Self-accelerating universe in Galileon cosmology

We present a cosmological model with a solution that self-accelerates at late times without signs of ghost instabilities on small scales. The model is a natural extension of the Brans-Dicke (BD) theory including a nonlinear derivative interaction, which appears in a theory with the Galilean shift symmetry. The existence of the self-accelerating universe requires a negative BD parameter but, thanks to the nonlinear term, small fluctuations around the solution are stable on small scales. General relativity is recovered at early times and on small scales by this nonlinear interaction via the Vainshtein mechanism. At late time, gravity is strongly modified and the background cosmology shows a phantomlike behavior and the growth rate of structure formation is enhanced. Thus this model leaves distinct signatures in cosmological observations and it can be distinguished from standard LCDM cosmology.

Figure 1

The effective equation of state $w_{\mathrm{eff}}$ as a function of redshift for various values of the Brans-Dicke parameter.

Figure 2

The comoving distance $r(z)$ as a function of redshift for different values of $\omega$.

Figure 3

Plot of $c_s$ as a function of redshift for various values of the Brans-Dicke parameter. As can be seen during the transition to the accelerating phase the perturbations go superluminal for $\omega \lesssim -190$.

Figure 4

The growth rate $\delta /a$ as a function of redshift for different values of the BD parameter, $\omega$.