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Both scalar fields and (generalized) Chaplygin gases have been widely used separately to characterize the dark sector of the universe. Here we investigate the cosmological background dynamics for a mixture of both these components and quantify the fractional abundances that are admitted by observational data from supernovae of type Ia and from the evolution of the Hubble rate. ...

In this paper the f(R) global monopole is reexamined. We provide an exact solution for the modified field equations in the presence of a global monopole for regions outside its core, generalizing previous results. Additionally, we discuss some particular cases obtained from this solution. We consider a setup consisting of a possible Schwarzschild black hole that absorbs the ...

The k-essence theory with a power-law function of \((\partial \phi )^2\) and Rastall’s non-conservative theory of gravity with a scalar field are shown to have the same solutions for the metric under the assumption that both the metric and the scalar fields depend on a single coordinate. This equivalence (called k–R duality) holds for static configurations with various symmetries ...

In this paper, we determine regular black hole solutions using a very general f(R) theory, coupled to a non-linear electromagnetic field given by a Lagrangian \(\mathcal {L}_\mathrm{NED}\). The functions f(R) and \(\mathcal {L}_\mathrm{NED}\) are in principle left unspecified. Instead, the model is constructed through a choice of the mass function M(r) presented in the metric ...

The Reduced Relativistic Gas (RRG) model was introduced by A. Sakharov in 1965 for deriving the cosmic microwave background (CMB) spectrum. It was recently reinvented by some of us to achieve an interpolation between the radiation and dust epochs in the evolution of the Universe. This model circumvents the complicated structure of the Boltzmann–Einstein system of equations and ...

We formulate a theory combining the principles of scalar–tensor gravity and Rastall’s proposal of a violation of the usual conservation laws. We obtain a scalar–tensor theory with two parameters \(\omega \) and \(\lambda \), the latter quantifying the violation of the usual conservation laws (\(\lambda = 1\) corresponding to the General Relativity limit). The only exact spherically ...