Anisotropy screening in Horndeski cosmologies

Abstract : We consider anisotropic cosmologies in a particular shift-symmetric Horndeski theory containing the $G^{\mu\nu}\partial_\mu\phi \partial_\nu\phi$ coupling, where $G^{\mu\nu}$ is the Einstein tensor. This theory admits stable in the future self-accelerating cosmologies whose tensor perturbations propagate with the velocity very close to the speed of light such that the theory agrees with the gravity wave observations. Surprisingly, we find that the anisotropies within the Bianchi I homogeneous spacetime model are screened at early time by the scalar charge, whereas at late times they are damped in the usual way, hence they are damped at all times. Therefore, contrary to what one would normally expect, the initial state of the universe close to the singularity cannot be anisotropic, although it seems it should be inhomogeneous. At the same time, we find that in the Bianchi IX case the universe can be strongly anisotropic close to the initial singularity even in the presence of a scalar charge.
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https://hal.archives-ouvertes.fr/hal-02458704
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Submitted on : Tuesday, January 28, 2020 - 9:15:39 PM
Last modification on : Tuesday, February 18, 2020 - 9:15:58 PM

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Alexei A. Starobinsky, Sergey V. Sushkov, Mikhail S. Volkov. Anisotropy screening in Horndeski cosmologies. 2020. ⟨hal-02458704⟩

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