## [1910.08821] Dark calling Dark: Interaction in the dark sector in presence of neutrino properties after Planck CMB final

 Authors: Weiqiang Yang, Supriya Pan, Rafael C. Nunes, David F. Mota Abstract: We investigate a well known scenario of interaction in the dark sector, where the vacuum energy is interacting with cold dark matter throughout the cosmic evolution in light of the cosmic microwave background (CMB) data from final Planck 2018 release. In addition to this minimal scenario, we generalize the model baseline by including the properties of neutrinos, such as the neutrino mass scale ($M_{\nu}$) and the effective number of neutrino species ($N_{\rm eff}$) as free parameters, in order to verify the possible effects that such parameters might generate on the coupling parameter, and vice versa. As already known, we confirm that in light of Planck 2018 data, such dark coupling can successfully solve the $H_0$ tension (with and without the presence of neutrinos). Concerning the properties of neutrinos, we find that $M_{\nu}$ may be wider than expected within the $\Lambda$CDM model and $N_{\rm eff}$ is fully compatible with three neutrino species (similar to $\Lambda$CDM prevision). The parameters characterizing the properties of neutrinos do not correlate with the coupling parameter of the interaction model. When considering the joint analysis of CMB from Planck 2018 and an estimate of $H_0$ from Hubble Space Telescope 2019 data, {\it we find an evidence for a non-null value of the coupling parameter at more than 3$\sigma$ confidence-level.} We discuss also the inclusion the baryon acoustic oscillations data in combination with Planck 2018 and implications for the scenarios addressed. Our main results updating the dark sectors' interaction and neutrino properties in the model baseline, represent a new perspective in this direction. Clearly, a possible new physics in light of some dark interaction between dark energy and dark matter can serve as an alternative to $\Lambda$CDM scenario to explain the observable Universe, mainly in light of the current tension on $H_0$. [PDF]  [PS]  [BibTex]  [Bookmark]

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### [1910.08821] Dark calling Dark: Interaction in the dark sector in presence of neutrino properties after Planck CMB final

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

This paper presents a study of the observational implications of a non-gravitational coupling between cold dark matter and dark energy. The authors briefly discuss the cosmological phenomenology of the model, which is an interacting vacuum scenario, although it is difficult to see what is new in this analysis with respect to previous works, as this model has been extensively studied before (e.g., Guo et al. 1702.04189, and references therein). They then present the constraints on the model as obtained with several observational datasets by including the neutrino mass and effective number of neutrinos. They argue that such a coupling can resolve the Hubble tension and that the posterior on $M_\nu$ can be wider than in ΛCDM.

A few specific remarks:

In the introduction, the authors state that Interacting Dark Energy (IDE) models have been found to be able to solve the cosmic coincidence problem. This is an incorrect statement in my opinion. There has been an abundance of parameter estimation studies that showed that the coupling needed to solve the coincidence problem is too large (excluded by data). Furthermore, recent research on the QFT (D'Amico et al. 1605.00996, Marsh 1606.01538) of interacting dark energy seems to point out that, in general, coupled models with background energy exchange suffer from serious problems coming from huge quantum corrections. For this reason (and also because usually IDE models with background energy exchange are severely constrained from the currently available data), claims about such models being able to alleviate the coincidence problem should be reconsidered.

The authors claim that the interaction in the dark sector is an excellent way to alleviate/solve the $H_0$ and $\sigma_8$ tensions. This statement is optimistic: some IDE models are interesting phenomenologically, but they are based on ad hoc couplings and extra free parameters such that ΛCDM is clearly preferred if one performs a Bayesian model selection analysis.

The authors claim that their chosen interaction function, $Q$, is the most natural and simple, but in fact, several authors (e.g. Ref. [12] in the manuscript, 0804.0232) have argued for the opposite: this form of $Q$ can be seen as problematic, as it raises the question how an interaction rate expected to depend on local interactions, is determined by a global quantity like the Hubble expansion. Some discussion of this seems necessary.

The authors may want to consider adding discussion and calculations on the covariance and gauge-invariance properties of their model. This has been shown to be necessary (see e.g. Valiviita et al. 0804.0232 and Clemson et al. 1109.6234) to construct meaningful models. A previous paper by Guo et al. (1702.04189) seems to have done a more thorough investigation of this.

It is unclear what assumptions for the nonlinear behaviour of the considered models are taken in this work. For example, what exactly is assumed in order to perform the BAO analysis and for the impact of massive neutrinos? Likelihoods are usually constructed under the ΛCDM assumption. If nonlinear scales are considered, the behaviour of an exotic model might be completely different.

It would be interesting to compare the results of this analysis with an uncoupled model where $w$ is allowed to vary (and massive neutrinos are included). Then, one could see if the results are due to the coupling or if they can be mimicked by a very different assumption/model extension.

There are now lab-based (e.g. from KATRIN) constraints on the neutrino mass which are cosmology independent. Are the constraints presented in this paper consistent with the lab-based experiments?

[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.]