Reconstruction of DBI-essence dark energy with f(R) gravity and its effect on black hole and wormhole mass accretion

In this paper, we have used the reconstructed Dirac–Born–Infeld (DBI)-essence dark energy density to modify the mass accretions of black holes and wormholes. In general, the black hole mass accretion does not depend on the metric or local Einstein geometry. That is why we have used a generalized mechanism by reconstructing the DBI-essence dark energy reconstruction with [Formula: see text] gravity. We have used some particular forms of the scale factor to analyze the accretion phenomena. We have shown the effect of cosmic evolution in the proper time variation of black hole mass accretion. Finally, we have studied the validity of energy conditions and analyzed the Type I–IV singularities for our reconstructed model.

Modified cosmology through Kaniadakis horizon entropy

We apply the gravity-thermodynamics conjecture, namely the first law of thermodynamics on the Universe horizon, but using the generalized Kaniadakis entropy instead of the standard Bekenstein–Hawking one. The former is a one-parameter generalization of the classical Boltzmann–Gibbs–Shannon entropy, arising from a coherent and self-consistent relativistic statistical theory. We obtain new modified cosmological scenarios, namely modified Friedmann equations, which contain new extra terms that constitute an effective dark energy sector depending on the single model Kaniadakis parameter K. We investigate the cosmological evolution, by extracting analytical expressions for the dark energy density and equation-of-state parameters and we show that the Universe exhibits the usual thermal history, with a transition redshift from deceleration to acceleration at around 0.6. Furthermore, depending on the value of K, the dark energy equation-of-state parameter deviates from $$\Lambda $$ Λ CDM cosmology at small redshifts, while lying always in the phantom regime, and at asymptotically large times the Universe always results in a dark-energy dominated, de Sitter phase. Finally, even in the case where we do not consider an explicit cosmological constant the resulting cosmology is very interesting and in agreement with the observed behavior.

Cosmology in the mimetic higher-curvature $$f(R,R_{\mu \nu }R^{\mu \nu })$$ gravity

In the framework of the mimetic approach, we study the $$f(R,R_{\mu \nu }R^{\mu \nu })$$ f ( R , R μ ν R μ ν ) gravity with the Lagrange multiplier constraint and the scalar potential. We introduce field equations for the discussed theory and overview their properties. By using the general reconstruction scheme we obtain the power law cosmology model for the $$f(R,R_{\mu \nu }R^{\mu \nu })=R+d(R_{\mu \nu }R^{\mu \nu })^p$$ f ( R , R μ ν R μ ν ) = R + d ( R μ ν R μ ν ) p case as well as the model that describes symmetric bounce. Moreover, we reconstruct model, unifying both matter dominated and accelerated phases, where ordinary matter is neglected. Using inverted reconstruction scheme we recover specific $$f(R,R_{\mu \nu }R^{\mu \nu })$$ f ( R , R μ ν R μ ν ) function which give rise to the de-Sitter evolution. Finally, by employing the perfect fluid approach, we demonstrate that this model can realize inflation consistent with the bounds coming from the BICEP2/Keck array and the Planck data. We also discuss the holographic dark energy density in terms of the presented $$f(R,R_{\mu \nu }R^{\mu \nu })$$ f ( R , R μ ν R μ ν ) theory. Thus, it is suggested that the introduced extension of the mimetic regime may describe any given cosmological model.

Force, metric, or mass: Disambiguating causes of uniform gravity

In addition to nonzero forces and nontrivial metrics, here I show that a nonconstant Higgs expectation value, which endows elementary particles with their masses, also leads to apparent universal particle accelerations and photon frequency shifts. When effects of the Higgs is attributed to spacetime curvatures, a spurious stress-energy tensor is required in Einstein’s equation. On cosmological scales, the spurious density coincides with the observed dark energy density. On smaller scales, effects of the Standard Model Higgs gradients are unlikely observable except near compact astrophysical bodies. To estimate the experimental precision required to disambiguate causes of apparent accelerations, I compare distinct effects of the force, metric, and Higgs profiles that cause uniform acceleration of a test particle. When the acceleration is caused by a force, the motion of all particles are hyperbolic with the same acceleration. However, when the cause is a metric, only a one-parameter family of particles undergo hyperbolic motion. In comparison, when the cause is a Higgs gradient, the trajectory of all particles are hyperbolic, but the acceleration is larger when the particle’s energy is higher. The discrepancies among the three causes are minuscule on laboratory scales, which makes experimental tests very challenging.

The scalar field models of Tsallis holographic dark energy with Granda-Oliveros cut-off in modified gravity

This research explores the Tsallis holographic quintessence, k-essence, and tachyon model of dark energy in the modified f(R, T) gravity framework with Granda-Oliveros cutoff. We have analyzed the energy density through ρΛ = (αH² + βH)^-δ+2. We study the correspondence between the quintessence, k-essence, and tachyon energy density with the Tsallis holographic dark energy density in a flat FRW Universe. The reconstruction is performed for the different values of Tsallis parameter δ in the region of ωΛ > -1 for the EoS parameter. This correspondence allows reconstructing the potentials and the dynamics for the scalar fields models, if we set some constraints for the model parameters, which describe the accelerated expansion of the Universe.

Explaining cosmological anisotropy: evidence for causal horizons from CMB data

The origin of power asymmetry and other measures of statistical anisotropy on the largest scales of the universe, as manifested in Cosmic Microwave Background (CMB) and large-scale structure data, is a long-standing open question in cosmology. In this paper we analyse the Planck Legacy temperature anisotropy data and find strong evidence for a violation of the Cosmological principle of isotropy, with a probability of being a statistical fluctuation of order ∼10−9. The detected anisotropy is related to large-scale directional ΛCDM cosmological parameter variations across the CMB sky, that are sourced by three distinct patches in the maps with circularly-averaged sizes between 40 to 70 degrees in radius. We discuss the robustness of our findings to different foreground separation methods and analysis choices, and find consistent results from WMAP data when limiting the analysis to the same scales. We argue that these well-defined regions within the cosmological parameter maps may reflect finite and casually disjoint horizons across the observable universe. In particular we show that the observed relation between horizon size and mean dark energy density within a given horizon is in good agreement with expectations from a recently proposed model of the universe that explains cosmic acceleration and cosmological parameter tensions between the high and low redshift universe from the existence of casual horizons within our universe.

Ghost dark energy in Rastall theory

Making use of the generalized form of the Ghost dark energy density, which has the functional form [Formula: see text] where [Formula: see text] represents the Hubble expanding rate, the present accelerated enlargement behavior of the cosmos is investigated from the Rastall theory perspective. After finding a relation for the Hubble cosmic expansion rate, we consider recent cosmology-independent measurements calculated for the expansion history of the cosmos to fit the model via the [Formula: see text]-analysis. Moreover, we discuss the cosmographic properties of the model with the help of some cosmological quantities. We show that our model is stable and consistent with the recent astrophysical data. Also, for our model, we investigate cosmological interpretations of thermodynamics.

Cosmic expansion with curved dark energy cosmology: Inevitability of cosmic doomsday

In this work, we have constructed deceleration–acceleration and future evolution of cosmic expansion with curved dynamical dark energy models. Closed and open spatial curvatures are calculated by assuming that dark energy density does not exceed 85% of the closure density and by obtaining lower bounds on the ratio of dark energy to matter density, in terms of equation of state of dark energy. The range of transition epoch [Formula: see text] realized for spatial curvature [Formula: see text] is consistent with model independent estimations coming from galactic ages, strong lensing, Type Ia supernovae and recent constraints coming from [Formula: see text] measurements in non-flat dynamical dark energy models. Two novel parametrizations of dark energy equation of state namely the logarithmic and oscillatory, which are singularity free at future point [Formula: see text] are used to study the deceleration parameter q(z). Irrespective of spatial curvature, cosmic doomsday has been found inevitable for both the parametrizations. The time evolution of logarithmic parametrization, being reminiscent of a quintom field (canonical[Formula: see text]phantom), is converted into dynamics of a canonical quintessence and a phantom field for the redshift range ([Formula: see text],[Formula: see text]) and ([Formula: see text], [Formula: see text]). It is found that irrespective of spatial curvature, the quintessence component becomes sub-dominant in future giving it’s way to the phantom component.

Interacting Rényi Holographic Dark Energy in the Brans-Dicke Theory

In this work, we construct an interacting model of the Rényi holographic dark energy in the Brans-Dicke theory of gravity using Rényi entropy in a spatially flat Friedmann-Lemaître-Robertson-Walker Universe considering the infrared cut-off as the Hubble horizon. In this setup, we then study the evolutionary history of some important cosmological parameters, in particular, deceleration parameter, Hubble parameter, equation of state parameter, and Rényi holographic dark energy density parameter in both nonflat Universe and flat Universe scenarios and also observe satisfactory behaviors of these parameters in the model. We find that during the evolution, the present model can give rise to a late-time accelerated expansion phase for the Universe preceded by a decelerated expansion phase for both flat and nonflat cases. Moreover, we obtain ω D → − 1 as z → − 1 , which indicates that this model behaves like the cosmological constant at the future. The stability analysis for the distinct estimations of the Rényi parameter δ and coupling coefficient b 2 has been analyzed. The results indicate that the model is stable at the late time.

Non-Extensive Thermodynamics Effects in the Cosmology of f(T) Gravity

Using f(T) gravitational theory, we construct modified cosmological models via the first law of thermodynamics by using the non-extensive thermodynamics framework, the effects of which are captured by the parameter δ. The resulting cosmological equations are modified compared to the standard Einstein-Hilbert ones, with the modifications coming from the f(T) gravitational theory and from the non-extensive parameter which quantifies the non-extensive thermodynamics effects quantified by the parameter δ, which when is set equal to unity, one recovers the field equations of f(T) gravity. We study in detail the cosmological evolution of the model in the presence of collisionless non-relativistic matter case, and we derive the exact forms of the dark energy density parameter and of the dark energy equation of state parameter, from which we impose constraints on the non-extensive thermodynamics parameter, δ, by using the Planck 2018 data on cosmological parameters. Accordingly, we repeat our calculations after including the relativistic matter along with the non-relativistic one, and we derive the new forms of the dark energy density parameter and of the dark energy equation of state parameter. Our study shows that the inclusion of non-extensive thermodynamic effects, quantified by the parameter δ, for a flat Friedmann-Robertson-Walker Universe, has measurable differences compared with the normal thermodynamics case. We confront our results with Type Ia supernovae observations for z≥0.4 and we obtain reasonably agreement with the observational data.