Gravitation and Cosmology最新文献

We consider the gravitational collapse of a quark binding string fluid in (f(R,T)) theory of gravity. A quark binding string state of a fluid is expected to occur as a combination of quark matter and strings in the initial phases of the Universe. Assuming the collapse of this quark fluid, the junction conditions are derived, taking famous FRW and Schwarzschild space-times as the interior and exterior regions, respectively. We have assumed that (f(R,T)=alpha R+beta T), ((alpha), (beta) are positive constants). The collapsing gravitational mass is calculated under the conditions of a trace of the energy momentum tensor and a constant scalar curvature. It is calculated how long and how far the apparent horizons form. The constant term (f(R_{0},T_{0})) is a factor which delays the collapse. The event horizon formation is followed by creation of an apparent horizon, which results in a black hole. Additionally, the existence of the string tension lengthens the period before the horizon forms. As a result, it is anticipated that the Universe’s black holes are expected to originate during the quark binding string phase.

We present a family of static and spherically symmetric wormholes in Rastall gravity. A unique characteristic of this modified theory is violation of the local conservation of the energy-momentum tensor. These wormholes are supported by applying a specific equation of state for matter satisfying the tracelessness constraint [Kar and Sahdev: Phys. Rev D 52, 2030 (1995)]. Next, we impose some restrictions on the redshift function and solve the field equations analytically for a classical traversable wormholes. Finally, we investigate some issues concerning the energy conditions and the volume integral quantifier in these time-interdependent geometries.

The existence of Schwarzschild black holes in the structure of a Swiss-cheese brane world has led to the conclusion that this specific brane-world scenario is more realistic than the FLRW branes. In this paper, we show that Logamediate inflation on the Swiss-cheese brane with a time-dependent cosmological constant (Lambda(H)) leads to a positive kinetic term and a negative potential with AdS minimum. The cosmic pressure (p) is always positive, but the energy density (rho) starts to get negative after finite time. However, there is a time interval where they both are positive. Although this behavior of (rho) can be considered as a drawback of the Swiss-cheese brane, where positive energy dominates the present universe, it has been suggested that the presence of some source of negative energy could have played a significant role in the early cosmic expansion. The model suffers from the eternal inflation problem which appears from the evolution of the first slow-roll parameter (epsilon). Due to the existence of the (rho^{2}) term, we have tested the new nonlinear energy conditions. The slow-roll parameters have been investigated and compared to Planck 15 results.

In the presence of an anisotropic fluid, we examine the irregularity factors for a spherically symmetric relativistic matter. In (f(mathcal{G},T^{2})) gravity, we investigate the equations of motion and dynamical relations using a systematic construction, where (T) stands for the trace of the energy-momentum tensor, and (mathcal{G}) is the Gauss–Bonnet term. With the use of the Weyl tensor, we examine two well-known differential equations that would lead to an analysis of the sources of inhomogeneities. In (f(mathcal{G},T^{2})) gravity, the irregularity factors are investigated by taking specific cases in the adiabatic and non-adiabatic regimes. We find that the conformal tensor and additional curvature terms compromise inhomogeneity for a pressureless nonradiating fluid and an isotropic fluid. In contrast to other cases, for a nonradiating anisotropic fluid, we observe that the term ((Pi+mathcal{E})) now accounts for the survival of density inhomogeneity, rather than just the Weyl tensor and the modified terms. The last case clearly illustrates how several components, namely, radiating terms, the fluid shear and the expansion scalar in the (f(mathcal{G},T^{2})) framework, are accountable for the formation of inhomogeneities from a homogeneous state of the structure. In the case (f(mathcal{G},T^{2})=0), all our results reduce to those of GR.

In this research, we focus on investigating the behavior of zero-spin scalar boson-antiboson particles in a specific space-time of ((1+2))-dimensional circularly symmetric and static traversable wormhole with cosmic strings, all under the influence of a quantum flux field. We start by deriving the wave equation from the Klein–Gordon equation, which governs the relativistic quantum motion of scalar bosons-antibosons. By solving this equation using the confluent Heun equation, we obtain the ground state energy level (E^{+}_{1,ell}) and the corresponding wave function (Psi^{+}_{1,ell}) as a particular case. The main findings of this study indicate that various factors, such as the presence of cosmic strings, the radius of the wormhole throat, and the quantum flux, have significant impacts on the behavior of scalar bosons-antibosons.

During typical general relativity courses, the gravitational fields generated by rotating objects and the so-called frame dragging effect are explained by emphasizing the presence of a gravitational Coriolis-like force term. It is well known that, in a rotating system, there is also a fictitious centrifugal force. In general, textbooks do not discuss also the possibility of a gravitational centrifugal-like force, and, in a recent paper, we have analyzed the presence of a repulsive force in the vicinity of a rotating mass. Now, however, we want to reviews some historical aspects of Mach’s Principle and to analyze the centrifugal gravitational term inside a rotating spherical shell, with a new simple approach.

This investigation focuses on a Bianchi type-V universe characterized by spatial homogeneity and anisotropy, wherein the cosmic medium consists of interacting dark matter and holographic dark energy. We obtain solutions to the field equations by considering the Hubble parameter (H(z)=(H_{0}/sqrt{2})sqrt{1+(1+z)^{2n}}), and constrain the model parameters. Employing Bayesian analysis and likelihood functions in conjunction with the Markov Chain Monte Carlo (MCMC) method, we determine the following model parameters: (H_{0}=71.3388^{+0.00010}_{-0.00094}), and (n=-1.08147^{+0.00010}_{-0.00010}). In this study, we constrain the model parameters by using the joint datasets ((H(z)+textrm{BAO}+textrm{Pantheon})). We explain the physical and geometric aspects of the model. We also examine the behavior of the velocity of sound and different energy conditions to test the viability of our cosmological model.

We examine almost Riemann solitons and almost gradient Riemann solitons in generalized Robertson–Walker space-times and perfect fluid space-times. At first, we prove that if a generalized Robertson–Walker space-time admits an almost Riemann soliton or an almost gradient Riemann soliton, then it becomes a perfect fluid space-time. Next, we observe that a space-time with an almost Riemann soliton whose potential vector field, is a conformal vector field, is an Einstein manifold, and if the potential vector field is a nonhomothetic conformal vector field, then space-time is of Petrov type O or N. In final, we prove that if a generalized Robertson–Walker space-time satisfies the definition of an almost Riemann soliton, and (Q.P=0) then it is an Einstein manifold, and consequently it is a perfect fluid space-time.

The 4D gravitational model with a real scalar field (varphi), Einstein and Gauss–Bonnet terms is considered. The action contains the potential (U(varphi)) and the Gauss–Bonnet coupling function (f(varphi)). For a special static spherically symmetric metric (ds^{2}=(A(u))^{-1}du^{2}-A(u)dt^{2}+u^{2}dOmega^{2}), with (A(u)>0) ((u>0) is a radial coordinate), we verify the so-called reconstruction procedure suggested by Nojiri and Nashed. This procedure presents certain implicit relations for (U(varphi)) and (f(varphi)) which lead to exact solutions to the equations of motion for a given metric governed by (A(u)). We confirm that all relations in the approach of Nojiri and Nashed for (f(varphi(u))) and (varphi(u)) are correct, but the relation for (U(varphi(u))) contains a typo which is eliminated in this paper. Here we apply the procedure to the (external) Schwarzschild metric with the gravitational radius (2mu) and (u>2mu). Using the “no-ghost” restriction (i.e., reality of (varphi(u))), we find two families of ((U(varphi),f(varphi))). The first one gives us the Schwarzschild metric defined for (u>3mu), while the second one describes the Schwarzschild metric defined for (2mu<u<3mu) ((3mu) is the radius of the photon sphere). In both cases the potential (U(varphi)) is negative.

We describe the possible forms of black hole images viewed by a distant observer (or a telescope) on the celestial sphere. These images are numerically calculated based on general relativity and the equations of motion in the Kerr–Newman metric. A black hole image is a gravitationally lensed image of the black hole event horizon. It may be viewed as a black spot on the celestial sphere, projected inside the position of a classical black hole shadow. In the nearest future it will be possible to verify modified gravity theories by observations of astrophysical black holes with Space Observatory Millimetron.