Dark-energy instabilities induced by gravitational waves

DOI10.1088/1475-7516/2020/05/002zbMATH Open1490.83041arXiv1910.14035OpenAlexW3105997240MaRDI QIDQ5069111

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Publication date: 8 April 2022

Published in: (Search for Journal in Brave)

Abstract: We point out that dark-energy perturbations may become unstable in the presence of a gravitational wave of sufficiently large amplitude. We study this effect for the cubic Horndeski operator (braiding), proportional to $alph{a}_{mB}$. The scalar that describes dark-energy fluctuations features ghost and/or gradient instabilities for gravitational-wave amplitudes that are produced by typical binary systems. Taking into account the populations of binary systems, we conclude that the instability is triggered in the whole Universe for $|alph{a}_{mB}|gtrsim10^{-2}$, i.e. when the modification of gravity is sizeable. The instability is triggered by massive black-hole binaries down to frequencies corresponding to $10^{10}$ km: the instability is thus robust, unless new physics enters on even longer wavelengths. The fate of the instability and the subsequent time-evolution of the system depend on the UV completion, so that the theory may end up in a state very different from the original one. The same kind of instability is present in beyond-Horndeski theories for $|alph{a}_{mH}|gtrsim10^{-20}$. In conclusion, the only dark-energy theories with sizeable cosmological effects that avoid these problems are $k$-essence models, with a possible conformal coupling with matter.

Full work available at URL: https://arxiv.org/abs/1910.14035

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