Self-Accelerating Universe in Galileon Cosmology

Abstract

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 non-linear 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 non-linear term, small fluctuations around the solution are stable on small scales. General relativity is recovered at early times and on small scales by this non-linear interaction via the Vainshtein mechanism. At late time, gravity is strongly modified and the background cosmology shows a phantom-like behaviour 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 $Λ$CDM cosmology.

Figures (4)

• Figure 1: The effective equation of state $w_{\rm eff}$ 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 $\frac{\delta}{a}$ as a function of redshift for different values of the BD parameter, $\omega$.