Gravitation and Cosmology最新文献

We study the density contrast equations for cold dark matter (CDM) in the cosmological radiation and dark energy (DE) background. We provide a general prescription for the derivation of the aforesaid density contrast equations of the CDM using the metric perturbation technique. In particular, in the early radiation domination, the density contrast equation, the so-called Mészáros equation is derived, considering a four-fluid model, while on the other hand, in the late time DE domination, the “w-Mészáros equation” is derived, using the two-fluid system of CDM and DE. In the first case, we find eight-parameter Lie symmetries, while in the second case we also obtain eight symmetry generators of the “w-Mészáros equation,” each for the values of the equation-of-state parameter (w=-2/3) and (-1). Finding group-invariant solutions using the invariant curve condition for both cases, we have investigated the sub-horizon evolution of density contrasts of the CDM and provided a qualitative study on the nature of evolution of the CDM perturbations. The density contrast of CDM shows no growth during the radiation dominated era, but growth is seen just at the time of matter-radiation equality. The freezing or stagnation of the density contrast of the CDM prior to the matter-radiation equilibrium is due to the rapid expansion of the radiation background at early time, while the decay of the density contrast with increasing scale factor, which results in suppression in the growth of the inhomogeneity, is due to the DE dominated accelerated expansion.

Dilatonic dyon-like black hole solutions with two (color) charges (Q_{1}) and (Q_{2}) (electric and magnetic ones) are considered in a gravitational 4D model with two scalar fields and two 2-forms. Two-dimensional dilatonic coupling vectors (vec{lambda}_{i}), (i=1,2), determining the model, obey the relation (vec{lambda}_{1}vec{lambda}_{2}=1/2). Circular null geodesics in the field of such black holes are explored. The master equation for the photon sphere radius (R) is derived. A conjecture is suggested on the existence and uniqueness of the solution to the master equation with (R>R_{g}), where (R_{g}) is the horizon radius. This conjecture is varified for certain special cases, e.g., for a charge symmetric configuration: (Q_{1}^{2}=Q_{2}^{2}). In this charge symmetric case, we present a relation for the spectrum of quasinormal modes of a test massless scalar field in the eikonal approximation, and an example of circular orbits of a massive particle.

In this study, based on the constraints of modified dispersion relation parameters from Ultra High Energy Cosmic Rays (UHECR), we investigate the wavelength shift of scattered photons in a Compton process. In other words, the main motivation here is to consider the effects of modified dispersion relation and Lorentz invariance violation on inverse Compton scattering of cosmic microwave background (CMB) photons and the GZK cutoff. Our calculations indicate that the change in wavelength depends on the incident photon wavelength. In addition, we obtain a threshold momentum of an electron as (|P|leq 10^{12}) eV. This threshold indicates the reasonable values for (|P|). Also, we find that with respect to the specified modified dispersion relation it is possible to extend the GZK cutoff in the UHECR spectrum beyond (10^{20}) eV, and we expect to detect cosmic rays from deep space having energies greater than (10^{20}) eV, as suggested in some experiments.

On the basis of the formulated and proven similarity properties of cosmological models based on a statistical system of degenerate scalarly charged fermions, as well as the previously identified mechanism of scalar-gravitational instability of cosmological models, a numerical and analytical study of the formation of supermassive black hole nuclei in the early Universe is carried out. A mathematical model of the evolution of spherical perturbations is constructed, making itt possible to reveal the main regularities of the process of evolution of collapsing masses and the dependence of the parameters of forming black holes on the fundamental parameters of the cosmological model and the wavelength of gravitational perturbations. In this case, the mass loss of the black hole due to quantum evaporation is taken into account. A stable tendency for the early formation of supermassive black hole nuclei in the class of cosmological models under study is shown, and a close connection between the growth of masses of spherical perturbations and the nature of the singular points of these models is shown.

The first inflationary stage of the Bianchi type IX metric is considered for the case of inflation near the maximum potential according to the old terminology, “new inflation.” The evolution of rotation is studied with dark energy modeled by an anisotropic fluid.

We illustrate the dynamical instability of charged spherical fluid configuration with anisotropic conditions in nonminimally coupled (f(R,T)) theory of gravitation, where (R) is the Ricci scalar and (T) is the trace of the energy momentum-tensor. We investigate both modified field equations and continuity equations to provide some extra degrees of freedom under the constraints of a specific considered form of (f(R,T)) gravity. We examine how small changes in geometric and material profiles affect the fluid’s collapsing structure via a perturbation scheme. We study the unstable eras using Newtonian ((mathbb{N})) and post-Newtonian (pN) approximations by applying some significant constraints on the collapse equation. Our findings demonstrate that the stiffness parameter (Gamma) has a substantial influence on the identification of unstable phases for our charged stellar geometry. We conclude that some correction terms that are dark source terms, which appear due to (f(R,T)) gravity, lead to an unstable structure/phase throughout the evolutionary process.

We study the linear stability of vacuum static, spherically symmetric solutions to the gravitational field equations of the Bergmann–Wagoner–Nordtvedt class of scalar-tensor theories (STT) of gravity, restricting ourselves to nonphantom theories, massless scalar fields and configurations with positive Schwarzschild mass. We consider only small radial (monopole) perturbations as the ones most likely to cause an instability. The problem reduces to the same Schrödinger-like master equation as is known for perturbations of Fisher’s solution of general relativity (GR), but the corresponding boundary conditions that affect the final result of the study depend on the choice of the STT and a particular solution within it. The stability or instability conclusions are obtained for the Brans–Dicke, Barker and Schwinger STT as well as for GR nonminimally coupled to a scalar field with an arbitrary parameter (xi).

From a classical analysis, we show that gravitational waves in a cosmological medium with the equation of state parameter (w=-1/3) can follow a London-like equation, implying that some gravitational wave solutions present a decay for certain wavelengths. This scenario, corresponding to a cosmic string cosmology, induces an attenuation temporal scale on the gravitational wave propagation. We discuss on how these solutions impose a limit on the wavelength of the waves that can propagate, which depends on the type of spatial curvature and the energy density content of this type of cosmology.

We introduce a new, well-mannered and stable model. To create the model, we used well-known Finch–Skea [1] ansatz for the metric potential (g_{rr}). For our model, we look into a number of physical variables, including the energy density, pressures, mass, anisotropy factor, gravitational redshifts, energy conditions, and stability analysis by utilizing graphs. This made sure that our suggested solutions behave as expected and represent physically plausible models for anisotropic matter distributions. We consider compact stars to compare our model with observational data: SAX J1808.4-3658, Her X-1, 4U 1608-52, PSR B1913 (+) 16, Cen X-3, LMC X-4, PSR J1614-2230, 4U1538-52, PSR J1903 (+) 327, SMC X-1, 4U 1820-30, Vela X-1.

The present investigation is focused on a curvatureless and torsionless modified nonmetricity theory of gravity in the context of string fluid. For that, we use an anisotropic, locally rotationally symmetric (LRS), Bianchi Type I space-time universe with the arbitrary function (f(Q)=Q+Lambda), where (Q) is the nonmetricity scalar and (Lambda) is the cosmological constant. We solve the field equations using a time-dependent deceleration parameter and obtain various cosmological parameters. To obtain the best-fit values of the model parameters, we use the observational constraints on Hubble function (H(z)) and the apparent magnitude (m(z)) using the observational datasets (H(z)) and SNe Ia. Using these values of model parameters, we investigate cosmological scenarios of the model. We observe the behavior of the variable dark energy equation of state (EoS) parameter (omega) in the context of a string fluid universe and find the present value of the effective EoS parameter (omega^{textrm{eff}}<-1) that corresponds to phantom dark energy. Also, we analyze the statefinder parameters and estimate the present age of the universe.