(Submitted on 7 May 2019)Constraints on the properties of the cosmological dark matter have previously been obtained in a model-independent fashion using the Generalised Dark Matter (GDM) framework. Here we extend that work in several directions: We consider the inclusion of WiggleZ matter power spectrum data, and show that this improves the constraints on the two perturbative GDM parameters,and , by a factor of 3, for a conservative choice of wavenumber range. A less conservative choice can yield an improvement of up to an order of magnitude compared to previous constraints.
In order to examine the robustness of this result we develop a GDM halo model to explore how non-linear structure formation could proceed in this framework, since currently GDM has only been defined perturbatively and only linear theory has been used when generating constraints. We then examine how the halo model affects the constraints obtained from the matter power spectrum data.
The less-conservative wavenumber range shows a significant difference between linear and non-linear modelling, with the latter favouring GDM parameters inconsistent withCDM, underlining the importance of careful non-linear modelling when using this data.
We also use this halo model to establish the robustness of previously obtained constraints, particularly those that involve weak gravitational lensing of the cosmic microwave background.
Additionally, we show how the inclusion of neutrino mass as a free parameter affects previous constraints on the GDM parameters.
Dark matter is not the only way that one can explain the cosmic background radiation signatures (which is what Lambda CDM does best). But, it is a paradigm that simply cannot be reasonably fit to very hard data from observations of galaxy dynamics. Sooner or later, the evidence is going to force the astronomy and physics community to recognize that gravity modification and not dark matter particle theories are what explains dark matter phenomena and probably also dark energy phenomena (which are described as a gravity modification, i.e. the cosmological constant, in the Lambda CDM cosmology model).
The introduction to this paper is also informative:
The introduction to this paper is also informative:
The ΛCDM cosmological model does a good job of reproducing the current cosmological observations. In this model, the standard model of particle physics is supplemented by a cosmological constant Λ and a dark matter particle. This dark matter particle is assumed to interact purely due to the influence of gravity and to have a negligible (initial) velocity dispersion, thus the name Cold Dark Matter (CDM). In perturbative calculations this is typically modelled as a pressure-less perfect fluid. As a result, many cosmological constraints on the dark matter density are, more correctly, constraints on the density of this pressure-less perfect fluid. More generally, CDM is evolved by solving the collision-less Boltzmann equation. This is done on large scales using cosmological perturbation theory (implemented in Boltzmann codes such as class and camb) and on smaller scales using N-body simulations and other non-linear methods.
Since we are entering the era of so-called “precision cosmology,” in which many cosmological parameters have been measured with 1% accuracy or better, it is timely to consider whether such an idealised and simple dark matter model is sufficient when analysing the data. There are many physical dark matter models that do not yield precisely CDM, for example Warm Dark Matter (WDM) [1–3] or ultra light axions (one example of Fuzzy Dark Matter (FDM)) [4, 5]. In addition, recent work on the Effective Field Theory of Large Scale Structure (EFTofLSS)  shows that even an ideal CDM candidate develops a more complicated energy momentum tensor, even on linear scales, once the non-linearities that inevitably form on small scales back-react on the large scales. This causes an effective pressure and viscosity on large scales.
From a non-cosmological perspective, despite a large number of direct and indirect detection experiments for dark matter, no convincing detections have been made, and many theoretically favoured regions of parameter space have been ruled out [7–12]. Thus, there are strong reasons to go beyond the simplest ways of modelling dark matter.
In , the Generalised Dark Matter (GDM) model (first proposed in ) was examined in some detail, notably how it relates to particular physical models. GDM adds to the CDM energy momentum tensor a background pressure, pressure perturbation and anisotropic stress. Closure relations are then postulated to match qualitative properties of known models, like massive neutrinos, and in order to de-correlate background and perturbative properties. GDM encompasses WDM, FDM and the EFTofLSS effects as well as other physical models, so it is sufficiently versatile for examining dark matter properties in a model independent fashion. In , all GDM parameters were constrained using Cosmic Microwave Background (CMB) data, supported by additional data on the cosmological expansion history (see section 4.3 in this paper and references therein for comparison to earlier works constraining partial or similar parameters to those we consider here, such as [16–18]). The results showed no evidence for any non-CDM properties of dark matter. This was expanded on in , where an improved freedom was given to one of the GDM parameters; this was used to demonstrate for the first time that there is no cosmological epoch where the data would favour a nonzero equation of state, and furthermore that there is no cosmological epoch where the data is consistent with zero dark matter density, thus showing the strength of the GDM approach to testing the CDM paradigm. An independent group subsequently  verified some of the results in , as well as using some late time matter clustering data; we will comment further on this later in the paper. Further work constraining the GDM parameters is now ongoing by other groups, see e.g. . It was noted in  that matter power spectrum data could not only improve the constraints on the GDM parameters, but also has the potential to break a degeneracy between two of them (see section II).
The robust use of such data requires a non-linear extension of the GDM model, which is not present in the literature. It was also noted in  that the inclusion of a non-linear extension to perturbation theory for ΛCDM makes a difference to the CMB lensing potential. This effect is of a similar magnitude, but opposite sign, to that of GDM with parameters saturating the constraints found in . In this paper we develop a halo model for GDM which allows us first to test the robustness of the results in , and second to use matter power spectrum data from the WiggleZ survey  to improve the constraints on the GDM parameters.Self-interacting neutrino models also don't work:
Constraining the Self-Interacting Neutrino Interpretation of the Hubble Tension(Submitted on 7 May 2019)Large, non-standard neutrino self-interactions have been shown to resolve thetension in Hubble constant measurements and a milder tension in the amplitude of matter fluctuations.
We demonstrate that interactions of the necessary size imply the existence of a force-carrier with a large neutrino coupling () and mass in the keV -- 100 MeV range. This mediator is subject to stringent cosmological and laboratory bounds, and we find that nearly all realizations of such a particle are excluded by existing data unless it carries spin 0 and couples almost exclusively to -flavored neutrinos. Furthermore, we find that the light neutrinos must be Majorana, and that a UV-complete model requires a non-minimal mechanism to simultaneously generate neutrino masses and appreciable self-interactions.