Dark Energy In Universe Research Articles

Scenario of FLRW Dark Energy Universe and f(Q) Gravity

In this paper, we discuss the accelerated expansion of the Universe in the context of f(Q) gravity theory, where Q is the non-metricity scalar. We consider a well-established parametrization of the Hubble parameter in order to determine the solution of the field equations in the FLRW universe. To obtain the best-fit value for the model parameters we use the combined Hubble, BAO, and Pantheon datasets. In order to analyze the cosmological parameters, we take two different f(Q) models into consideration, Specifically, f(Q)=λQ2 and f(Q)=Q+mQ2, where λ is a constant and m is the free parameter. Additionally, we examine how statefinder analysis, the jerk parameter, and the Om diagnostic parameter behave. Finally, we came to the conclusion that the parameters of both f(Q) models show quintessential behavior and that the cosmos is in an accelerating phase.

Anisotropic dark energy models in f(R,Lm)-gravity

In this investigation, we have observed an anisotropic Bianchi type-I dark energy universe model in [Formula: see text] gravity with observational constraints, where [Formula: see text] is an arbitrary function of Ricci-scalar curvature [Formula: see text] and matter Lagrangian [Formula: see text]. For our investigation, we have considered a specific form of this function as [Formula: see text] with [Formula: see text] as model free parameters and [Formula: see text] is the “cosmological constant”. We have solved the field equations analytically and established a relationship between energy parameters [Formula: see text] and [Formula: see text] and obtained Hubble function [Formula: see text] in terms of redshift [Formula: see text]. We have compared this Hubble function with two observational datasets Union 2.1 plus bined and [Formula: see text] for obtaining the best fit values of model parameters and energy parameters, by applying [Formula: see text]-test. We have analyzed and discussed the results using these best fit values throughout the paper. We have found a accelerating transit phase phantom dark energy model which tends to [Formula: see text]CDM model at late-time universe.

Anisotropic dark energy universe in f(Q, T) gravity with observational constraints

Aim of this paper is to investigate an anisotropic locally rotationally symmetric (LRS) Bianchi type-I space–time in the context of the recently proposed f( Q, T) gravity, where Q is the non-metricity scalar and T is energy–momentum tensor. We have considered f( Q, T) = α Q + β T a linear form, where α and β are model parameters. We have analyzed the exact solution of LRS Bianchi type-I space–time by assuming relation between metric potential A = B n , where n is arbitrary non-zero real number. To study the anisotropic nature of the dynamical dark energy, we assume that the skewness parameters are time dependent and n ≠ 1. We have constrained to our model by using observational Hubble dataset. Onwards, discussed the physical behavior of cosmological parameters such as energy density, pressure, EoS parameter, deceleration parameter and, Energy conditions.

Detectable universes inside regular black holes

While spacetime in the vicinity outside astrophysical black holes is believed to be well understood, the event horizon and the interior remain elusive. Here, we discover a degenerate infinite spectrum of novel general relativity solutions with the same mass-energy and entropy that describe a dark energy universe inside an astrophysical black hole. This regular cosmological black hole is stabilized by a finite tangential pressure applied on the dual cosmological-black hole event horizon, localized up to a quantum indeterminacy. We recover the Bekenstein–Hawking entropy formula from the classical fluid entropy, calculated at a Tolman temperature equal to the cosmological horizon temperature. We further calculate its gravitational quasi-normal modes. We find that cosmological black holes are detectable by gravitational-wave experiments operating within the mu mathrm{Hz}–mathrm{Hz} range, like LISA space-interferometer.

Energy Basic State Field of the Universe (I)

This paper proposes the hypothesis that cosmic vacuum is full of energy basic state field (EBSF), and expounds its physical connotation based on cosmological constant of general relativity and Dirac’s negative energy sea. Cosmic vacuum is a special kind of physical object with complexity that can be characterized by quantum super-fluidity; it forms Dark energy in Universe. The rationality and correctness of this hypothesis are demonstrated through the analysis in terms of energy basic distribution on the background of cosmic scale and energy scale, quantum vacuum field, the evolution of EBSF state into static quantum’s state (particles or quasiparticle) and so on. Also it estimates the vacuum energy value in the energy basic state field to the same order of energy as the energy value for driving the accelerated expansion of the universe.

Creation and annihilation of a wormhole in a matter—dark energy universe

In this paper, we consider the creation/annihilation of a wormhole in the matter and dark energy universe based on wormhole cosmology. Several cases for the generation of a wormhole are reviewed, and the events are found to happen by controlling the input phantom (ghost) fields. However, when we adopt the wormhole cosmological model, a throat is created, expands, and disappears at late time with the hubble horizon in a matter-phantom-based universe background.

Cosmological model parameter dependence of the matter power spectrum covariance from the DEUS-PUR Cosmo simulations

ABSTRACT Future galaxy surveys will provide accurate measurements of the matter power spectrum across an unprecedented range of scales and redshifts. The analysis of these data will require one to accurately model the imprint of non-linearities of the matter density field. In particular, these induce a non-Gaussian contribution to the data covariance that needs to be properly taken into account to realize unbiased cosmological parameter inference analyses. Here, we study the cosmological dependence of the matter power spectrum covariance using a dedicated suite of N-body simulations, the Dark Energy Universe Simulation–Parallel Universe Runs (DEUS-PUR) Cosmo. These consist of 512 realizations for 10 different cosmologies where we vary the matter density Ωm, the amplitude of density fluctuations σ8, the reduced Hubble parameter h, and a constant dark energy equation of state w by approximately $10{{\ \rm per\ cent}}$. We use these data to evaluate the first and second derivatives of the power spectrum covariance with respect to a fiducial Λ-cold dark matter cosmology. We find that the variations can be as large as $150{{\ \rm per\ cent}}$ depending on the scale, redshift, and model parameter considered. By performing a Fisher matrix analysis we explore the impact of different choices in modelling the cosmological dependence of the covariance. Our results suggest that fixing the covariance to a fiducial cosmology can significantly affect the recovered parameter errors and that modelling the cosmological dependence of the variance while keeping the correlation coefficient fixed can alleviate the impact of this effect.

Halo Counts-in-cells for Cosmological Models with Different Dark Energy

Abstract We examine the counts-in-cells (CiC) probability distribution functions (PDFs) that describe dark matter halos in the Dark Energy Universe Simulations (DEUS). We describe the measurements between redshifts z = 0 to z = 4 on both linear and nonlinear scales. The best fits of the gravitational quasi-equilibrium distribution (GQED), the negative binomial distribution (NBD), the Poisson-Lognormal distribution (PLN), and the Poisson-Lognormal distribution with a bias parameter (PLNB) are compared to simulations. The fits agree reasonably consistently over a range of redshifts and scales. To distinguish quintessence (RPCDM) and phantom (wCDM) dark energy from Λ dark energy, we present a new method that compares the model parameters of the CiC PDFs. We find that the mean and variance of the halo CiC on 2–25h −1 Mpc scales between redshifts 0.65 < z < 4 show significant percentage differences for different dark energy cosmologies. On 15–25 h −1 Mpc scales, the g parameter in NBD, ω parameter in PLN, and b and C b parameters in PLNB show larger percentage differences for different dark energy cosmologies than on smaller scales. On 2–6 h −1 Mpc scales, the kurtosis and the b parameter in the GQED show larger percentage differences for different dark energy cosmologies than on larger scales. For cosmologies explored in the DEUS, the percentage differences between these statistics for the RPCDM and wCDM dark energy cosmologies relative to ΛCDM generally increases with redshift from a few percent to significantly larger percentages at z = 4. Applying our method to simulations and galaxy surveys can provide a useful way to distinguish among dark energy models and cosmologies in general.

Ray tracing the integrated Sachs-Wolfe effect through the light cones of the dark energy universe simulation-full universe runs

The late integrated Sachs-Wolfe (ISW) effect correlates the Cosmic Microwave Background (CMB) temperature anisotropies with foreground cosmic large-scale structures. As the correlation depends crucially on the growth history in the era of dark energy, it is a key observational probe for constraining the cosmological model. Here we present a detailed study based on full-sky and deep light cones generated from very large volume numerical N-body simulations, which allow us to avoid the use of standard replica techniques, while capturing the entirety of the late ISW effect on the large scales. We post-process the light cones using an accurate ray-tracing method and construct full-sky maps of the ISW temperature anisotropy for three different dark energy models. We quantify in detail the extent to which the ISW effect can be used to discriminate between different dark energy scenarios when cross-correlated with the matter distribution or the CMB lensing potential. We also investigate the onset of non-linearities, the so-called Rees-Sciama effect which provides a complementary probe of the dark sector. We find the signal of the lensing-lensing and ISW-lensing correlation of the three dark energy models to be consistent with measurements from the Planck satellite. Future surveys of the large-scale structures may provide cross-correlation measurements that are sufficiently precise to distinguish the signal of these models. Our methodology is very general and can be applied to any dark energy or modified gravity scenario as long as the metric seen by photons can still be characterized by a Weyl potential.

What Can the Anthropic Principle Tell Us about the Future of the Dark Energy Universe

An anthropic explanation for evident smallness of the value of the dark energy implies the existence of a time-dependent component of the scalar field, serving, together with a negative-valued cosmological constant, as one of two components to the overall density of the dark energy. The observers (i.e. us) might then only evolve in those regions of the universe where the sum of those two components (a positive and a negative ones) is sufficiently close to zero. However, according to Vilenkin and Garriga, the scalar field component has to slowly but surely diminish in time. In about a trillion years this process will put a cap to the now-observable accelerated expansion of the universe, leading to a subsequent phase of impending collapse. However, the vanishing scalar field might also produce some rather unexpected singularities for a finite non-zero scale factor. We analyse this possibility on a particular example of Sudden Future Singularities (SFS) and come to a startling conclusion: the time required for SFS to arise must be "comparable" to the lifetime of observable universe.

Investigating cosmology with equation of state

The aim of this paper is to investigate the field equations of modified [Formula: see text] theory of gravity, where R and [Formula: see text] represent the Ricci scalar and scalar potential, respectively. We consider the Friedmann–Robertson–Walker space–time for finding some exact solutions by using different values of equation of state parameter. In this regard, different possibilities of the exact solutions have been discussed for dust universe, radiation universe, ultra-relativistic universe, sub-relativistic universe, stiff universe, and dark energy universe. Mainly power law and exponential forms of the scale factor are chosen for the analysis.

Study of Entropy-Corrected Models Using Dark Energy in the Framework of a Complementary Gravitational Field

The dark energy (DE) model of a complementary gravitational field is used to address the entropy-corrected cosmological model. The dominant DE universe can originate from an indirect coupling between vacuum energy and another complementary gravitational field. This idea is used together with entropy-corrected models to study the evolution of DE in the universe. The zero point energy is calculated from one-loop corrections and used to reconstruct the scale factor and the Hubble parameter. It is then used to evaluate the equation of state (EoS) parameter for both interacting and non-interacting DE and the cosmic pressure. The square of sound speed, used to assess the stability of DE models, and the deceleration parameter are obtained. Further, the geometrical statefinder parameters s and r and a new diagnostic statefinder parameter, om(z), are evaluated and used to distinguish between the energy densities of various DE models. It is found that the studied cosmological functions show strong variation with cosmic time, and numerical results show a good agreement with observations.

Dynamics of anisotropic dark energy universe embedded in one-directional magnetized fluid

In this work, we have constructed an anisotropic dark energy cosmological model in a two-fluid situation, such as the usual dark energy and the magnetized fluid. We have assumed the dark energy pressure to be anisotropic in different spatial directions. In order to develop the mathematical formalism of the model, we have considered the scale factor as hybrid scale factor which is a combination of both power law and exponential volumetric expansion law. The physical parameters are derived and analyzed and found to be in agreement with the observational limits.

Holographic dark energy models in LTB inhomogeneous universe

The holographic principle is investigated for the generalized Lemaitre–Tolman–Bondi (LTB) model of an inhomogeneous and an isotropic dark energy. The inhomogeneity is considered as spherically symmetric and locally flat overdense of dark energy with a few Gpc in size and has a core size on the scale of apparent horizon. Two particular forms of refined parabolic arbitrary scale function R(r, t) are used in the LTB metric and some cosmological functions are calculated and plotted with the cosmological time and the distance from the center of the inhomogeneity. By choosing specific values of some numerical parameters of the models, we showed that some of these functions are in agreement with recent observations and have behavior similar to that of the cosmological constant dark energy universe.

Kantowski–Sachs modified holographic Ricci dark energy model in Saez–Ballester theory of gravitation

We have considered Kantowski–Sachs space–time in the presence of matter and anisotropic modified holographic Ricci dark energy components in the scalar–tensor theory of gravitation formulated by Saez and Ballester (Phys. Lett. A, 113, 467, 1986) and derived the field equations of the theory. We have used (i) hybrid expansion law proposed by Akarsu et al. (JCAP, 022, 2014), (ii) a relation between metric potentials, and (iii) modified holographic Ricci dark energy density given by Chen and Jing (Phys. Lett. B, 679, 144, 2009) to obtain an exact solution of the field equations that describes a Kantowski–Sachs holographic modified Ricci dark energy universe in this theory. Physical and kinematical parameters are also computed and their physical behavior is discussed.

Cosmological perturbations in an effective and genuinely phantom dark energy Universe

We carry out an analysis of the cosmological perturbations in general relativity for three different models which are good candidates to describe the current acceleration of the Universe. These three set-ups are described classically by perfect fluids with a phantom nature and represent deviations from the most widely accepted $\Lambda$CDM model. In addition, each of the models under study induce different future singularities or abrupt events known as (i) Big Rip, (ii) Little Rip and (iii) Little Sibling of the Big Rip. Only the first one is regarded as a true singularity since it occurs at a finite cosmic time. For this reason, we refer to the others as abrupt events. With the aim to find possible footprints of this scenario in the Universe matter distribution, we not only obtain the evolution of the cosmological scalar perturbations but also calculate the matter power spectrum for each model. We have carried the perturbations in the absence of any anisotropic stress and within a phenomenological approach for the speed of sound. We constrain observationally these models using several measurements of the growth rate function, more precisely $f\sigma_8$, and compare our results with the observational ones.

On the uniqueness of solution to generalized Chaplygin gas

The main object of the paper is finding a unique solution to Riemann problem for generalized Chaplygin gas model. That is a model of the dark energy in Universe introduced in the last decade. It permits an infinite mass concentration so one has to consider solutions containing the Dirac delta function. Although it was easy to construct solution to any Riemann problem, the usual admissibility conditions, overcompressiveness, do not exclude unwanted delta-type waves when a classical solution exists. We are using Shadow Wave approach in order to solve that uniqueness problem since they are well adopted for using Lax entropy–entropy flux conditions and there is a rich family of convex entropies.

Viscous holographic dark energy universe with Nojiri-Odintsov cut-off

In this paper we consider a toy model of large scale universe, when one of the dark components is a generalized holographic DE with a Nojiri-Odintsov cut-off. On the other hand, we made another assumption concerning to the companion of considered DE model due to impossibility of exact parameterization of dark side of the large scale universe. In particular, we assume an inhomogeneous viscous dark fluid discussed in literature very actively, to be possible candidate for the companion. In addition to cosmographic analysis we discuss the results from $Om$ and two point $Om$ analysis and estimated present day values of statefinder parameters $(r,s)$ and $(\omega^{\prime }_{de}, \omega_{de})$ . Moreover, validity of generalized second law of thermodynamics is studied showing that theoretical results are well consistent with observational data. Constraints on cosmological parameters are due to Planck 2015 experiments.

The role of Lagrange multiplier in Gauss–Bonnet dark energy

We review accelerating cosmology in Gauss–Bonnet gravity with Lagrange multiplier constraint studied in [S. Capozziello, A. N. Makarenko and S. D. Odintsov, Phys. Rev. D 87 (2013) 084037, arXiv: 1302.0093 [gr-qc], S. Capozziello, M. Francaviglia and A. N. Makarenko, Astrophys. Space Sci. 349 (2014) 603–609, arXiv: 1304.5440 [gr-qc]. Several examples of dark energy universes are presented. We can get new dark energy solutions (with additional scalar) as well as certain limits to earlier found accelerating solutions.

О вязкой Вселенной голографической темной энергии с обрезками Ножири-Одинцова

О вязкой Вселенной голографической темной энергии с обрезками Ножири-Одинцова