999$ at $z=3.12\times10^{8}$, and (c) $\alpha=1.9$, $\beta=0.8$, $\epsilon_{2}=1$, $\epsilon_{4}=1$, and $x_{{\rm dS}}=1$ with initial conditions $r_{1}=10^{-5}$, $r_{2}=10^{-35}$, and $\Omega_{r}=0.99999$ at $z=3.12\times10^{8}$.