339 research outputs found
Qualitative Approach to Semi-Classical Loop Quantum Cosmology
Recently the mechanism was found which allows avoidance of the cosmological
singularity within the semi-classical formulation of Loop Quantum Gravity.
Numerical studies show that the presence of self-interaction potential of the
scalar field allows generation of initial conditions for successful slow-roll
inflation. In this paper qualitative analysis of dynamical system,
corresponding to cosmological equations of Loop Quantum Gravity is performed.
The conclusion on singularity avoidance in positively curved cosmological
models is confirmed. Two cases are considered, the massless (with flat
potential) and massive scalar field. Explanation of initial conditions
generation for inflation in models with massive scalar field is given. The
bounce is discussed in models with zero spatial curvature and negative
potentials.Comment: Online at http://www.iop.org/EJ/abstract/1475-7516/2004/07/01
Approach of the NGC 1977 star cluster to the TOI-2796 host star
We study of possible encounters in past epochs of the open star cluster NGC
1977 with host stars. For this purpose, the age of the cluster was determined
based on our catalog data. Stars with planetary systems were selected from the
NASA Archive. The age of the cluster was determined using the color - absolute
magnitude diagram and the isochron system. By extending the track of the
movement of the cluster and stars in past epochs, 10 Myr. The time of the
maximum approach 32 pc of the host star with planetary system TOI-2796 with the
NGC 1977 are found. The place of approach in the sky is shown, this point can
be considered as the place of appearance of interstellar comets. Thus, the
result of our work is that the we found approach of the host star to the
cluster entailed effects associated with the gravitational influence of the
cluster on the nuclei of comets located in the outer parts of the Oort cloud of
the planetary system. The effect of approach on comets is estimated
The linearization method and new classes of exact solutions in cosmology
We develop a method for constructing exact cosmological solutions of the
Einstein equations based on representing them as a second-order linear
differential equation. In particular, the method allows using an arbitrary
known solution to construct a more general solution parameterized by a set of
3\textit{N} constants, where \textit{N} is an arbitrary natural number. The
large number of free parameters may prove useful for constructing a theoretical
model that agrees satisfactorily with the results of astronomical observations.
Cosmological solutions on the Randall-Sundrum brane have similar properties. We
show that three-parameter solutions in the general case already exhibit
inflationary regimes. In contrast to previously studied two-parameter
solutions, these three-parameter solutions can describe an exit from inflation
without a fine tuning of the parameters and also several consecutive
inflationary regimes.Comment: 7 page
Mathematical Modeling of a Solar Arrays Deploying Process at Ground Tests
This paper focuses on the creating of a mathematical model of a solar array deploying process during ground tests. Lagrange equation was used to obtain the math model. The distinctive feature of this mathematical model is the possibility of taking into account the gravity compensation system influence on the construction in the deploying process and the aerodynamic resistance during ground tests
Pressure-induced phase transition of Bi2Te3 into the bcc structure
The pressure-induced phase transition of bismuth telluride, Bi2Te3, has been
studied by synchrotron x-ray diffraction measurements at room temperature using
a diamond-anvil cell (DAC) with loading pressures up to 29.8 GPa. We found a
high-pressure body-centered cubic (bcc) phase in Bi2Te3 at 25.2 GPa, which is
denoted as phase IV, and this phase apperars above 14.5 GPa. Upon releasing the
pressure from 29.8 GPa, the diffraction pattern changes with pressure
hysteresis. The original rhombohedral phase is recovered at 2.43 GPa. The bcc
structure can explain the phase IV peaks. We assumed that the structural model
of phase IV is analogous to a substitutional binary alloy; the Bi and Te atoms
are distributed in the bcc-lattice sites with space group Im-3m. The results of
Rietveld analysis based on this model agree well with both the experimental
data and calculated results. Therefore, the structure of phase IV in Bi2Te3 can
be explained by a solid solution with a bcc lattice in the Bi-Te (60 atomic%
tellurium) binary system.Comment: 12 pages, 5 figure
Thermalization of a nonequilibrium electron-positron-photon plasma
Starting from a nonequilibrium configuration we analyse the essential role of
the direct and the inverse binary and triple interactions in reaching an
asymptotic thermal equilibrium in a homogeneous isotropic
electron-positron-photon plasma. We focus on energies in the range 0.1--10 MeV.
We numerically integrate the integro-partial differential relativistic
Boltzmann equation with the exact QED collisional integrals taking into account
all binary and triple interactions in the plasma. We show that first, when
detailed balance is reached for all binary interactions on a timescale
sec, photons and electron-positron pairs establish
kinetic equilibrium. Successively, when triple interactions fulfill the
detailed balance on a timescale sec, the plasma reaches
thermal equilibrium. It is shown that neglecting the inverse triple
interactions prevents reaching thermal equilibrium. Our results obtained in the
theoretical physics domain also find application in astrophysics and cosmology.Comment: 4 pages, 3 figures, Phys. Rev. Lett., to appea
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