Narrow Results

We investigate the new agegraphic dark energy models with generalized uncertainty principle (GUP). It turns out that although the GUP affects the early universe, it does not change the current and future dark energy-dominated universe significantly. Furthermore, this model could describe the matter-dominated universe in the past only when the parameter n is chosen to be n > n_c, where the critical value is determined to be n_c = 2.799531478.

We construct a dark energy model where a scalar field non-minimally coupled to gravity plays the role of the dark component. We compare cosmological consequences of this non-minimal coupling of the scalar field and gravity in the spirit of the dark energy paradigm in Jordan and Einstein frames. Some important issues such as phantom divide line crossing, existence of the bouncing solutions and the stability of the solutions are compared in these two frames. We show that while a non-minimally coupled scalar field in the Jordan frame is a suitable dark energy component with capability to realize phantom divide line crossing, its conformal transformation in the Einstein frame does not have this capability. The conformal transformation from Jordan frame to Einstein frame transforms the equation of state parameter of the dark energy component to its minimal form with a redefined scalar field and in this case it is impossible to realize a phantom phase with possible crossing of the phantom divide line.

This paper is devoted to check validity of the laws of thermodynamics for locally rotationally symmetric (LRS) Bianchi type I (BI) universe model which is filled with combination of dark matter and dark energy (DE). We take two types of DE models, i.e. generalized holographic DE (HDE) and generalized Ricci DE (RDE). It is proved that the first and generalized second law of thermodynamics (GSLT) are valid on the apparent horizon for both the models. Further, we take fixed radius L of the apparent horizon with original holographic or RDE. We conclude that the first and GSLT do not hold on the horizon of fixed radius L for both the models.

This paper is devoted to check the validity of laws of thermodynamics for Kaluza–Klein universe in the state of thermal equilibrium, composed of dark matter and dark energy. The generalized holographic dark energy and generalized Ricci dark energy models are considered here. It is proved that the first and generalized second law of thermodynamics are valid on the apparent horizon for both of these models. Further, we take a horizon of radius L with modified holographic or Ricci dark energy. We conclude that these models do not obey the first and generalized second law of thermodynamics on the horizon of fixed radius L for a specific range of model parameters.

It has been found that the geometrical diagnostic methods can break the degeneracy for dark energy models. In this paper, we investigate the Om diagnostic, the statefinder hierarchy $S_n$ and the composite null diagnostic $\{S_n,𝜖\}$ for the Tsallis holographic dark energy models with interactions. We find that model parameters and the forms of interaction will influence the values of diagnostic parameters or the trends of the evolutionary trajectories for each model. Moreover, the statefinder hierarchy $S_3^{(1)}$ together with $\{S_3^{(1)},𝜖\}$ could give good diagnostic results. Furthermore, we also obtain some issues of cosmological structure by means of the composite null diagnostic.

The reconstruction scenario of well-established dark energy models such as pilgrim dark energy model and generalized ghost dark energy with Hubble horizon and $f(T,{𝒯})$ models is being considered. We have established $f(T,{𝒯})$ models and analyzed their viability through equation of state parameter and $\omega -{\omega }^{\prime }$ (where prime denotes derivative with respect to $lna$) plane. The equation of state parameter evolutes the universe in three different phases such as quintessence, vacuum and phantom. However, the $\omega -{\omega }^{\prime }$ plane also describes the thawing as well as freezing region of the universe. The recent observational data also favor our results.

In this work, we investigated the influence on the evolution of gas of thin tubes of a massless scalar field, in particular, on its ability to accelerated expansion, of such characteristics of thin tubes forming gas as thickness (core radius), the moment of their formation and spatial shape. The study showed that the formation of thin tubes of a scalar field that form gas becomes possible only some time after the singular state of the Universe. At the same time, the thinner the tubes form a gas, and the later in the process of evolution they were formed, the faster such a gas can achieve the equation of state $p=-\rho$.

In this paper, we investigate the statefinder, the deceleration and equation of state parameters when universe is composed of generalized holographic dark energy or generalized Ricci dark energy for Bianchi I universe model. These parameters are found for both interacting as well as noninteracting scenarios of generalized holographic or generalized Ricci dark energy with dark matter and generalized Chaplygin gas. We explore these parameters graphically for different situations. It is concluded that these models represent accelerated expansion of the universe.

The propagation of weak gravitational waves on the background of dark energy is studied. The consideration is carried out within the framework of an approximate approach where the cosmological scale factor is expanded as a power series for relatively small values of the redshift corresponding to the epoch of the present accelerated expansion of the universe. For several different dark energy models, we obtain dispersion relations for gravitational waves which can be used to estimate the viability of every specific model by comparing with observational data.

In order to explain the current acceleration of the universe, the fine-tuning problem of the cosmological constant $\mathrm{\Lambda }$ and the cosmic coincidence problem, different alternative models have been proposed in the literature. We use the most recent observational data from CMB (Planck 2018 final data release) and LSS (SDSS, WiggleZ, VIPERS) to constrain dynamical dark energy (DE) models. The CMB shift parameter, which traditionally has been used to determine the main cosmological parameters of the standard model $\mathrm{\Lambda }\mathrm{\text{CDM}}$, is employed in addition to data from redshift-space distortions through the growth parameter $A(z)=f(z){\sigma }_8(z)$ to constrain the mass variance ${\sigma }_8$. BAO data are also used to study the history of the cosmological expansion and the main properties of DE. From the evolution of $q(z)$, we found a slowdown of acceleration behavior at low redshifts, and by using the Akaike and Bayesian Information Criteria (AIC, BIC), we discriminate different models to find those that are better suited to the observational data, finding that the interacting dark energy (IDE) model is the most favored by observational data, including information from SNIa and Hz. The analysis shows that the IDE model is followed closely by EDE and $\mathrm{\Lambda }\mathrm{\text{CDM}}$ models, which in some cases fit better the observational data with individual probes.

Sign changeable interactions continue to gain serious attention in modern cosmology. On the other hand, recent interest towards nonlinear interactions gave us a motivation to consider various sign changeable nonlinear interactions. In this paper, we will consider cosmological models involving suggested new sign changeable nonlinear interactions. To describe background dynamics of the large scale universe $f(T)$ gravity is used i.e the universe is governed by torsional modified gravity. In our models, we adopted barotropic EoS and pressureless fluid to model dark energy and dark matter, respectively. In addition to other assumptions about the models, particular form of $f(T)$ is considered to perform phase space analysis.

Thermodynamics of extended gravity static spherically symmetric black hole solutions is investigated. The energy issue is discussed making use of the derivation of Clausius relation from equations of motion, evaluating the black hole entropy by the Wald method and computing the related Hawking temperature.