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تسجيل مستخدم جديدInhomogeneous and interacting vacuum energy
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Vacuum energy is a simple model for dark energy driving an accelerated expansion of the universe. If the vacuum energy is inhomogeneous in spacetime then it must be interacting. We present the general equations for a spacetime-dependent vacuum energy in cosmology, including inhomogeneous perturbations. We show how any dark energy cosmology can be described by an interacting vacuum+matter. Different models for the interaction can lead to different behaviour (e.g., sound speed for dark energy perturbations) and hence could be distinguished by cosmological observations. As an example we present the cosmic microwave microwave background anisotropies and the matter power spectrum for two differe
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Vacuum energy remains the simplest model of dark energy which could drive the accelerated expansion of the Universe without necessarily introducing any new degrees of freedom. Inhomogeneous vacuum energy is necessarily interacting in general relativi
ty. Although the four-velocity of vacuum energy is undefined, an interacting vacuum has an energy transfer and the vacuum energy defines a particular foliation of spacetime with spatially homogeneous vacuum energy in cosmological solutions. It is possible to give a consistent description of vacuum dynamics and in particular the relativistic equations of motion for inhomogeneous perturbations given a covariant prescription for the vacuum energy, or equivalently the energy transfer four-vector, and we construct gauge-invariant vacuum perturbations. We show that any dark energy cosmology can be decomposed into an interacting vacuum+matter cosmology whose inhomogeneous perturbations obey simple first-order equations.
The dynamics of interacting dark matter-dark energy models is characterized through an interaction rate function quantifying the energy flow between these dark sectors. In most of the interaction functions, the expansion rate Hubble function is consi
dered and sometimes it is argued that, as the interaction function is a local property, the inclusion of the Hubble function may influence the overall dynamics. This is the starting point of the present article where we consider a very simple interacting cosmic scenario between vacuum energy and the cold dark matter characterized by various interaction functions originated from a general interaction function: $Q= Gammarho_{c}^{alpha }rho_{x}^{1-alpha -beta}(rho_{c}+rho_{x})^{beta}$, where $rho_c$, $rho_x$ are respectively the cold dark matter density and vacuum energy density; $alpha$, $beta$ are real numbers and $Gamma$ is the coupling parameter with dimension equal to the dimension of the Hubble rate. We investigate four distinct interacting cosmic scenarios and constrain them both theoretically and observationally. Our analyses clearly reveal that the interaction models should be carefully handled.
We present a phase-space analysis of the qualitative dynamics cosmologies where dark matter exchanges energy with the vacuum component. We find fixed points corresponding to power-law solutions where the different components remain a constant fractio
n of the total energy density and given an existence condition for any fixed points with nonvanishing energy transfer. For some interaction models we find novel fixed points in the presence of a third noninteracting fluid with constant equation of state, such as radiation, where the interacting vacuum+matter tracks the evolution of the third fluid, analogous to tracker solutions previously found for self-interacting scalar fields. We illustrate the phase-plane behavior, determining the equation of state and stability of the fixed points in the case of a simple linear interaction model, for interacting vacuum and dark matter, including the presence of noninteracting radiation. We give approximate solutions for the equation of state in matter- or vacuum-dominated solutions in the case of small interaction parameters.
Since physics of the dark sector components of the Universe is not yet well-understood, the phenomenological studies of non-minimal interaction in the dark sector could possibly pave the way to theoretical and experimental progress in this direction.
Therefore, in this work, we intend to explore some features and consequences of a phenomenological interaction in the dark sector. We use the Planck 2018, BAO, JLA, KiDS and HST data to investigate two extensions of the base $Lambda$CDM model, viz., (i) we allow the interaction among vacuum energy and dark matter, namely the I$Lambda$CDM model, wherein the interaction strength is proportional to the vacuum energy density and expansion rate of the Universe, and (ii) the I$Lambda$CDM scenario with free effective neutrino mass and number, namely the $ u$I$Lambda$CDM model. We also present comparative analyses of the interaction models with the companion models, namely, $Lambda$CDM, $ uLambda$CDM, $w$CDM and $ u w$CDM. In both the interaction models, we find non-zero coupling in the dark sector up to 99% CL with energy transfer from dark matter to vacuum energy, and observe a phantom-like behavior of the effective dark energy without actual ``phantom crossing. The well-known tensions on the cosmological parameters $H_0$ and $sigma_8$, prevailing within the $Lambda$CDM cosmology, are relaxed significantly in these models wherein the $ u$I$Lambda$CDM model shows consistency with the standard effective neutrino mass and number. Both the interaction models find a better fit to the combined data compared to the companion models under consideration.
Recent measurements of the Cosmic Microwave Anisotropies power spectra measured by the Planck satellite show a preference for a closed universe at more than $99 %$ Confidence Level. Such a scenario is however in disagreement with several low redshift
observables, including luminosity distances of Type Ia Supernovae. Here we show that Interacting Dark Energy (IDE) models can ease the discrepancies between Planck and Supernovae Ia data in a closed Universe. Therefore IDE cosmologies remain as very appealing scenarios, as they can provide the solution to a number of observational tensions in different fiducial cosmologies. The results presented here strongly favour broader analyses of cosmological data, and suggest that relaxing the usual flatness and vacuum energy assumptions can lead to a much better agreement among theory and observations.
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