Cosmological perturbations in $k$-essence model

Abstract

Subhorizon approximation is often used in cosmological perturbation theory. In this paper, however, it is shown that the subhorizon approximation is not always a good approximation at least in case of $k$-essence model. We also show that the sound speed given by $k$-essence model exerts a huge influence on the time evolution of the matter density perturbation, and the future observations could clarify the differences between the $\Lambda$CDM model and $k$-essence model.

21 pages, sentences and equations are corrected, conclusions are changed a little

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• On the necessity of the revisions for the cosmological matter perturbations from the general relativity

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