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Register a new userViscous dark matter growth in (neo-)Newtonian cosmology
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We assume cold dark matter to possess a small bulk-viscous pressure which typically attenuates the growth of inhomogeneities. Explicit calculations, based on Eckarts theory of dissipative processes, reveal that for viscous cold dark matter the usual Newtonian approximation for perturbation scales smaller than the Hubble scale is no longer valid. We advocate the use of a neo-Newtonian approach which consistently incorporates pressure effects into the fluid dynamics and correctly reproduces the general relativistic dynamics. This result is of interest for numerical simulations of nonlinear structure formation involving nonstandard dark-matter fluids. We obtain upper limits on the magnitude of the viscous pressure by requiring that relevant perturbation amplitudes should grow sufficiently to enter the nonlinear stage.
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We discuss the possibility to implement a viscous cosmological model, attributing to the dark matter component a behaviour described by bulk viscosity. Since bulk viscosity implies negative pressure, this rises the possibility to unify the dark sector. At the same time, the presence of dissipative effects may alleviate the so called small scale problems in the $Lambda$CDM model. While the unified viscous description for the dark sector does not lead to consistent results, the non-linear behaviour indeed improves the situation with respect to the standard cosmological model.
In this work, we study the extended viscous dark energy models in the context of matter perturbations. To do this, we assume an alternative interpretation of the flat Friedmann-Lema^itre-Robertson-Walker Universe, through the nonadditive entropy and the viscous dark energy. We implement the relativistic equations to obtain the growth of matter fluctuations for a smooth version of dark energy. As result, we show that the matter density contrast evolves similarly to the $Lambda$CDM model in high redshift; in late time, it is slightly different from the standard model. Using the latest geometrical and growth rate observational data, we carry out a Bayesian analysis to constrain parameters and compare models. We see that our viscous models are compatible with cosmological probes, and the $Lambda$CDM recovered with a $1sigma$ confidence level. The viscous dark energy models relieve the tension of $H_0$ in $2 sim 3 sigma$. Yet, by involving the $sigma_8$ tension, some models can alleviate it. In the model selection framework, the data discards the extended viscous dark energy models.
We study how the cosmological constraints from growth data are improved by including the measurements of bias from Dark Energy Survey (DES). In particular, we utilize the biasing properties of the DES Luminous Red Galaxies (LRGs) and the growth data provided by the various galaxy surveys in order to constrain the growth index ($gamma$) of the linear matter perturbations. Considering a constant growth index we can put tight constraints, up to $sim 10%$ accuracy, on $gamma$. Specifically, using the priors of the Dark Energy Survey and implementing a joint likelihood procedure between theoretical expectations and data we find that the best fit value is in between $gamma=0.64pm 0.075$ and $0.65pm 0.063$. On the other hand utilizing the Planck priors we obtain $gamma=0.680pm 0.089$ and $0.690pm 0.071$. This shows a small but non-zero deviation from General Relativity ($gamma_{rm GR}approx 6/11$), nevertheless the confidence level is in the range $sim 1.3-2sigma$. Moreover, we find that the estimated mass of the dark-matter halo in which LRGs survive lies in the interval $sim 6.2 times 10^{12} h^{-1} M_{odot}$ and $1.2 times 10^{13} h^{-1} M_{odot}$, for the different bias models. Finally, allowing $gamma$ to evolve with redshift [Taylor expansion: $gamma(z)=gamma_{0}+gamma_{1}z/(1+z)$] we find that the $(gamma_{0},gamma_{1})$ parameter solution space accommodates the GR prediction at $sim 1.7-2.9sigma$ levels.
A suitable nonlinear interaction between dark matter with an energy density $rho_{M}$ and dark energy with an energy density $rho_{X}$ is known to give rise to a non-canonical scaling $rho_{M} propto rho_{X}a^{-xi}$ where $xi$ is a parameter which generally deviates from $xi =3$. Here we present a covariant generalization of this class of models and investigate the corresponding perturbation dynamics. The resulting matter power spectrum for the special case of a time-varying Lambda model is compared with data from the SDSS DR9 catalogue. We find a best-fit value of $xi = 3.25$ which corresponds to a decay of dark matter into the cosmological term. Our results are compatible with the $Lambda$CDM model at the 2$sigma$ confidence level.
We consider Tsallis cosmology as an approach to thermodynamic gravity and derive the bound on the Tsallis parameter to be $beta<2$ by using the constraints derived from the formation of the primordial light elements, Helium, Deuterium and Litium, from the observational data from Big Bang Nucleosynthesis (BBN) which allows only a very tiny deviation from General Relativity (GR). Next we consider thermal dark matter (DM) freeze-out mechanism in Tsallis cosmological era and derive bounds on the Tsallis parameter from the observed DM relic abundance to be $1-beta < 10^{-5}$.
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