Narrow Results

This paper explores cosmic evolution in $f(R,T)$ gravity considering two distinct models with minimal coupling that are linear and quadratic in $T$; $f(R,T)=R+\alpha T$ and $f(R,T)=R+\alpha T^2$. The flat FRW space-time under the matter and radiation distribution has been considered to complete the analysis. In this background, the dynamical equations for dust in terms of dimensionless variables are developed. Via solutions of these dynamical equations, we develop significant cosmological parameters to compare the obtained results with the $\mathrm{\Lambda }$CDM model and analyze them graphically. It is found that these parameters efficiently explain the expanding phenomenon of the universe for constrained values of the involved model parameters using some recent observational data. Moreover, the results are found to be stable via the square speed of sound. The matter and radiation fixed points are calculated by an autonomous set of dynamical equations, which are the stable and unstable saddle points. Also, the growth of matter perturbation in the $f(R,T)$ theory has been discussed. Under this scenario, it is also checked that exact solutions of the dynamical equations are not possible for non-minimal coupling between geometry and matter.

Using the absolute ages of passively evolving galaxies observed at different redshifts, one can obtain the differential ages, the derivative of redshift z with respect to the cosmic time t (i.e. dz/dt). Thus, the Hubble parameter H(z) can be measured through the relation H(z) = -(dz/dt)/(1+z). By comparing the measured Hubble parameter at different redshifts with the theoretical one containing free cosmological parameters, one can constrain current cosmological models. In this paper, we use this method to present the constraint on a spatially flat Friedman–Robert–Walker universe with a matter component and a holographic dark energy component, in which the parameter c plays a significant role in this dark energy model. Firstly we consider three fixed values of c = 0.6, 1.0 and 1.4 in the fitting of data. If we set c free, the best fitting values are c = 0.26, Ω_m0 = 0.16, h = 0.9998. It is shown that the holographic dark energy behaves like a quintom-type at the 1σ level. This result is consistent with some other independent cosmological constrains, which imply that c < 1.0 is favored. We also test the results derived from the differential ages using another independent method based on the lookback time to galaxy clusters and the age of the universe. It shows that our results are reliable.

In this paper, we use a set of observational H(z) data (OHD) to constrain the ΛCDM cosmology. This data set can be derived from the differential ages of the passively evolving galaxies. Meanwhile, the -parameter, which describes the Baryonic Acoustic Oscillation (BAO) peak, and the newly measured value of the Cosmic Microwave Background (CMB) shift parameter are used to present combinational constraints on the same cosmology. The combinational constraints favor an accelerating flat universe while the flat ΛCDM cosmology is also analyzed in the same way. We obtain a result compatible with that by many other independent cosmological observations. We find that the observational H(z) data set is a complementarity to other cosmological probes.

In this paper, the cosmological "constant" and the Hubble parameter are considered in the Weyl theory of gravity, by taking them as functions of r and t, respectively. Based on this theory and in the linear approximation, we obtain the values of H₀ and Λ₀ which are in good agreement with the known values of the parameters for the current state of the universe.

We introduce a set of two-parameter models for the dark energy equation of state (EOS) w(z) to investigate time-varying dark energy. The models are classified into two types according to their boundary behaviors at the redshift z = (0, ∞) and their local extremum properties. A joint analysis based on four observations (SNe + BAO + CMB + H₀) is carried out to constrain all the models. It is shown that all models get almost the same and the cosmological parameters (Ω_M, h, Ω_bh²) with the best-fit results (0.28, 0.70, 2.24), although the constraint results on two parameters (w₀, w₁) and the allowed regions for the EOS w(z) are sensitive to different models and a given extra model parameter. For three of Type I models which have similar functional behaviors with the so-called CPL model, the constrained two parameters w₀ and w₁ have negative correlation and are compatible with the ones in CPL model, and the allowed regions of w(z) get a narrow node at z~0.2. The best-fit results from the most stringent constraints in Model Ia give which may compare with the best-fit results in the CPL model. For four of Type II models which have logarithmic function forms and an extremum point, the allowed regions of w(z) are found to be sensitive to different models and a given extra parameter. It is interesting to obtain two models in which two parameters w₀ and w₁ are strongly correlative and appropriately reduced to one parameter by a linear relation w₁∝(1+w₀).

One of the most outstanding problems of the standard model of cosmology today is the problem of cosmological constant/dark energy. It corresponds to about 73% of the energy content of the universe gone missing. I hereby postulate a modified FRW metric for our universe, which animates a universe spinning very slowly with an angular frequency that is equal to the Hubble's constant. It is shown by a simple argument that in such a universe there will be an overlooked rotational energy whose average value is identically equal to the matter-energy content of this universe as observed by a coordinate observer. The corresponding Friedmann equation is derived, whereby the overlooked rotational energy naturally becomes the cosmological constant/dark energy term without artificially/mysteriously adding any such term, as is commonly done. The proposed model also produces Hubble's law naturally.

In this paper, we consider the interacting generalized dark energy with cold dark matter and analyze the behavior of evolution parameter via dark energy and interacting parameters. It is found that the evolution parameter crosses the phantom divide line in most of the cases of integration constants. We also establish the correspondence of scalar field models (quintessence, k-essence and dilaton) with this dark energy model in which scalar fields show the increasing behavior. The scalar potential corresponds to attractor solutions in quintessence case.

Recently, the Chiral Cosmological Model (CCM) coupled to cold dark matter (CDM) has been investigated as $\sigma$CDM model to study the observed accelerated expansion of the Universe. Dark sector fields (as Dark Energy content) coupled to cosmic dust were considered as the source of Einstein gravity in Friedmann–Robertson–Walker (FRW) cosmology. Such model had a beginning at the matter-dominated era. The purposes of our present investigation are two-fold: To extend “life” of the $\sigma$CDM for earlier times to radiation-dominated era and to take into account variation of the exponential potential $V=V_{0}exp\left(\right.-\sqrt{\lambda }\frac{\phi }{M_{\mathrm{\text{P}}}}\left)\right.+V_{0}exp\left(\right.-\sqrt{\lambda }\frac{\chi }{M_{\mathrm{\text{P}}}}\left)\right.$ via variation of the interaction parameter $\lambda$.

We use Markov Chain Monte Carlo (MCMC) procedure to investigate possible values of initial conditions constrained by the measured amount of the dark matter, dark energy and radiation component today.

Our analysis includes dark energy contribution to critical density, the ratio of the kinetic and potential energies, deceleration parameter, effective equation of state (EoS) and evolution of DE EoS with variation of coupling constant $\lambda$. A comparison with the $\Lambda$CDM model was performed. A new feature of the model is the existence of some values of potential coupling constant, leading to a $\sigma$CDM solution without transition into accelerated expansion epoch.

Accelerating dynamics from Born–Infeld-f(R) gravity are studied in a simplified conformal approach without matter. Explicit unification of inflation with late-time acceleration is realized within this singular inflation approach, which is similar to Odintsov–Oikonomou singular f(R) inflation. Our model turns out to be consistent with the latest release of Planck data.

Among various dark energy models, Tsallis holographic dark energy model shows the dynamical enthusiasm to describe the transition phase of the universe. In this paper, we consider Tsallis holographic dark energy with event and apparent horizon as an infrared cutoff in the framework of dynamical Chern–Simon modified gravity and non-flat FRW universe. We explore Hubble, equation of state and deceleration parameters and found that Hubble parameter lies in the range $80_{-60}^{+60}$ and $120_{-40}^{+40}$ for event and apparent horizon trajectories, respectively. It is mentioned here that the equation of state parameter lies within the range $-1.3\mp 0.4$ (event) and $-1.125\mp 0.125$ (apparent). Also, deceleration parameter for both cases show accelerated and decelerated phase of universe as well as cosmological constant. Moreover, we also checked the stability of our model through square speed of sound, which shows the positive behavior (exhibits the stability of the model). Finally, we observe that the generalized second law of thermodynamics remains valid in both cases of horizon.

In this paper, we study the behavior of non-interacting and interacting pilgrim dark energy (DE) for non-flat FRW model in Brans–Dicke (BD) theory. We consider the future event horizon as well as logarithmic form of BD scalar field $\psi \propto ln(\gamma +\delta a)$, where $\gamma >1$, $\delta >0$ and $a$ is the scale factor. We evaluate some well-known cosmological parameters such as equation of state as well as deceleration parameter and ${\omega }_{\mathrm{\Lambda }}-{\omega }_{\mathrm{\Lambda }}^{\prime }$ plane as well as statefinder parameters. We discuss graphical behavior of these parameters through pilgrim DE parameter $\mu =-1.2,1,1.2$ for both non-interacting as well as interacting case with interacting parameter $b^2=0.02,0.5,1$. It is found that the equation of state parameter gives consistent results with the current cosmic behavior while the deceleration parameter represents transition from decelerated to accelerated phase. The cosmological planes represent different DE regions. Finally, we discuss stability of the model through squared speed of sound in both cases.

We investigate the consequences of incepting the Bianchi type I metric in the $f(R,T)$ gravity theory field equations. We particularly derive solutions for a matter-dominated universe. From such a scenario, it is possible to predict a late-time de Sitter universe. Moreover, depending on the numerical fitting function for the scale factor, the universe is predicted to bounce and evade the Big Bang singularity.

By assuming generalized nonlinear and linear interaction term between dark matter and dark energy, we investigate the cosmic accelerated expansion of the universe. For this reason, we suppose a flat fractal universe platform as well as Tsallis holographic dark energy model. The Hubble horizon is being adopted as an infrared cutoff and extracted different cosmological parameters as well as plane. It is observed that equation-of-state parameter exhibits the quintom-like nature while (${\omega }_d$–${\omega }_d^{\prime }$) lies in thawing and freezing regions for different parametric values for both the cases. Furthermore, the squared sound speed shows stable behavior for nonlinear interaction term but shows the partially stable behavior for linear term. For both cases, the deceleration parameter leads to the accelerated phase of the universe and the consequences are comparable with observational data. The results for $r$–$s$ plane, leads to the quintessence and phantom region of the universe for nonlinear case while this plane represents the Chaplygin gas behavior for linear term. The $Om$ diagnostic also shows the satisfying results.

Measurements for the expansion rate of the universe disagree. Indeed, local measurements suggest a higher value of the Hubble constant than those performed through the cosmic microwave background. This fact led to a very interesting debate within the scientific community. The paper is not devoted to give solutions to the problem of “Hubble tension”. The aim of this paper is, on the contrary, to deduce the ${\mathrm{\Omega }}_{0m}{h}^2$ cosmological parameter from a theoretical point of view, using only two experimental data: the temperature of CMB today and the temperature of photons near the decoupling time.

Locally Rotationally Symmetric (LRS) Bianchi type-I metric is examined in the presence of perfect fluid. Exact solutions of Einstein’s field equations (EFE) have been studied by taking into account a hyperbolic scale factor. We observed that the model has initial singularity. It is found that the Universe approaches isotropy at late times. Through state finder pair $\{r,s\}$, it is observed that at late cosmic time, the model behaves analogous to $\mathrm{\Lambda }$CDM model. Energy conditions of the model are studied and it is found that null energy condition (NEC), weak energy condition (WEC) and dominant energy conditions (DEC) are satisfied for our model while SEC is violated. We investigate some physically and geometrically realistic models in order to develop a viable cosmological model.

The early Universe was characterized by the presence of heavy particles that decoupled at different temperatures leading to different phases of the Universe. This had some consequences on the time evolution of the thermodynamic and the cosmological parameters characterizing each phase of the early Universe. In this study, we derive the analytic expressions of the equations governing the time evolution of these parameters using the equations of state of the MIT bag model describing the quark–gluon plasma era. In addition, using the equations of state derived from considering the recent results of the lattice QCD simulations, we solve numerically the differential equation governing the time evolution of the energy density in the early Universe. The time evolution of the parameters under concern including the energy density, entropy density, temperature, pressure in addition to Hubble parameter and scale factor can then be estimated as will be presented in this work.

In this paper, $F(\nu )$ cosmology is proposed for the accelerating universe with asymptotic de Sitter expansion in terms of Hankel function index $\nu$. To some extent, both the initial expansion during early inflation and the current accelerated expansion can be studied with a vacuum cosmic fluid, i.e. $\mathrm{\Lambda }$ in the pure de Sitter phase. Observational data further support the notion of a quasi-vacuum fluid, rather than a pure vacuum, contributing to the quasi-de Sitter acceleration in both the early and late universe. By examining the asymptotic expansion of the Henkel function as an approximate solution of the Mukhanov–Sasaki equation, we seek a more detailed study of quasi-de Sitter solutions in cosmology containing vacuum-like fluid.

We report on the status of primordial nucleosynthesis in light of recent results on CMB anisotropies from WMAP experiment. Theoretical estimates for nuclei abundances, along with the corresponding uncertainties, are evaluated using a new numerical code, where all nuclear rates usually considered have been updated using the most recent available data. Moreover, additional processes neglected in previous calculations have been included. The combined analysis of CMB and primordial nucleosynthesis prediction for Deuterium gives an effective number of relativistic degrees of freedom in good agreement with the simplest scenario of three nondegenerate neutrinos. Our findings seem to point out possible systematics affecting ⁴He mass fraction measurements, or the effect of exotic physics, like a slightly degenerate relic neutrino background.

The quantum contributions to the gravitational action are relatively easy to calculate in the higher derivative sector of the theory. However, the applications to the post-inflationary cosmology and astrophysics require the corrections to the Einstein–Hilbert action and to the cosmological constant, and those we cannot derive yet in a consistent and safe way. At the same time, if we assume that these quantum terms are covariant and that they have relevant magnitude, their functional form can be defined up to a single free parameter, which can be defined on the phenomenological basis. It turns out that the quantum correction may lead, in principle, to surprisingly strong and interesting effects in astrophysics and cosmology.

A modified theory of gravity with the function F(R) = Rexp(αR) instead of Ricci scalar R in the Einstein–Hilbert action is considered and analyzed. The action of the model is converted into Einstein–Hilbert action at small value of the parameter α. From local tests we obtain a bound on the parameter α ≤ 10^-6cm². The Jordan and Einstein frames are investigated and the potential of the scalar field in Einstein's frame is found. The mass of a scalar degree of freedom as a function of curvature is obtained. The static solutions of the model are found corresponding to the Schwarzschild–de Sitter space. We show that the de Sitter space is unstable but a solution with zero curvature is stable. The cosmological parameters of the model are calculated. It was demonstrated that the model passes the matter stability test.