Dark Energy Density Parameter Research Articles

Cosmology of f(Q) gravity in non-flat Universe

We investigate the cosmological implications of f(Q) gravity, which is a modified theory of gravity based on non-metricity, in non-flat geometry. We perform a detailed dynamical-system analysis keeping the f(Q) function completely arbitrary. As we show, the cosmological scenario admits a dark-matter dominated point, as well as a dark-energy dominated de Sitter solution which can attract the Universe at late times. However, the main result of the present work is that there are additional critical points which exist solely due to curvature. In particular, we find that there are curvature-dominated accelerating points which are unstable and thus can describe the inflationary epoch. Additionally, there is a point in which the dark-matter and dark-energy density parameters are both between zero and one, and thus it can alleviate the coincidence problem. Finally, there is a saddle point which is completely dominated by curvature. In order to provide a specific example, we apply our general analysis to the power-law case, showing that we can obtain the thermal history of the Universe, in which the curvature density parameter may exhibit a peak at intermediate times. These features, alongside possible indications that non-zero curvature could alleviate the cosmological tensions, may serve as advantages for f(Q) gravity in non-flat geometry.

Observational constraints on Yukawa cosmology and connection with black hole shadows

We confront Yukawa modified cosmology, proposed in Jusufi et al. [arXiv:2304.11492], with data from Supernovae Type Ia (SNe Ia) and Hubble parameter (OHD) observations. Yukawa cosmology is obtained from a Yukawa-like gravitational potential, with coupling parameter α and wavelength parameter λ, which gives rise to modified Friedmann equations. We show that the agreement with observations is very efficient, and within 1σ confidence level we find the best-fit parameters λ=2693−1262+1191Mpc, α=0.416−0.326+1.137, and a graviton mass of mg=2.374−0.728+2.095×10−42GeV. Additionally, we establish a connection between the effective dark matter and dark energy density parameters and the angular radius of the black hole shadow of the SgrA and M87 black holes in the low-redshift limit, consistent with the Event Horizon Telescope findings.

Possible combinations of early and late time cosmologies through BAO scales

The theoretical scenarios of early universe and late time accelerated expansion constitute an open problem in cosmology. Relative Baryon Acoustic Oscillation (BAO) scales measured by 6dFGS, mathrm SDSS+2dFGRS, BOSS DR11 Lyalpha and BOSS DR11 QSO Lyalpha are used to investigate the parameter spaces of combinations of pre-recombination physics and late time cosmology. For pre-recombination physics we fix mathrm N_{eff}=3 and 4 and incorporate an Early Dark Energy (EDE) component. For late time cosmologies we consider curved Lambda CDM model, Dynamical Dark Energy (DDE) model with CPL parameterisation (omega _0,omega _1) of equation of state, Coupled Dark Energy (CDE) model with constant coupling and curved DGP brane-world scenario. Combination of a curved Lambda CDM (Omega _{textrm{k}}Lambda CDM) model with mathrm N_{eff}-EDE cosmology is found to be possible. For DDE, the relative BAO scales are achievable through omega _1 preferring the region omega _1< 0. For mathrm N_{eff}-EDE-CDE, the BAO scales are generated for both dark energy-dark matter and dark matter-dark energy conversion, however with exceptionally large value of dark energy density parameter and constant equation of state with omega _0 > -1. Curved DGP cosmology (Omega _textrm{k}DGP) is found to join with N_{textrm{eff}}-EDE cosmology through curvature parameter Omega _{mathrm{k(0)}}in [-0.002, -0.1] and cross-over parameter Omega _{mathrm{r_c}} in [0.08, 0.45]. Cosmic ages are calculated for the derived parameters of Omega _{textrm{k}}Lambda CDM, flat DDE and Omega _{textrm{k}}DGP models and compared with the ages of few globular clusters available in literature. Curved cosmologies are found to be promising.

Kaniadakis agegraphic dark energy

In this manuscript, we present a novel dark energy scenario based on Kaniadakis entropy, which is a generalization of Boltzmann–Gibbs entropy that arises from relativistic statistical theory and is characterized by a parameter K which quantifies the deviations from standard expressions, and we use the age of the Universe as infrared cutoff. The cosmological characteristics of the proposed dark energy model, as well as the evolution of the cosmos filled with pressure-free matter and the ensuing dark energy candidates, are investigated. The interaction as well as non-interaction among the two sectors will also be considered. The differential equation for the dark energy density parameter, including the expression of the equation of state and deceleration parameters, are derived. The analysis of deceleration parameter clearly shows the universe to transit from decelerated to accelerated phase around z≈0.6. The most recent 30 H(z) data samples and the 580 Union 2.1 SNIa data set were also used to limit the theoretical results. The squared sound speed is also plotted against redshift z to check the stability behavior of the model for both the cases.

Transit string dark energy models in f(Q) gravity

In this paper, we have investigated an anisotropic cosmological model in [Formula: see text] gravity with string fluid in LRS Bianchi type-I universe. We have considered the arbitrary function [Formula: see text], where [Formula: see text] is model free parameter and [Formula: see text] is the cosmological constant. We have established a relationship between matter energy density parameter [Formula: see text] and dark energy density parameter [Formula: see text] through Hubble function using constant equation of state parameter [Formula: see text]. We have made observational constraint on the model using [Formula: see text]-test with observed Hubble datasets [Formula: see text] and SNe Ia datasets, and obtained the best fit values of cosmological parameters. We have used these best fit values in the result and discussion. We have discussed our result with cosmographic coefficients and found a transit phase dark energy model. Also, we analyzed the Om diagnostic function for anisotropic universe and found that our model is quintessence dark energy model.

Theoretical analysis on the Rényi holographic dark energy in the fractal cosmology

In this paper, we investigate the Rényi holographic dark energy model with the interaction between dark energy and dark matter within the framework of the fractal cosmology in which the Hubble horizon is considered as the IR cutoff. In this setting, we derive the evolution equation of the Rényi holographic dark energy density parameter, the equation of state (EoS) parameter and deceleration parameter. We find that the model in the fractal cosmology can explain the accelerated expansion of the universe. In addition, we discuss the statefinder diagnosis of this model, plotting the curves of [Formula: see text] and [Formula: see text] with redshift [Formula: see text] and the evolutionary trajectories of [Formula: see text]. We find that statefinder can ideally break the degeneracy of different coupling parameter values in this model. Moreover, we find that the statefinder pair [Formula: see text] performs better than [Formula: see text] and [Formula: see text] in this model.

New Tsallis holographic dark energy with future event horizon as IR-cutoff in non-flat Universe

In this work, new Tsallis holographic dark energy (NTHDE) with future event horizon as IR-cutoff is constructed in a non-flat Friedmann–Lemaitre–Robertson–Walker Universe. The accelerating expansion phase of the universe is described by using deceleration parameter, equation of state parameter and density parameter by using different values of NTHDE parameter “[Formula: see text]” and “[Formula: see text]”. We specifically derive the differential equations for the dark-energy density parameter (DP) and hence the equation of state parameter (EoS) by taking into account closed and open spatial geometry. In both a closed and an open universe, the equation of state parameter exhibits pure quintessence behavior for [Formula: see text], quintom behavior for [Formula: see text], and [Formula: see text]CDM model recovery for [Formula: see text]. We can see the phase changes from deceleration to acceleration at [Formula: see text] by tracking the evolution of the deceleration parameter. As inferred from the evolution of the Hubble parameter, NTHDE in a non-flat universe precisely matches Hubble data. Stability of our model by analyzing the squared speed of sound is investigated as well.

Probing Quintessence using BAO imprint on the cross-correlation of weak lensing and post-reionization H i 21 cm signal

ABSTRACT In this work, we investigate the possibility of constraining a thawing Quintessence scalar field model for dark energy. We propose using the imprint of baryon acoustic oscillation on the cross-correlation of post-reionization 21-cm signal and galaxy weak lensing convergence field to tomographically measure the angular diameter distance DA(z) and the Hubble parameter H(z). The projected errors in these quantities are then used to constrain the Quintessence model parameters. We find that independent 600 h radio interferometric observation at four observing frequencies 916, 650, 520, and 430 MHz with an SKA-1-Mid like radio telescope in cross-correlation with a deep weak lensing survey covering half the sky may measure the binned DA and H at a few per cent level of sensitivity. The Monte Carlo analysis for a power-law thawing Quientessence model gives the 1 − σ marginalized bounds on the initial slope λi, dark energy density parameter Ωϕ0 and the shape of the potential Γ at 8.63, 10.08, and $9.75{{\ \rm per\ cent}}$, respectively. The constraints improve to 7.66, 4.39, and $5.86{{\ \rm per\ cent}}$, respectively, when a joint analysis with supernovae and other probes is performed.

New Tsallis agegraphic dark energy

The proposed model is a study of the nature of dark energy through nonextensive Tsallis entropy. The method is based on the Karolyhazy relation which is a combined idea from quantum physics and general relativity. Dark energy is the energy density of quantum fluctuations in spacetime. This is the key idea behind proposing agegraphic dark energy (ADE) models here. The parameter [Formula: see text] is used to measure the quantitative distinction from the standard scenario. To look at the cosmological implications of the hypothesized dark energy model, as well as the expansion of the universe filled with zero pressure matter and the resulting dark energy alternatives, the role of IR cutoff is played by age of the universe. The dynamic behavior of dark energy density parameter is carried out. The expressions for the equation of state parameter and deceleration parameter are obtained. The analysis is performed by taking into account a no flow as well as a flow of energy among the dark matter and dark energy sectors of the universe.

Kaniadakis holographic dark energy and cosmology

We construct a holographic dark energy scenario based on Kaniadakis entropy, which is a generalization of Boltzmann-Gibbs entropy that arises from relativistic statistical theory and is characterized by a single parameter K which quantifies the deviations from standard expressions, and we use the future event horizon as the Infrared cutoff. We extract the differential equation that determines the evolution of the effective dark energy density parameter, and we provide analytical expressions for the corresponding equation-of-state and deceleration parameters. We show that the universe exhibits the standard thermal history, with the sequence of matter and dark-energy eras, while the transition to acceleration takes place at zapprox 0.6. Concerning the dark-energy equation-of-state parameter we show that it can have a rich behavior, being quintessence-like, phantom-like, or experience the phantom-divide crossing in the past or in the future. Finally, in the far future dark energy dominates completely, and the asymptotic value of its equation of state depends on the values of the two model parameters.

Power-law holographic dark energy and cosmology

We formulate power-law holographic dark energy, which is a modified holographic dark energy model based on the extended entropy relation arising from the consideration of state mixing between the ground and the excited ones in the calculation of the entanglement entropy. We construct two cases of the scenario, imposing the usual future event horizon choice, as well as the Hubble one. Thus, the former model is a one-parameter extension of standard holographic dark energy, recovering it in the limit where power-law extended entropy recovers Bekenstein–Hawking one, while the latter belongs to the class of running vacuum models, a feature that may reveal the connection between holography and the renormalization group running. For both models we extract the differential equation that determines the evolution of the dark-energy density parameter and we provide the expression for the corresponding equation-of-state parameter. We find that the scenario can describe the sequence of epochs in the Universe evolution, namely the domination of matter followed by the domination of dark energy. Moreover, the dark-energy equation of state presents a rich behavior, lying in the quintessence regime or passing into the phantom one too, depending on the values of the two model parameters, a behavior that is richer than the one of standard holographic dark energy.

FRW cosmological models with Barrow holographic dark energy in Brans–Dicke theory

In this paper, we have generalized the behaviors of transit cosmological model under the observational data with Barrow holographic dark energy. We consider the scale factor field as [Formula: see text] to get exact solutions for the field equations in a non-flat FRW universe in Brans–Dicke theory. The values of the model parameters [Formula: see text] and [Formula: see text] are obtained by best fitting of 46 observational Hubble data (OHD) points in the range [Formula: see text]. The derived model exhibits a transition scenario for open, flat and closed universe. The EoS parameter shows a quintom-like behavior, lies both quintessence ([Formula: see text]) and phantom ([Formula: see text]) regions and crosses the phantom divide. The matter and dark energy density parameters [Formula: see text], scalar field [Formula: see text] and other cosmological parameters provide the results consistent with the recent observational datasets. Some other physical and geometrical behaviors of BHDE are also described and the satisfactory behaviors are found with current observations (OHD+JLA).

New Tsallis holographic dark energy

Tsallis entropy is a generalization of the Boltzmann–Gibbs entropy in statistical theory which uses a parameter delta to measure the deviation from the standard scenario quantitatively. Using concepts of Tsallis entropy and future event horizon, we construct a new Tsallis holographic dark energy model. The parameters c and delta will be used to characterize various aspects of the model. Analytical expressions for various cosmological parameters such as the differential equation describing the evolution of the effective dark energy density parameter, the equation of state parameter and the deceleration parameter are obtained. The equation of state parameter for the current model exhibits the pure quintessence behaviour for c>1, quintom behaviour for c<1 whereas the Lambda CDM model is recovered for c=1. To analyze the thermal history of the universe, we obtained the expression for the deceleration parameter and found that for z approx 0.6, the phase transits from deceleration to acceleration.

Barrow holographic dark energy in a nonflat universe

We construct Barrow holographic dark energy in the case of nonflat universe. In particular, considering closed and open spatial geometry we extract the differential equations that determine the evolution of the dark-energy density parameter, and we provide the analytical expression for the corresponding dark energy equation-of-state parameter. We show that the scenario can describe the thermal history of the universe, with the sequence of matter and dark energy epochs. Comparing to the flat case, where the phantom regime is obtained for relative large Barrow exponents, the incorporation of positive curvature leads the universe into the phantom regime for significantly smaller values. Additionally, in the case of negative curvature we find a reversed behavior, namely for increased Barrow exponent we acquire algebraically higher dark-energy equation-of-state parameters. Furthermore, we confront the scenario with Hubble parameter measurements and supernova type Ia data. Hence, the incorporation of slightly non-flat spatial geometry to Barrow holographic dark energy improves the phenomenology while keeping the new Barrow exponent to smaller values.

Theoretical analysis on the Rényi holographic dark energy in the Finsler–Randers cosmology

In this work, we study the cosmological consequences of Rényi holographic dark energy model in the framework of the Finsler–Randers cosmology in which the Hubble horizon is considered as the IR cutoff. In this setup, we derive the evolution equation for the Rényi holographic dark energy density parameter, the equation of state (EoS) parameter and deceleration parameter. Our study shows that this model can describe the current accelerating universe in both noninteracting and interacting scenarios, and also a transition occurs from the deceleration phase to the accelerated phase at the late-time. Moreover, we discuss the statefinder diagnosis of this model, meanwhile, plot the curves of r and s versus redshift z and the evolutionary trajectories of [Formula: see text]. We find that statefinder can not only break the degeneracy of different coupling parameter values in this model, but also effectively distinguish the difference between the Rényi holographic dark energy model and the [Formula: see text]CDM model. In addition, we find that the statefinder pair [Formula: see text] performs better than [Formula: see text] and [Formula: see text] in this model.

Configurational entropy in Chaplygin gas models

The present work employs the Linder parametrization of a constant growth index \cite{linder/index} to investigate the evolution of growth rate of clustering and the dissipation of configurational entropy in some of the most widely studied Chaplygin gas models, such as the generalized Chaplygin gas and the modified Chaplygin gas. The model parameters of the Chaplygin gas models are found to play a vital role in the evolution of growth rate, dark energy density parameter, EoS parameter, and configurational entropy. Furthermore, the work communicates the rate of change of configurational entropy to attain a minimum which depend solely on the choice of model parameters and that there exist suitable parameter combinations giving rise to a viable dissipation of configurational entropy, and therefore certifying its time derivative to hit a minimum at a scale factor which complies with the current observational constraints on the redshift of transition from a dust to an accelerated Universe and thereby making Chaplygin gas models a viable candidate for dark energy.

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.

Barrow holographic dark energy

We formulate Barrow holographic dark energy, by applying the usual holographic principle at a cosmological framework, but using the Barrow entropy instead of the standard Bekenstein-Hawking one. The former is an extended black-hole entropy that arises due to quantum-gravitational effects which deform the black-hole surface by giving it an intricate, fractal form. We extract a simple differential equation for the evolution of the dark energy density parameter, which possesses standard holographic dark energy as a limiting sub-case, and we show that the scenario can describe the universe thermal history, with the sequence of matter and dark energy eras. Additionally, the new Barrow exponent $\Delta$ significantly affects the dark energy equation of state, and according to its value it can lead it to lie in the quintessence regime, in the phantom regime, or experience the phantom-divide crossing during the evolution.

Modified cosmology through spacetime thermodynamics and Barrow horizon entropy

We present modified cosmological scenarios that arise from the application of the “gravity-thermodynamics” conjecture, using the Barrow entropy instead of the usual Bekenstein-Hawking one. The former is a modification of the black hole entropy due to quantum-gravitational effects that deform the black-hole horizon by giving it an intricate, fractal structure. We extract modified cosmological equations which contain new extra terms that constitute an effective dark-energy sector, and which coincide with the usual Friedmann equations in the case where the new Barrow exponent acquires its Bekenstein-Hawking value. We present analytical expressions for the evolution of the effective dark energy density parameter, and we show that the universe undergoes through the usual matter and dark-energy epochs. Additionally, the dark-energy equation-of-state parameter is affected by the value of the Barrow deformation exponent and it can lie in the quintessence or phantom regime, or experience the phantom-divide crossing. Finally, at asymptotically large times the universe always results in the de-Sitter solution.