Gravitation and Cosmology最新文献

We consider the inverse function of the scale factor (a(t)) of the Friedmann equations, discussing also the asymptotics of cosmological parameters under various assumptions on the (Lambda)CDM model. Using algebraic dependencies between the cosmological parameters and the representation of the inverse function of the scale factor (a(t)) as an elliptic integral with a parameter, we compute in a uniform way some special events in the universe’s evolution.

We investigate the possibility of a future singularity due to accelerating expansion of the Universe in a gravitational theory that comprises the Ricci scalar (R) and the Gauss–Bonnet invariant (mathcal{G}), known as (F(R,mathcal{G})) gravity, which can be viewed in the quadratic form. Three models are presented using Hubble parameters to represent a finite and infinite future. The model parameters are analyzed on the basis of their physical and geometrical properties. This study also explores the properties of the modified gravitational theory, and neither a future singularity nor a little or pseudo-rip are posed as threats to the fate of the Universe. We present scalar perturbation approaches to perturbed evolution equations and demonstrate their stability.

We have constructed a dark energy cosmological model in the framework of the generalized Brans Dicke (GBD) theory with a self-interacting potential. The source of dark energy is considered through a unified dark fluid (UDF) characterized by a linear equation of state (EoS). A time-varying deceleration parameter simulating the cosmic transit behavior has been introduced to get the dynamical behavior of the model. The (H(z)) data have been explored to constrain the model parameters and to study the dynamical aspects of the Brans-Dicke parameter and the scalar field.

We revisit the cyclic Universe scenario in scalar field FRW cosmology and check its applicability for a nonminimally coupled scalar field. We show that for the most popular case of a quartic potential and the standard nonminimal coupling this scenario does work. On the other hand, we identify certain cases where the cyclic model fails to work and present the corresponding reasons for that.

Using the Radon–Nikodym theorem concerning the relation between any two measures, as well as the methods employed in loop quantum gravity, it is shown that in gravitation one can quantize any measure which is not even associated with metric. We have considered the simplest case where the proportionality coefficient between two operators of measure (the Radon–Nikodym derivative) is some function. The result is that in the right-hand side of the commutation relations for the measure the fundamental length becomes a variable quantity, and this can lead to smoothing the singularity. The case where the Radon–Nikodym derivative is an operator is also under discussion. Classical and quantum theories in the background of space endowed with quantum measure are under consideration.

A model of a thick fluid disk around a binary black hole is considered. A binary black hole is described by the Majumdar-Papapetrou solution. The hydrodynamic equations in this metric are written out. Exact analytical solutions are presented. A generalization to the case of a toroidal magnetic field is carried out.

We consider a Bianchi Type I universe filled with cold dark matter and non-interacting Tsallis holographic dark energy in the framework of (f(R)) theory of gravity. We take the GO scale as an IR cutoff and find exact solutions of the field equations by considering a linearly varying deceleration parameter. The evolution of different cosmologically relevant parameters are studied graphically, and Statefinder diagnostics is performed in the light of recent cosmological observations. We find that our model corresponds to current accelerated expansion scenarios, and the function (f(R)approx R) implies that our model resembles General Relativity.

The paper considers Higgs inflation in the Jordan and Einstein pictures. In Jordan’s picture, the dynamic equations are presented in terms of the Higgs field and a superpotential. In the approximation of small field values and in the slow-rolling mode, examples of solutions are found, and verifiability by cosmological parameters is analyzed. Dynamic equations are also obtained in the Einstein picture, and a nonsingular solution is found for a weak Higgs field, whose cosmological parameters slightly differ from the observed data. To analyze the cosmological dynamics, the set of equations in Einstein’s picture is reduced to the form of an autonomous set of equations. Singular points of the dynamical system are found, and a phase portrait near these points is presented for different values of the nonminimal interaction parameter (xi). The case of large values of (xi) is also considered.

We investigate some bouncing cosmological models in an isotropic and homogeneous space-time with in (F(R)) theory of gravity. Two functional forms of (F(R)) have are studied with a bouncing scale factor. The dynamical parameters are derived and analyzed along with cosmographic parameters. A violation of sthe trong energy conditions in both bouncing models is also shown. We show that both models exhibit stable behavior with respect to cosmic time.

The effective Einstein equations on a 3-brane embedded in a 5-dimensional Riemann–Cartan bulk space-time are revisited. Addressing the shortcomings in the hitherto published junction conditions on the brane in the presence of torsion, we have elaborated on our general form of the junction conditions recently published. Applying our general junction conditions, we formulate the effective Einstein equations on a (Z_{2}) symmetric brane in a standard form highlighting the difference to those published so far.