A. R. Gomes, Luca Amendola
We find the most general coupled scalar field Lagrangian linear in $\Box\phi$ and with a general kinetic term that contains cosmological scaling solutions, i.e. solutions on which the ratio of matter to field density and the equation of state remains constant. Scaling solutions of this kind may help solving the coincidence problem since in this case the presently observed ratio of matter to dark energy does not depend on initial conditions but rather on the theoretical parameters. Extending previous results we find that it is impossible to join in a single solution a matter era and the scaling attractor. This is an additional step towards finding the most general scaling Lagrangian within the Horndeski class, i.e. general scalar-tensor models with second order equations of motion.
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http://arxiv.org/abs/1306.3593
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