On sign-changeable interaction in FLRW cosmology

We investigate models of interacting dark matter and dark energy for the\nuniverse in a spatially flat Friedmann-Robertson-Walker (FRW) space-time. We\nfind the "source equation" for the total energy density and determine the\nenergy density of each dark component. We introduce an effective one-fluid\ndescription to evidence that interacting and unified models are related with\neach other, analyze the effective model and obtain the attractor solutions. We\nstudy linear and nonlinear interactions, the former comprises a linear\ncombination of the dark matter and dark energy densities, their first\nderivatives, the total energy density, its first and second derivatives and a\nfunction of the scale factor. The latter is a possible generalization of the\nlinear interaction consisting of an aggregate of the above linear combination\nand a significant nonlinear term built with a rational function of the dark\nmatter and dark energy densities homogeneous of degree one. We solve the\nevolution equations of the dark components for both interactions and examine\nexhaustively several examples. There exist cases where the effective one-fluid\ndescription produces different alternatives to the $\\La$CDM model and cases\nwhere the problem of coincidence is alleviated. In addition, we find that some\nnonlinear interactions yield an effective one-fluid model with a Chaplygin gas\nequation of state, whereas others generate cosmological models with de Sitter\nand power-law expansions. We show that a generic nonlinear interaction induces\nan effective equation of state which depends on the scale factor in the same\nway that the variable modified Chaplygin gas model, giving rise to the "relaxed\nChaplygin gas model".\n

Using the Tsallis generalized entropy, holographic hypothesis and also considering the Hubble horizon as the IR cutoff, we build a holographic model for dark energy and study its cosmological consequences in the Brans–Dicke framework. At first, we focus on a non-interacting universe, and thereinafter, we study the results of considering a sign-changeable interaction between the dark sectors of the cosmos. Our investigations show that, compared with the flat case, the power and freedom of the model in describing the cosmic evolution is significantly increased in the presence of the curvature. The stability analysis also indicates that, independent of the universe curvature, both the interacting and non-interacting cases are classically unstable. In fact, both the classical stability criterion and an acceptable behavior for the cosmos quantities, including the deceleration and density parameters as well as the equation of state, are not simultaneously obtainable.

We have presented FRW cosmological model in the framework of Brans-Dicke theory. This paper deals with a new proposed form of deceleration parameter and cosmological constant Λ. The effect of bulk viscosity is also studied in the presence of modified Chaplygin gas equation of state (p=Aρ-B/ρn). Furthermore, we have discussed the physical behaviours of the models.

FLRW cosmology in Weyl type f(Q) gravity and observational constraints

In this paper, we study two different models of dark energy based on Chaplygin gas equation of state. The first model is the variable modified Chaplygin gas while the second one is the extended Chaplygin gas. Both models are considered in the framework of higher order $f(R)$ modified gravity. We also consider the case of time varying gravitational constant $G$ and $\Lambda$ for both models. We investigate some cosmological parameters such as the Hubble, the deceleration and the equation of state parameters. Then we showed that the model that we considered, extended Chaplygin gas with time-dependent $G$ and $\Lambda$, is consistent with the observational data. Finally we conclude with the discussion of cosmological perturbations of our model.

In this paper, we have established a scalar field dark energy model in the flat FLRW universe, with the aim of studying the evolution of cosmic acceleration. A parameterization of the deceleration parameter [Formula: see text] is considered. We derive constraints on cosmological parameters by applying sophisticated Markov Chain Monte Carlo (MCMC) methods through the combination of various cosmological datasets such as Baryon Acoustic Oscillation (BAO) data points, Cosmic Chronometer (CC) measurements, and Standard Candle (SC) datasets from Pantheon Type Ia supernovae (SNe Ia), Quasars and gamma-ray bursts. This analysis allows us to determine a transition redshift, from the decelerated to the accelerated universe, with a value of [Formula: see text] and the current value of the deceleration parameter is [Formula: see text]. The dynamical behavior of quintessence is confirmed by the Equation of State (EoS) parameter, where [Formula: see text], indicating a subtle deviation from the cosmological constant. At lower redshifts [Formula: see text], our model shows strong agreement with the [Formula: see text]CDM model, while clear deviations are observed at higher redshifts [Formula: see text]. This study, through an analysis of cosmographic parameters such as energy density [Formula: see text], pressure [Formula: see text], and the scalar field EoS, emphasizes the potential of scalar field models as leading candidates for dark energy. Furthermore, we observe that the model yields a slightly higher value of the Hubble constant [Formula: see text] for certain dataset combinations, indicating that it may partially alleviate the Hubble tension.

In this paper, we study an isotropic and homogeneous Friedmann–Lemaitre–Robertson–Walker cosmological model in Weyl type [Formula: see text] gravity with a dynamic deceleration parameter. The solution of the field equations is found using a variable deceleration parameter, viz., [Formula: see text], with q the deceleration parameter, n and m constants, and H the Hubble parameter [Tiwari et al., Eur. Phys. J. Plus 132, 1 (2017)]. Then we calculate the values of the model parameters that are most likely to accord with the most recent observational data sets. We make use of the 57 CC points, SN and BAO datasets. We have contracted a graphical representation of the dynamical behavior of the cosmological parameters (the Hubble parameter, the deceleration parameter, the energy density and the equation of state) in terms of the redshift. To investigate the various eras of the universe, the evolution of the statefinder trajectory pairs [Formula: see text] and [Formula: see text] are studied. It is shown that the main energy component of the model is ordinary matter, which behaves initially as the SCDM model and then later as a fluid resembling quintessence, which behaves as the well-known [Formula: see text]CDM model. A suitable cosmological model is constructed after analyzing the physical parameters of the model, the [Formula: see text] diagnostic analysis.

The coupling between dark sectors provides a possible approach to mitigate the coincidence problem of cosmological standard model. In this paper, dark energy is treated as a fluid with a constant equation of state, whose coupling with dark matter is proportional the Hubble parameter and energy density of dark energy, that is, $\bar{Q}=3\xi_x\bar{H}\bar{\rho}_x$. Particularly, we consider the Hubble expansion rate to be perturbed in the perturbation evolutions of dark sectors. Using jointing data sets which include cosmic microwave background radiation, baryon acoustic oscillation, type Ia supernovae, and redshift-space distortions, we perform a full Monte Carlo Markov Chain likelihood analysis for the coupled model. The results show that the mean value with errors of interaction rate is: $\xi_x=0.00305_{-0.00305-0.00305-0.00305}^{+0.000645+0.00511+0.00854}$ for $Q^{\mu}_A\parallel u^{\mu}_c$; $\xi_x=0.00317_{-0.00317-0.00317-0.00317}^{+0.000628+0.00547+0.00929}$ for $Q^{\mu}_A\parallel u^{\mu}_x$, which means that the recently cosmic observations favored small interaction rate which is up to the order of $10^{-3}$. Moreover, in contrast to the coupled model with unperturbed expansion rate, we find perturbed Hubble expansion rate could bring about negligible impact on the model parameter space.

We investigate constraints on dark energy fluctuations using type Ia supernovae. If dark energy is not in the form of a cosmological constant, that is if the equation of state w≠−1, we expect not only temporal, but also spatial variations in the energy density. Such fluctuations would cause local variations in the universal expansion rate and directional dependences in the redshift-distance relation. We present a scheme for relating a power spectrum of dark energy fluctuations to an angular covariance function of standard candle magnitude fluctuations. The predictions for a phenomenological model of dark energy fluctuations are compared to observational data in the form of the measured angular covariance of Hubble diagram magnitude residuals for type Ia supernovae in the Union2 compilation. The observational result is consistent with zero dark energy fluctuations. However, due to the limitations in statistics, current data still allow for quite general dark energy fluctuations as long as they are in the linear regime.

We present an investigation of a scalar field dark energy model in the context of the FLRW universe, focusing on the parameterization of the deceleration parameter q(z) to study the evolution of cosmic acceleration. By employing extensive observational datasets—including 30 independent cosmic chronometer measurements, 17 additional baryon acoustic oscillation data points, and standard candle datasets from Pantheon Type Ia supernovae, Quasars, and Gamma-Ray Bursts—we provide constraints on cosmological parameters using advanced Markov chain Monte Carlo methods. Our analysis identifies a transition redshift of zt=0.62, marking the shift from decelerated to accelerated expansion, with a current deceleration parameter of q0=-0.59. The equation of state parameter confirms the dynamical behavior of quintessence, deviating slightly from a cosmological constant. Furthermore, the model demonstrates strong consistency with ΛCDM at lower redshifts while revealing distinct deviations at higher redshifts, which provides valuable insights into the late-time dynamics of the universe. By examining the evolution of cosmography parameters, energy density, pressure, and the scalar field equation of state, this study contributes the relevance of scalar field models as promising candidates for dark energy.

We study entropic cosmology with new nonadditive entropy of the apparent horizon $S_K=S_{BH}/(1+\sigma S_{BH})^2$ where $S_{BH}$ is the Bekenstein--Hawking entropy. Making use of the thermodynamics-gravity correspondence a new model of dark energy is analyzed. We obtain the generalised Friedmann equations for Friedmann--Lema\^{i}tre--Robertson--Walker spatially flat universe with the barotropic matter fluid possessing the equation of state (EoS) $w=p/\rho$. The dark energy pressure $p_D$, the energy density $\rho_D$, and the deceleration parameter $q$ of the universe are computed. At some model parameters $w$ and $\sigma$ we obtain the normalized density parameter of the matter $\Omega_{m}\approx 0.315$ and the deceleration parameter $q_0\approx -0.535$ for the current epoch that are in agreement with the Planck data. We show that our model has the phantom divide for the EoS of dark energy. The model under consideration, with the help of the thermodynamics-gravity correspondence, describes the universe inflation and the late time of the universe acceleration. It is proven that the suggested entropic cosmology being equivalent to cosmology within the teleparallel gravity with the function $F(T)$ which we computed.

We study interacting modified Chaplygin gas, which has shear and bulk viscosities. We assume bulk viscosity is a function of density, and also consider sign-changeable interaction. Interaction is assumed between modified Chaplygin gas and baryonic matter. Then, the effects of viscosities on the cosmological parameters, such as energy, density, Hubble expansion parameter, scale factor, and deceleration parameter, are investigated. We can consider this model as a toy model of our universe.

Regarding the Veneziano ghost of QCD and its generalized form, we consider a Friedmann-Robertson-Walker (FRW) universe filled by a pressureless matter and a dark energy component interacting with each other through a mutual sign-changeable interaction of positive coupling constant. Our study shows that, at the late time, for the deceleration parameter we have $q\rightarrow-1$, while the equation of state parameter of the interacting ghost dark energy (GDE) does not cross the phantom line, namely $\omega_D\geq-1$. We also extend our study to the generalized ghost dark energy (GGDE) model and show that, at late time, the equation of state parameter of the interacting GGDE also respects the phantom line in both flat and non-flat universes. Moreover, we find out that, unlike the non-flat universe, we have $q\rightarrow-1$ at late time for flat FRW universe. In order to make the behavior of the underlying models more clear, the deceleration parameter $q$ as well as the equation of state parameter $w_D$ for flat and closed universes have been plotted against the redshift parameter, $z$. All of the studied cases admit a transition in the expansion history of universe from a deceleration phase to an accelerated one around $z\approx 0.6$

Can luminosity distance measurements probe the equation of state of dark energy?

An exponential equation of state of dark energy in the light of 2018 CMB Planck data