Gravitation and Cosmology最新文献

We study the cosmological implications of a barotropic fluid satisfying an affine equation of state (EoS) in general relativity (GR) and (f(Q)) gravity. We describe the impact of the affine EoS on the cosmic evolution in the model and derive observational constraints on the model parameters. The models of GR and (f(Q)) gravity may provide a scenario in which the universe transits from decelerated into accelerated expansion. The model parameters are constrained by Bayesian analysis based on the (chi^{2}) minimization technique with the observational data of the Cosmic chronometer and Type Ia Supernovae. The affine EoS model in GR possesses a quintessence kind of dark energy, while it has a phantom type of dark energy in (f(Q)) gravity. The present-day values of the cosmological parameters along with the current age of the universe are compatible with observations. We also probe the possibility of setting up the solution of the GR model into (f(Q)) gravity.

The influence of low-frequency gravitational waves coupled to electromagnetic waves in material media on test masses is investigated. The propagation of coupled gravitational waves in rarefied gases and cold magnetized plasma is considered. It has been shown that under specific conditions the amplitude of coupled gravitational waves in media reaches values of the order of the amplitude of transverse gravitational waves from external astrophysical sources. The specific properties of longitudinal gravitational waves coupled to electromagnetic waves in a medium are considered in relation to transverse gravitational waves from external sources as well.

For a static and spherically symmetric space-time, we investigate a class of exact solutions that arise when two fundamental geometric constraints are imposed simultaneously: Karmarkar’s condition and vanishing of the Weyl tensor. These conditions restrict the curvature in such a way that the space-time becomes conformally flat and belongs to the family of embedding class-I solutions. Even though the subsequent solutions, namely, the Schwarzschild interior solution and the de Sitter solution are well known, the novelty of our presentation is that these solutions are shown to be a direct consequence of the imposed geometric constraints. The physical matter composition becomes highly constrained by the associated geometry under such conditions. The Schwarzschild interior solution describes space-time of an incompressible fluid sphere, while the de Sitter solution corresponds to a vacuum energy dominated configuration. Interestingly, pressure anisotropy as well as the ‘‘complexity factor’’ vanish identically once Karmarkar’s condition and the conformal flatness conditions are applied simultaneously. As these two geometric constraints alone are sufficient to determine the background space-time uniquely, application of Karmarkar’s condition might not be a suitable method for the development of realistic stellar models in conformally flat space-time unless one invokes other factors into consideration, such as time-dependent metric potentials.

This study offers a probable solution to the singularity problems borne with the conventional Big Bang cosmology by analyzing the dynamics of a bouncing cosmological model within the paradigm of a remodeled gravity framework. We peruse the cosmic bounce conditions and denote the cosmological parameters as functions of cosmic time. Furthermore, we employ the redshift parameter and express the cosmological parameters in terms of redshift graphically to remodel an (f(R)) gravity model. An accelerated attribute of the Universe expansion is unveiled through these graphs. We ultimately reconstruct the (f(R)) gravity model for a genuinely bouncing cosmology corresponding to a hyperbolic scale factor. Conclusively, we analyze the speed of sound to explore stability. The late-time stability is proved by plots of the sound speed against redshift.

A study of the behavior of cosmological models near singular points has been conducted, based on a previously proposed model of an asymmetric scalar Higgs doublet with potential interactions between the components. Classes of global behavior of cosmological models and the relationship between the nature of singular points and these classes were identified: the presence of initial and final singularities and rebound points.

We revisit the problem of relaxation in a scalar gravitational field theory proposing a novel numerical solution to the problem. We predict the approach to equilibrium of a spherically symmetric field-particle system initially excited in a non-equilibrium state, where the particle is in an unstable circular orbit around the origin. We explicitly work out a quasistatic approximation in an unstable circular orbit to the exact dynamical problem. Our toy model could give some relevant insights in the full General Relativity realistic model as theoretical effort is carried on to solve it numerically. For example we propose to use the method of images to tackle a numerical solution of the resulting wave equation.

We establish a new (SU(N)) symmetric model of interaction between the fields of four types: a multiplet of vector fields, which describes the so-called color aether, a multiplet of pseudoscalar fields, which is associated with a multi-component cosmic dark matter, gauge and gravitational fields. The extended Lagrangian of the model contains a new constructive element based on the covariant (SU(N)) symmetric divergence of the multiplet of vector fields; this new element, being a multiplet of scalars from the point of view of space-time transformations, and a color vector from the point of view of the (SU(N)) group space, gives us a possibility to properly formulate a multi-axionic extension of the Peccei-Quinn theory. A hypothesis of a spontaneous polarization of the multi-axionic color aether in the early Universe is presented. A set of self-consistent master equations of the model is derived. An application to cosmology is considered: the obtained master equations are solved for a truncated test model based on the Bianchi-I space-time platform.

Einstein taught us through general relativity that mathematics/geometry is the language of nature. He also taught us that mass can be converted into energy. Our work advocates for the conversion of energy into matter, such as mass, charge, and the cosmological constant, discussed within the context of colliding waves in general relativity. The Big Bang is thought to be the prototype example of creating matter at large from an infinite concentration of energy. Our proposal provides an alternative example by using the power of nonlinear collision processes in which energy transmutes into solid matter. Our motto in this paper is: Collision is creation, so let there be light to collide and create!

We analyze a quantum-mechanical counterpart of Newtonian cosmology and show that the effects of zero-point motion eliminate the classical density singularity. Quantum effects are particularly significant for closed Universes where without the cosmological constant the energy spectrum and space curvature are quantized. When a small positive cosmological constant is included, these states become quasistationary and decay via tunneling. Corresponding metastable Universes evolve in three stages: a long period of gestation followed by rapid tunneling expansion further followed by slower Hubble expansion. This closely resembles an inflation scenario of modern cosmology.

We explore anisotropic cosmological models within the framework of modified gravity theory, specifically (f(R,L_{m})) gravity, where the gravitational Lagrangian depends on both the Ricci scalar (R) and the matter Lagrangian (L_{m}). We adopt the locally rotationally symmetric (LRS) Bianchi type I metric to investigate anisotropic cosmic evolution, which allows for directional dependence in the expansion rates while retaining analytical tractability. The modified field equations are derived using a particular form of the function (f(R,L_{m})=R/2+L_{m}^{alpha}), where (alpha) quantifies the strength of the curvature-matter coupling. To obtain exact solutions, we consider power-law forms for the directional scale factors and study the cosmological dynamics under three distinct physically motivated assumptions on the equation of state: (p+rho=0), (rho-p=0), and (rho+3p=0). In each case, the field equations are systematically reduced to algebraic forms and solved analytically. The resulting solutions exhibit both isotropic and anisotropic behavior, depending on the specific relationship between the expansion exponents and the coupling parameter (alpha). For (p+rho=0), the solutions suggest an effective cosmological constant-like behavior, with possible higher-dimensional interpretations where some dimensions remain static or compactified. Under the (rho-p=0) condition, a richer structure of solutions emerges, admitting both static, partially expanding, and fully anisotropic power-law evolutions, where anisotropy is directly controlled by the coupling parameter (alpha). The analysis under (rho+3p=0) also yields anisotropic power-law solutions, where expansion and contraction coexist in different spatial directions. Overall, this work demonstrates the richness of cosmological behavior in (f(R,L_{m})) gravity and highlights its potential to address key open problems in modern cosmology.