MG15 - Talk detail

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The Gleyzes-Langlois-Piazza-Vernizzi (GLPV) theories up to quartic order are the general scheme of scalar-tensor theories allowing the possibility for realizing the tensor propagation speed c_t equivalent to 1 on the isotropic cosmological background. We propose a dark energy model in which the late-time cosmic acceleration occurs by a simple k-essence Lagrangian analogous to the ghost condensate with cubic and quartic Galileons in the framework of GLPV theories. We show that a wide variety of the variation of the dark energy equation of state w including the entry to the region w<−1 can be realized without violating conditions for the absence of ghosts and Laplacian instabilities. The approach to the tracker equation of state w=−2 during the matter era, which is disfavored by observational data, can be avoided by the existence of a quadratic k-essence Lagrangian. We study the evolution of nonrelativistic matter perturbations for the model c_t=1 and show that the two quantities mu and Sigma, which are related to the Newtonian and weak lensing gravitational potentials respectively, are practically equivalent to each other. For the case in which the deviation of w from −1 is significant at a later cosmological epoch, the values of mu and Sigma tend to be larger at low redshifts.

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