## [1910.14035] Dark-Energy Instabilities induced by Gravitational Waves

 Authors: Paolo Creminelli, Giovanni Tambalo, Filippo Vernizzi, Vicharit Yingcharoenrat 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 $\alpha_{\rm B}$. 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 $|\alpha_{\rm B} |\gtrsim 10^{-2}$, 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 depends on the UV completion, but the theory around the final state will be qualitatively different from the original one. The same kind of instability is present in beyond-Horndeski theories for $|\alpha_{\rm H}| \gtrsim 10^{-20}$. In conclusion, the only viable dark-energy theories with sizeable cosmological effects are $k$-essence models, with a possible conformal coupling with matter. [PDF]  [PS]  [BibTex]  [Bookmark]

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### [1910.14035] Dark-Energy Instabilities induced by Gravitational Waves

This paper was commented on through Cosmo Comments. The following comments can also be viewed as annotations on the paper via Hypothesis.

This article reveals an interesting mechanism, which seems to be at work in Horndeski theories featuring kinetic braiding, such as the cubic Galileon model. Namely, the scalar field standing for dark energy is unstable in the presence of gravitational waves (GWs) with a sufficiently high amplitude. The article concludes that Horndeski models whose EFT parameter $\alpha_B$ is larger than 1% are excluded.

As a non-specialist of Horndeski theories, I tried my best to understand the physics of this destabilization mechanism, but I ended up quite puzzled. Thus, I would like to ask the authors a few questions; hopefully, this dialogue will be useful for other members of the community.

Let me first try to summarize what I think is the origin of the instability – again, to the best of my understanding, which may be incorrect. The important term seems to be the last one of Eq. (3.2), which is quadratic in $\Gamma_{\mu\nu}$. If this term is large enough, the perturbations of the scalar field, denoted $\delta\pi$ feature a ghost. This appears quite clearly in Eqs. (3.19) and (3.30), where the second-order differential operator of the equation of motion for $\delta\pi$ becomes elliptical in that case.

Since $\Gamma_{\mu\nu}\propto\dot{h}_{ij}$, the time derivative of the GW amplitude, $\Gamma^{\mu\nu}\Gamma_{\mu\nu}$ may be understood as the energy density of the GW. Whatever its interpretation, it is a space-time curvature term, because it is the square of Christoffel symbols. However, what surprises me is that the cubic Galileon $\pi$ does not directly couple to curvature. Thus, how can curvature appear in its effective Lagrangian, Eq. (3.2)?

I guess that the reason is the following. While $\pi$ is not directly coupled to curvature, the gravitational potentials $\Phi$ and $\Psi$ are. However, in the sub-Hubble regime, $\pi\propto\Phi=\Psi$ according to Eq. (2.5) of the article. Replacing $\Phi$ and $\Psi$ by $\pi$ would then lead to $\pi$ being effectively coupled to curvature.

My questions to the authors are the following:
1. Is the above sound or erroneous?
2. What is the domain of validity of Eq. (2.5)?
3. If (2.5) is very general, then $\pi$ should inherit all the couplings of $\Phi$ and $\Psi$, including with matter. In other words, I suppose that if one had included the matter action in this work, one would get a term $\propto \pi T$, where $T$ is the matter energy-momentum tensor, in Eq. (2.8). In other words, there would now be a significantly larger source of instability for the model: the powerful curvature provoked by the presence of matter. This would significantly improve the constraint on $\alpha_B$, wouldn’t it?

[These comments were shared with us by a member of the cosmology community. They do not necessarily reflect the opinion of the Cosmo Comments team.]

Filippo Vernizzi
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Affiliation: IPhT - CEA Saclay

### Re: [1910.14035] Dark-Energy Instabilities induced by Gravitational Waves

Thanks for commenting.

1. Yes, you are right. What you describe is the mixing of with and , which appears in theories with so-called "kinetic braiding". (This is different from the mixing that one has in Brans-Dicke theories. For instance in this case.) We would not describe it as a coupling to curvature, since you do not have second derivatives of the metric.

2. This equation is valid on sub-Hubble regime but it is qualitatively the same on super-Hubble scales.

3. After demixing, indeed couples to the stress-energy tensor, although not to its trace . (This coupling is what leads to effects in the LSS.) On the other hand, this coupling does not create instabilities; it gives rise to non-relativistic solutions that become nonlinear (Vainshtein effect) but always stable. Our instability requires a sizable gravitational wave (more properly the combination appearing in our paper). In our discussion we are far away from matter sources so matter doesn't matter.

Filippo, Giovanni, Paolo and Vicharit