Navegando por Autor "Wilhelm, Zimdahl Winfried Ernst"
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Item Early-time thermalization of cosmic components? : a hint for solving cosmic tensions.(2021) Velten, Hermano Endlich Schneider; Costa, Ingrid; Wilhelm, Zimdahl Winfried ErnstWe study an expanding two-fluid model of nonrelativistic dark matter and radiation, which are allowed to interact during a certain time span and to establish an approximate thermal equilibrium. Such an interaction, which generates an effective bulk viscous pressure at background level, is expected to be relevant for times around the transition from radiation to matter dominance. We quantify the magnitude of this pressure for dark-matter particle masses within the range 1 eV ≲ mχ ≲ 10 eV around the matter-radiation equality epoch (i.e., redshift zeq ∼ 3400) and demonstrate that the existence of a transient bulk viscosity has consequences which may be relevant for addressing current tensions of the standard cosmological model: (i) the additional (negative) pressure contribution modifies the expansion rate around zeq, yielding a larger H0 value, and (ii) large-scale structure formation is impacted by suppressing the amplitude of matter overdensity growth via a new viscous friction-term contribution to the M ́eszáros effect. As a result, the H0 and S8 tensions of the current standard cosmological model are both significantly alleviated.Item Early-time thermalization of cosmic components? A hint for solving cosmic tensions.(2021) Velten, Hermano Endlich Schneider; Costa, Ingrid; Wilhelm, Zimdahl Winfried ErnstWe study an expanding two-fluid model of nonrelativistic dark matter and radiation, which are allowed to interact during a certain time span and to establish an approximate thermal equilibrium. Such an interaction, which generates an effective bulk viscous pressure at background level, is expected to be relevant for times around the transition from radiation to matter dominance. We quantify the magnitude of this pressure for dark-matter particle masses within the range 1 eV ≲ mχ ≲ 10 eV around the matter-radiation equality epoch (i.e., redshift zeq ∼ 3400) and demonstrate that the existence of a transient bulk viscosity has consequences which may be relevant for addressing current tensions of the standard cosmological model: (i) the additional (negative) pressure contribution modifies the expansion rate around zeq, yielding a larger H0 value, and (ii) large-scale structure formation is impacted by suppressing the amplitude of matter overdensity growth via a new viscous friction-term contribution to the M ́eszáros effect. As a result, the H0 and S8 tensions of the current standard cosmological model are both significantly alleviated.Item Matter growth in extended ΛCDM cosmology.(2019) Wilhelm, Zimdahl Winfried Ernst; Velten, Hermano Endlich Schneider; Algoner Jorge, William CelestinoOn the basis of a previously established scalar–tensor extension of the ΛCDM model, we develop an effective fluid approach for the matter growth function. This extended ΛCDM (henceforth eΦΛCDM) cosmology takes into account deviations from the Standard Model both via a modified background expansion and by the inclusion of geometric anisotropic stresses as well as of perturbations of the geometric dark-energy equivalent. The background dynamics is governed by an explicit analytic expression for the Hubble rate in which modifications of the Standard Model are given in terms of a single constant parameter [W. C. Algoner, H. E. S. Velten and W. Zimdahl, J. Cosmol. Astropart. Phys. 1611 (2016) 034]. To close the system of fluid-dynamical perturbation equations, we introduce two phenomenological parameters through which the anisotropic stress is related both to the total energy density perturbation of the cosmic substratum and to relative perturbations in the effective two-component system. We quantify the impact of deviations from the standard background, of anisotropic stresses and of nonvanishing perturbations of the effective dark-energy component on the matter growth rate function fσ8 and confront the results with recent redshift-space distortion (RSD) measurements.Item Matter growth in imperfect fluid cosmology.(2019) Wilhelm, Zimdahl Winfried Ernst; Velten, Hermano Endlich Schneider; Algoner Jorge, William CelestinoExtensions of Einstein’s General Relativity (GR) can formally be given a GR structure in which additional geometric degrees of freedom are mapped on an effective energy-momentum tensor. The corresponding effective cosmic medium can then be modeled as an imperfect fluid within GR. The imperfect fluid structure allows us to include, on a phenomenological basis, anisotropic stresses and energy fluxes which are considered as potential signatures for deviations from the cosmological standard Λ-cold-dark-matter (ΛCDM) model. As an example, we consider the dynamics of a scalar-tensor extension of the standard model, the eΦΛCDM model. We constrain the magnitudes of anisotropic pressure and energy flux with the help of redshift-space distortion (RSD) data for the matter growth function f σ8.Item On (non-)dynamical dark energy.(2020) Wilhelm, Zimdahl Winfried Ernst; Fabris, Júlio César; Velten, Hermano Endlich Schneider; González Herrera, Manuel RamónThe current Universe is composed by a mixture of relativistic species, baryonic matter, dark matter and dark energy which evolve in a non-trivial way at perturbative level. An advanced description of the cosmological dynamics should include non-standard features beyond the simplistic approach idealized by the standard cosmology in which cosmic components do not interact, are adiabatic and dissipationless. We promote a full perturbative analysis of linear scalar perturbations of a non- interacting cosmological model containing baryons, dark matter (both pressureless) and a scalar field allowing for the presence of relative entropic perturbations between the three fluids. Assuming an effective scalar-field sound speed equal to one and neglecting anisotropic stresses we establish a new set of equations for the scalar cosmological perturbations. As a consequence of this new approach, we show that tiny departures from a constant scalar field equation of state wS = −1 damage structure formation in a non-acceptable manner. Hence, by strongly constraining wS our results provide compelling evidence in favor of the standard cosmological model and rule out a large class of dynamical dark energy models.Item O universo dinâmico de Friedmann.(2022) Velten, Hermano Endlich Schneider; Wilhelm, Zimdahl Winfried ErnstApresenta-se uma tradução do alemão, do artigo seminal de Alexander Friedmann publicado em 1922, onde encontram-se as bases de um modelo cosmológico dinâmico.