This paper tries to obtain a simple picture of several aspects of the mode structure in relativistic non-equilibrium thermodynamics. Its pedagogical focus is on the relation between long-wavelength perturbation modes of the causal Müller-Israel-Stewart (MIS) theory and those of the traditional Eckart theory. Principally, this issue was clarified in a series of papers by Hiscock and Lindblom (see [8-10]). Here, I put together some essential features which do not require the entire formalism of the complete theory.

Apresenta-se uma tradução do alemão, do artigo seminal de Alexander Friedmann publicado em 1922, onde encontram-se as bases de um modelo cosmológico dinâmico.

Os dados observacionais atuais indicam que aproximadamente 95% do substrato cosmológico é invisível e só se manifesta através de sua ação gravitacional. A conclusão mais aceita, baseada na teoria da relatividade geral de Einstein, é que estes 95% formam um “setor escuro", de natureza não bariônica. Este setor é normalmente dividido em energia escura e matéria escura. Energia escura é uma componente exótica com uma pressão negativa que domina dinamicamente o Universo atual. Na teoria de Einstein uma pressão efetiva negativa é necessária para entender a expansão acelerada do Universo, detectada em 1998. Matéria escura, por outro lado, é matéria sem pressão, necessária para explicar a origem das estruturas cósmicas. A natureza da matéria escura e da energia escura é objeto de intensos estudos em todo o mundo, tanto do ponto de vista teórico, quanto observacional. Este artigo, baseado numa palestra para alunos do IFES Guarapari no mês de outubro de 2019, visa dar uma introdução geral nos problemas atuais da cosmologia.

We study an expanding two-fluid model of non-relativistic dark matter and radiation which are allowed to interact during a certain time span and to establish an approximate thermal equilibrium. Such 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 particles masses within the range 1eV . mχ . 10eV 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 Mészáros effect. As a result, both the H0 and the S8 tensions of the current standard cosmological model are significantly alleviated.

Os dados observacionais atuais indicam que aproximadamente 95% do substrato cosmológico é invisível e só se manifesta através de sua ação gravitacional. A conclusão mais aceita, baseada na teoria da relatividade geral de Einstein, é que estes 95% formam um “setor escuro", de natureza não bariônica. Este setor é normalmente dividido em energia escura e matéria escura. Energia escura é uma componente exótica com uma pressão negativa que domina dinamicamente o Universo atual. Na teoria de Einstein uma pressão efetiva negativa é necessária para entender a expansão acelerada do Universo, detectada em 1998. Matéria escura, por outro lado, é matéria sem pressão, necessária para explicar a origem das estruturas cósmicas. A natureza da matéria escura e da energia escura é objeto de intensos estudos em todo o mundo, tanto do ponto de vista teórico, quanto observacional. Este artigo, baseado numa palestra para alunos do IFES Guarapari no mês de outubro de 2019, visa dar uma introdução geral nos problemas atuais da cosmologia.

The 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.

Models with non-gravitational interactions between the dark matter and dark energy components are an alternative to the standard cosmological scenario. These models are characterized by an interaction term, and a frequently used parameterization is Q = 3ξ H ρ_x, where H is the Hubble parameter and ρ_x is the dark energy density. Although current observations support such a model for negative values of the interaction parameter ξ, we show here that this interval of values of ξ leads the model to predict a violation of the Weak Energy Condition (WEC) for the dark matter density, regardless of the value of the equation-of-state parameter of the dark energy component. This violation is accompanied by unphysical instabilities of matter perturbations.

Extensions 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 ( Λ CDM) model. As an example, we consider the dynamics of a scalar-tensor extension of the standard model, the e Φ Λ CDM 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 σ 8 .

On the basis of a previously established scalar–tensor extension of the [Formula: see text]CDM model, we develop an effective fluid approach for the matter growth function. This extended [Formula: see text]CDM (henceforth [Formula: see text]CDM) 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 [Formula: see text] and confront the results with recent redshift-space distortion (RSD) measurements.

We reconsider the dynamics of the Universe in the presence of interactions in the cosmological dark sector. A class of interacting models is introduced via a real function fr of the ratio r between the energy densities of the (pressureless) cold dark matter (CDM) and dark energy (DE). The subclass of models for which the ratio r depends only on the scale factor is shown to be equivalent to unified models of the dark sector, i.e. models for which the CDM and DE components can be combined in order to form a unified dark fluid. For specific choices of the function fr we recover several models already studied in the literature. We analyze various special cases of this type of interacting models using a suitably modified version of the CLASS code combined with Monte Python in order to constrain the parameter space with the data from supernova of type SNe Ia (JLA), the Hubble constant H0, cosmic chronometers (CC), baryon acoustic oscillations (BAO) and data from the Planck satellite (Planck TT). Our analysis shows that even if data from the late Universe ( H0, SNe Ia and CC) indicate an interaction in the dark sector, the data related to the early Universe (BAO and Planck TT) constrain this interaction substantially, in particular for cases in which the background dynamics is strongly affected.