Gravitation and Cosmology最新文献

We investigate the implications of the modified gravity theory (f(R,L_{m})) on the cosmological evolution. By examining the nonlinear model (f(R,L_{m})={R}/{2}+(alpha R+1)L_{m}), we explore the impact of a nonminimal coupling between curvature and matter on the cosmic expansion. Using a parametrized deceleration parameter dependent on the redshift (z), we analyze the Friedmann–Lemaître–Robertson–Walker (FLRW) universe in the (f(R,L_{m})) framework. Through observational constraints derived from Cosmic Chronometers (CC), Type Ia Supernovae (SNIa), and Baryon Acoustic Oscillations (BAO), we perform a detailed comparison with the standard (Lambda)CDM model. Our results show that the (f(R,L_{m})) model is consistent with observational data, but deviations from the (Lambda)CDM model emerge in its geometric structure, highlighting the potential of (f(R,L_{m})) gravity in explaining the dark energy and cosmic acceleration.

Cosmological models based on an asymmetric scalar Higgs doublet (the canonical (Phi) and phantom (phi) fields) with potential interaction between the components are proposed. A qualitative analysis of the corresponding dynamic systems is performed, and their transformation properties with respect to similarity transformations of fundamental constants are revealed. The asymptotic behavior of this class of cosmological models near cosmological singularities is investigated. Numerical simulations reveal a number of interesting features of these models, in particular, the possibility of a fairly long “waiting phase,” during which the Universe is almost Euclidean, as well as the presence of bounce points, near which there occur strong oscillations of the scalar potentials.

A mathematical model of the evolution of the Universe, based on an asymmetric doublet of classical and phantom scalar Higgs fields with a kinetic connection between the components, is constructed and studied. A detailed qualitative analysis is carried out, the properties of the modelCs symmetry and invariance with respect to the similarity transformation of fundamental constants are proven. The principles of numerical modeling are formulated, and an example of numerical modeling of the model evolution is given for a specific set of fundamental constants and initial conditions. The asymptotic behavior of the model near cosmological singularities is studied, and It is shown that it then manifests itself as a perfect fluid with an extremely rigid equation of state.

We explore the cosmological tests of the deceleration parameter (q=K-gamma/H), where (K) and (gamma) are arbitrary constants in a Bianchi type-III cosmological model in the framework of the Saez–Ballester theory of gravitation. Under the assumption that the shear scalar ((sigma)) is proportional to the expansion scalar ((theta)), we evaluate the physical and kinematic parameters, the jerk parameter, and the deceleration parameter (DP) in the early deceleration phase ((q>0)) and the current acceleration phase ((q<0)). The energy conditions and the (Om(z)) analysis for our spatially homogeneous and anisotropic Bianchi type-III model are discussed in details. Furthermore, we perform a (chi^{2}) test to obtain the best fit values of the model parameters in the derived model by using (46,H(z)) data sets in the redshift range (0leq zleq 2.36) and also analyze the present age of the universe.

We suggest commutation relations for a quantum measure. In one version of these relations, the right-hand side takes account of the presence of curvature of space; in the simplest case, this yields the action of general relativity. We consider the cases of quantization of the measure on spaces of constant curvature and show that in this case the commutation relations for the quantum measure are analogues of commutation relations in loop quantum gravity. It is assumed that, in contrast to loop quantum gravity, a triangulation of space is a necessary trick for quantizing such a nonlocal quantity as a measure; and at that, the space remains a smooth manifold. We consider the self-consistent problem of interaction of the quantum measure and classical gravitation. It is shown that this inevitably leads to the emergence of modified gravities. Also, we consider the problem of defining the Euler–Lagrange equations for a matter field in the background of a space endowed with quantum measure.

We present a general vacuum solution of the modified Gauss–Bonnet gravity equations for the Friedmann–Lemaître–Robertson–Walker metric. We use an ansatz to reduce the gravitational equations to an ordinary differential equation for the function (F=F(G)). The solution obtained depends on an arbitrary function and is new. As an example, we take the arbitrary function in the form of a power one and analyze the solutions for both (F(G)) and the main cosmological physical quantities as the scale factor, Hubble rate, the Gauss–Bonnet term and the scalar curvature.

We examine the Hawking radiation of a Schwarszchild-type black hole in Starobinsky–Bel–Robinson (SBR) gravity and calculate the corrected Hawking temperature using the tunneling method. We then discuss the deviation of our Hawking temperature from the standard Schwarszchild result. We relate the corrections to the Hawking temperature beyond the semiclassical approximation. We highlight that starting with a modification of the classical black hole geometry and calculating the semiclassical Hawking temperature yields temperature corrections comparable to those obtained when the classical background is kept unchanged and beyond-semiclassical terms in the temperature are included.

Numerical modeling of a mathematical model of the cosmological evolution of an asymmetric scalar doublet with kinetic interaction between the components was carried out. A wide range of values of fundamental parameters and initial conditions of the model are considered. Various types of behavior have been identified: models with an infinite inflationary past and future—with and without a rebound point, models with a finite past and infinite future, with an infinite past and finite future (Big Rip), as well as models with a finite past and future. Based on numerical analysis, the behavior of models near the initial singularity and the Big Rip is studied; it is shown that in both cases the barotropic coefficient tends to unity, which corresponds to an extremely rigid state of matter near the singularities. A numerical example of the cosmological generation of the classical component of a scalar doublet by its phantom component is given. An assessment was made of the creation of the velocity of fermion pairs by a scalar field near the rebound points, and it has been shown that a scalar field at the cold stage of the Universe can ensure creation of the required number of massive scalarly charged fermions.

Our aim is to link Bekenstein’s quantized form of the area of the event horizon to the Hamiltonian of the non-Hermitian Swanson oscillator which is known to be (mathbb{PT})-symmetric. We achieve this by employing a similarity transformation that maps the non-Hermitian quantum system to a scaled harmonic oscillator. Our procedure is standard and well known. To this end, we consider the unconstrained reduced Hamiltonian which is directly expressed in terms of the Schwarzschild mass and implies a periodic character of the conjugate momentum (which represents the asymptotic time coordinate), the period being the inverse Hawking temperature. This leads to quantization of the event-horizon area in terms of the harmonic-oscillator levels. Next, in the framework of the Swanson oscillator, we proceed to derive novel expressions for the Hawking temperature and the black hole entropy. Notably, the logarithmic area-correction term (-(1/2)ln)(area) is consistent with our results, whereas (-(3/2)ln)(area) is not.

The stage of a super-early (primordial) Universe is considered on the basis of Poincare gauge theory of gravity in the presence of a cosmological scalar field with an extremely weak self-action. For this purpose, field equations were obtained for a homogeneous and isotropic metric. A solution has been found at which the scale factor experiences an inflationary increase, and the effective cosmological constant has a sharp decrease from a huge value near the Big Bang to a small modern value. This fact solves the “cosmological constant problem.” It is predicted that the effective cosmological constant decreases extremely slowly throughout the lifetime of the Universe.