Cosmological data favor Galileon ghost condensate over $\Lambda$CDM

Abstract

We place observational constraints on the Galileon ghost condensate model, a dark energy proposal in cubic-order Horndeski theories consistent with the gravitational-wave event GW170817. The model extends the covariant Galileon by taking an additional higher-order field derivative $X^2$ into account. This allows for the dark energy equation of state $w_{\rm DE}$ to access the region $-2<w_{\rm DE}<-1$ without ghosts. Indeed, this peculiar evolution of $w_{\rm DE}$ is favored over that of the cosmological constant $\Lambda$ from the joint data analysis of cosmic microwave background (CMB) radiation, baryonic acoustic oscillations (BAOs), supernovae type Ia (SNIa) and redshift-space distortions (RSDs). Furthermore, our model exhibits a better compatibility with the CMB data over the $\Lambda$-cold-dark-matter ($\Lambda$CDM) model by suppressing large-scale temperature anisotropies. The CMB temperature and polarization data lead to an estimation for today's Hubble parameter $H_0$ consistent with its direct measurements at 2$\sigma$. We perform a model selection analysis by using several methods and find a statistically significant preference of the Galileon ghost condensate model over $\Lambda$CDM.