Abstract
We show that starting from initial conditions with stable perturbations, evolution of a galileon scalar field results in the appearance of a ghost later on. To demonstrate this, we consider a theory with k-essence and cubic galileon Lagrangians on a fixed Minkowski background. Explicit analytical solutions of simple waves are constructed, on top of which a healthy scalar degree of freedom is shown to be converted onto a ghost. Such a transformation is smooth and moreover perturbations remain hyperbolic all the time (until a caustic forms). We discuss a relation between the ghost and the appearance of closed causal curves for such solutions.
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Babichev, E. Emergence of ghosts in Horndeski theory. J. High Energ. Phys. 2020, 38 (2020). https://doi.org/10.1007/JHEP07(2020)038
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DOI: https://doi.org/10.1007/JHEP07(2020)038