Narrow Results

Quintessence issues can be achieved by taking into account higher order curvature invariants into the effective action of gravitational field. Such an approach is naturally related to fundamental theories of quantum gravity which predict higher order terms in loop expansion of quantum fields in curved space-times. In this framework, we obtain a class of cosmological solutions which are fitted against cosmological data. We reproduce encouraging results able to fit high redshift supernovae and WMAP observations. The age of the universe and other cosmological parameters are discussed in this context.

In this work, we propose a non-interacting model of Barrow holographic dark energy (BHDE) using Barrow entropy in a spatially flat FLRW Universe considering the IR cutoff as the Hubble horizon. We study the evolutionary history of important cosmological parameters, in particular, deceleration parameter, equation of state (EoS) parameter, the BHDE and matter density parameter, and also observe satisfactory behaviors in the BHDE model. The stability of the BHDE model has been examined by squared sound speed $v_s^2$. In addition, to describe the accelerated expansion of the Universe, the correspondence of the BHDE model with the quintessence scalar field has been reconstructed.

We study the shadow of the rotating black hole with quintessential energy (i) in vacuum, (ii) in the presence of plasma with radial power-law density. For the vacuum case, the quintessential field parameter of the rotating black hole significantly changes the shape of the shadow. With increasing quintessential field parameter, the radius of the shadow also increases. With the increase of the radius of the shadow of the rotating black hole, the quintessential field parameter causes decrease of the distortion of the shadow shape: in the presence of the quintessential field parameter, the shadow of the fast rotating black hole becomes too close to the circle. We assume the distant observer of the black hole shadow to be located near the so-called static radius where the gravitational attraction of the black hole is just balanced by the cosmic repulsion. The shape and size of the shadow of quintessential rotating black hole surrounded by plasma depends on (i) plasma parameters, (ii) black hole spin and (iii) quintessential field parameter. With the increase of the plasma refraction index, the apparent radius of the shadow increases. However, for the large values of the quintessential field parameter, the change of the black hole shadow shape due to the presence of plasma is not significant, i.e. the effect of the quintessential field parameter dominates over the plasma effect.

This paper is devoted to formulate a new model of quintessence anisotropic compact stars in the modified $f(R,G)$ gravity. Dynamical equations in modified theory consisting of anisotropic fluid along with quintessence field have been evaluated by adopting analytical solution of Krori–Barua. In order to determine the unknown constraints of Krori–Barua metric observational data of different stars, $4U1820$-$30$, $HerX-1$, $SAXJ1808$-$3658$ has been taken into account. To solve the dynamical equations Starobinsky-like model, $f(R,G)=R+\lambda R^2+G$ of modified gravity has been used. The outcome of the results depicts that all the examined celestial bodies are free from central singularity and are physically stable. Different physical parameters, such as energy density, energy conditions, evolution of quintessence and compactness factor, have been reviewed in detail.

We consider a model of interacting cosmological constant/quintessence, where dark matter and dark energy behave as, respectively, two coexisting phases of a fluid, a thermally excited Bose component and a condensate, respectively. In a simple phenomenological model for the dark components interaction we find that their energy density evolution is strongly coupled during the universe evolution. This feature provides a possible way out for the coincidence problem affecting many quintessence models.

We point out that the physics at the extreme IR — cosmology — might provide tests of the physics of the extreme UV — the Weak Gravity Conjecture. The current discrepancies in the determination of $H_0$ may hint at a modification of $\mathrm{\Lambda }$CDM. An extension which may fit better comprises an early contribution to dark energy which “decays” into relativistic matter. On the other hand, the discourse on WGC to date suggests that fields which support cosmic acceleration may produce relativistic matter after they traverse a $\sim$ Planckian distance in field space. We explain how this offers a simple realization of the requisite cosmic phenomenology. Thus, if the resolution of $H_0$ discrepancies is really early dark energy that ends with a shower of relativistic matter and the current ideas on WGC are indicative, this may be a rare opportunity to link the two extreme limits of quantum field theory.

The thermodynamic and spectroscopic behavior of Schwarzschild black hole surrounded by quintessence are studied. We have derived the thermodynamic quantities and studied their behavior for different values of quintessence parameter. We put the background spacetime into the Kruskal-like coordinate to find the period with respect to Euclidean time. Also assuming that the adiabatic invariant obeys Bohr–Sommerfeld quantization rule, detailed study of area spectrum and entropy spectrum have been done for special cases of the quintessence state parameter. We find that the spectra are equally spaced.

In this paper we study the properties of Schwarzschild black hole surrounded by quintessence matter. The main objective of the paper is to show the existence of Nariai type black hole for special values of the parameters in the theory. The Nariai black hole with the quintessence has the topology dS₂ ×S₂ with dS₂ with a different curvature than what would be expected for the Schwarzschild–de Sitter degenerate black hole. Temperature and the entropy for the Schwarzschild–de Sitter black hole and the Schwarzschild-quintessence black hole are compared. The temperature and the curvature are computed for general values of the state parameter ω.

By considering the back-reaction of the spacetime through the spherically symmetric quantum fluctuations of the background metric, Kazakov and Solodukhin removed the singularity of the Schwarzschild black hole. This regular Schwarzschild black hole has a spherical central region with a radius of the order of the Planck length. On the other hand, due to the positively accelerating expansion of the Universe, it seems that there exists a universal repulsive force known as dark energy. In the framework of quantum field theories, the quintessence field is a candidate model for investigating and modeling dark energy. Accordingly, by taking into account the quintessential matter field in the background of the Schwarzschild black hole, Kiselev gained the metric of this black hole surrounded by quintessence. By combining these two above ideas, in this study, we consider the quantum-corrected Schwarzschild black hole surrounded by quintessence to investigate null and time-like geodesics structure. Generally, this study points out that black holes are quantum-gravitational objects. We will show that the accelerated expansion of the Universe, instead of dark energy, happens because of the presence of quantum effects in this setup. Also, due to the presence of the central Planck-size sphere, the regular black hole has been possessed a shifting over radial coordinate in its inner structure.

Black hole thermodynamic stability can be determined by studying the nature of heat capacity of the system. For Schwarzschild black hole the heat capacity is negative, but in the quintessence field, this system shows a second-order phase transition, implying the existence of a stable phase. We further discuss the equation of state of the present system. While analyzing the quasinormal modes (QNM), we find that the massive scalar QNM frequencies in the complex ω plane shows a dramatic change when we plot it as a progressive function of quintessence state parameter. We also find the Hawking temperature of the system via the method of tunneling.

In this work, we investigate the thermodynamic and the stability of the regular Hayward black hole surrounded by quintessence. Using the metric of the black hole surrounded by quintessence and the new approach of the holographic principle, we derive the expression of the Unruh–Verlinde temperature. Hawking temperature and specific heat are derived using the first law of black holes thermodynamics. Gibbs free energy is also evaluated. The behaviors of these quantities show that, the parameter of the regular Hayward black hole $\beta$ induces a decreasing of the Hawking temperature of the black hole, and that decrease is accentuated when increasing the magnitude of $\beta$ and the normalization factor related to the density of quintessence. For the lower entropies, the black hole passes from the unstable phase to the stable one by a first-order thermodynamics phase transition. When increasing the entropy, a second phase transition occurs. This new phase transition is a second-order thermodynamics phase transition and brings the black hole to unstable state. It results that, when increasing of magnitude of $\beta$, the phase transition points are shifted to the higher entropies. Moreover, the phenomena of phase transitions are preserved by adding the quintessence. Furthermore, when increasing the normalization factor of quintessence, the first-order transition point is shifted to higher entropies, while the second-order thermodynamics phase transition point is shifted to lower entropies.

We study the properties of the null geodesics of four-dimensional Einstein–Gauss–Bonnet (4D EGB) black holes surrounded by quintessence matter, recently derived by Shah et al. [Phys. Lett. B818 (2021) 136383]. Due to the quintessence correction, we discuss the radial and nonradial geodesics. For the case of nonradial geodesics we obtain the effective potential, photon sphere and impact parameter associated to the null geodesics in detail. We analyze the different possible orbits of massless particles (i.e. photons) such as unstable circular orbits and unbounded orbits and investigate the role of the Gauss–Bonnet coupling $\alpha$ and the quintessence parameter q on the null geodesic trajectories. Moreover, we study the effects of the model parameters on the shadow cast by such black holes. The results are compared to that of Schwarzschild black hole with and without quintessence matter and 4D EGB black holes without quintessence. As a physical application of null geodesics, the deflection angle is calculated and the effect of the quintessence matter on it is investigated.

In the present work, we construct the Tsallis holographic quintessence model of dark energy in $f(R,T)$ gravity with Hubble horizon as infrared (IR) cut-off. In a flat Friedmann–Robertson–Walker (FRW) background, the correspondence among the energy density of the quintessence model with the Tsallis holographic density permits the reconstruction of the dynamics and the potentials for the quintessence field. The suggested Hubble horizon IR cut-off for the Tsallis holographic dark energy (THDE) density acts for two specific cases: (i) THDE 1 and (ii) THDE 2. We have reconstructed the Tsallis holographic quintessence model in the region ${\omega }_{\mathrm{\Lambda }}>-1$ for the equation of state (EoS) parameter for both the cases. we investigate the behavior of several well-known statefinder quantities, like the deceleration parameter, the jerk and the parameter $s$. In addition, the quintessence phase of the THDE models is analyzed with swampland conjecture to describe the accelerated expansion of the Universe.

We obtain the solution corresponding to a static and spherically symmetric black hole with a cloud of strings (Letelier spacetime) immersed in a quintessential fluid. We discuss some aspects of its thermodynamics and complete proceeding studies in the spacetime of Schwarzschild with quintessence and a solid deficit angle, which is mathematically analogous to the solution we obtained. We also present a discussion about Hawking radiation of particles, in the background under consideration and compare with related studies in the literature.

In this paper, by using the effective potential for the photons, we analyze the null geodesics and all kinds of orbits corresponding to the energy levels for the Reissner–Nordström black hole surrounded by quintessence with and compare our results with those obtained for the Schwarzschild black hole surrounded by quintessence matter. We also investigate the circular orbits and calculate the angle of deflection of the photons.

Starting from a regular, static and spherically symmetric spacetime, we present a stellar model formed by two sources of ordinary and quintessence matter both with anisotropic pressures. The ordinary matter, with density $\rho$, is formed by a fluid with a state equation type Chaplygin $P_r(\rho )=\mu c^2\rho -\nu /(c^2\rho )$ for the radial pressure. And the quintessence matter, with density ${\rho }_q$, has a state equation $P_r({\rho }_q)=-c^2{\rho }_q$ for the radial pressure and $P_r({\rho }_q)=-(1+3w)c^2{\rho }_q/2$ for the tangential pressure with $-1<w<-\frac{1}{3}$. The model satisfies the required conditions to be physically acceptable and additionally the solution is potentially stable, i.e. $v_t^2-v_r^2<0$ according to the cracking concept, and it also satisfies the Harrison–Zeldovich–Novikov criteria. We describe in a graphic manner the behavior of the solution for the case in which the mass is $1.3M_{\odot }$ and radius $R=8.849$km which matches the star EXO 1785-248, from where we obtain the maximum density ${\rho }_c=1.206510^{18}\mathrm{\text{kg}}/{\mathrm{\text{m}}}^3$ for the values of the parameters $\mu =0.15535$, $w=-0.33334$.

Using the Hamilton–Jacobi method, we study optical shadows of rotating Bardeen-AdS black holes in the presence of internal and external field contributions. Precisely, we examine the shadow using one-dimensional real curves. By establishing the equations of motion of such AdS black holes without external dark sectors, we first analyze the shadow configurations in terms of a reduced moduli space involving only internal parameter contributions including the charge of the nonlinear electrodynamics. Among others, we find that this parameter serves as a geometric quantity controlling the shadow shape. Then, we graphically discuss the associated astronomical observables. Enlarging the moduli space, we analyze the behaviors of the quintessential black hole solutions in terms of the involved parameters controlling the dark field sector. Concretely, we observe that the dark energy not only affects the size but also deforms the shadow shape. Finally, we provide a possible link with observations from Event Horizon Telescope by showing certain constraints on the involved moduli space in the light of the M87${}^{\ast }$ picture.

This work studies the theoretical construction of charged quintessence thin-shell wormholes using Israel thin-shell approach. The stability of these wormhole solutions is investigated by taking linear, logarithmic and Chaplygin gas models as a constituent of exotic matter at thin-shell. The presence of wormhole stability regions particularly relies on the physically justifiable values of charge and quintessence parameter. It is noted that the increasing value of charge seems as an effective component for stable regions while the rise in negativity of the quintessence parameter gives more stable wormhole configurations.

We calculate the Casimir energy of a massless scalar field in a cavity formed by nearby parallel plates orbiting a rotating spherical body surrounded by quintessence, investigating the influence of the gravitational field on that energy, at zero temperature. This influence includes the effects due to the spacetime dragging caused by the source rotation as well as those ones due to the quintessence. We show that the energy depends on all the involved parameters, as source mass, angular momentum and quintessence state parameter, for any radial coordinate and polar angle. We show that at the north pole the Casimir energy is not influenced by the quintessential matter. At the equatorial plane, when the quintessence is canceled, the result obtained in the literature is recovered. Finally, constraints in the quintessence parameters are obtained from the uncertainty in the current measurements of Casimir effect.

In this paper, we have researched tachyon field, k-essence and quintessence dark energy (DE) models for Friedmann–Robertson–Walker (FRW) universe with varying G and $\mathrm{\Lambda }$ in f(R, T) gravitation theory. The theory of f(R, T) is proposed by Harko et al. [Phys. Rev. D84, 024020, 2011]. In this theory, R is the Ricci scalar and T is the trace of energy–momentum tensor. For the solutions of field equations, we have used linearly varying deceleration parameter (LVDP), the equation of state (EoS) and the ratio between $\mathrm{\Lambda }$ and Hubble parameter. Also, we have discussed some physical behavior of the models with various graphics.