Holographic dark energy model and scalar-tensor theories

We study the holographic dark energy model in a generalized scalar tensor theory. In a universe filled with cold dark matter and dark energy, the effect of potential of the scalar field is investigated in the equation of state parameter. We show that for a various types of potentials, the equation of state parameter is negative and transition from deceleration to acceleration expansion of the universe is possible.     

The Quantum Evolution of the Closed Friedman Universe with “Bouncing Off”

The homogeneous and isotropic closed Friedmanuniverse evolution in higher-order gravity theories isconsidered. The model takes into account vacuumpolarisation of conformal and nonconformal fields. That leads to the following addition in the Einsteinlagrangian: R² ln |R/R₀|. Near theregular minimum of the scale factor the model has ananalytical solution depending on an integration constantC. If |R/R₀| > 1, the solution passes through the regularminimum, experiences inflation with a decreasing valueof R and approaches to the critical value R =R₀. In the interval |R/R₀| < 1,the solutions have non-linear oscillations (i.e. the scalaronstage). On this stage of the evolution the universe isfilled with relativistic plasma. The continuoustransition through the critical point R = R₀is possible in only one type of solution, the separatrix.Though other solutions have no features in this point,they experience the discontinuity in derivatives of R.It is unsuitable since higher-order gravity theories are considered. Thus the measure of continuoussolutions giving a hot universe is negligible. Howeversolutions of the model can be continued in the imaginarytime. In such a case the Euclidean action will have a non-zero value because of the spaceclosed boundedness of the universe and the finiteness ofthe imaginary time interval (instanton). The last allowsus to calculate the probability of the quantum tunnelling of the Friedman universe from theinflation region into the scalaron region.     

Chaotic motion in axially symmetric potentials with oblate quadrupole deformation

By computing the Poincaré?s surfaces of section and Lyapunov exponents, we study the effect of introducing an oblate quadrupole in the dynamics associated with two generic spherical potentials of physical interest: the central monopole and the isotropic harmonic oscillator. In the former case we find saddle points in the effective potential, in contrast to the statements presented by Guéron and Letelier in [E. Guéron, P.S. Letelier, Phys. Rev. E 63 (2001) 035201]. The results we show in the second case have application in nuclear or atomic physics. In particular, we find values of oblate deformation leading to a disappearance of shell structure in the single-particle spectrum.     

Spin rotation by Earthʼs gravitational field in a “frozen-spin” ring

Detailed calculations of spin rotation by the Earth?s gravitational field in a frozen-spin ring are presented in three different coordinate systems and used (a) to show that the systematic error caused by gravitation in a proposed electric dipole moment measurement can be unambiguously determined, and (b) to propose measuring the spin-gravity effect in a dedicated frozen-spin ring using electrons.     

The Energy of the Universe in Teleparallel Gravity

The teleparallel versions of the Einstein and the Landau-Lifshitz energy-momentum complexes of the gravitational field are obtained. By using these complexes, the total energy of the universe, which includes the energy of both the matter and the gravitational fields, is then obtained. It is shown that in the case of a closed universe, the total energy vanishes independently of the pseudotensor used, as well as of the three dimensionless coupling constants of teleparallel gravity.     

Relativistic compact objects in isotropic coordinates

We present a matrix method for obtaining new classes of exact solutions for Einstein’s equations representing static perfect fluid spheres. By means of a matrix transformation, we reduce Einstein’s equations to two independent Riccati-type differential equations for which three classes of solutions are obtained. One class of the solutions corresponding to the linear barotropic-type fluid with an equation of statep =γρ is discussed in detail.     

New methodical developments for GRANIT

New methodical developments for the GRANIT spectrometer address further improvements of the critical parameters of this experimental installation, as well as its applications to new fields of research. Keeping in mind an extremely small fraction of ultra cold neutrons (UCN) that could be bound in gravitational quantum states, we look for methods to increase statistics due to: developing UCN sources with maximum phase-space density, counting simultaneously a large fraction of neutrons using position-sensitive detectors, and decreasing detector backgrounds. Also we explore an eventual application of the GRANIT spectrometer beyond the scope of its initial goals, for instance, for reflectometry with UCN.     

What kind of science is cosmology?

In recent years, by theory and observation cosmology has advanced substantially. Parameters of the concordance or ΛCDM cosmological model are given with unprecedented precision (“precision cosmology”). On the other hand, 95% of the matter content of the universe are of an unknown nature. This awkward situation motivates the present attempt to find cosmology's place among the (exact) natural sciences. Due to its epistemic and methodical particularities, e.g., as a mathematized historical science, cosmology occupies a very special place. After going through some of the highlights of cosmological modeling, the conclusion is reached that knowledge provided by cosmological modeling cannot be as explicative and secure as knowledge gained by laboratory physics.     

Stability for the Gravitational Vlasov–Poisson System in Dimension Two

We consider the two dimensional gravitational Vlasov–Poisson system. Using variational methods, we prove the existence of stationary solutions of minimal energy under a Casimir type constraint. The method also provides a stability criterion of these solutions for the evolution problem.