186 research outputs found
Gravitational Wave Signals from Chaotic System: A Point Mass with A Disk
We study gravitational waves from a particle moving around a system of a
point mass with a disk in Newtonian gravitational theory. A particle motion in
this system can be chaotic when the gravitational contribution from a surface
density of a disk is comparable with that from a point mass. In such an orbit,
we sometimes find that there appears a phase of the orbit in which particle
motion becomes to be nearly regular (the so-called ``stagnant motion'') for a
finite time interval between more strongly chaotic phases. To study how these
different chaotic behaviours affect on observation of gravitational waves, we
investigate a correlation of the particle motion and the waves. We find that
such a difference in chaotic motions reflects on the wave forms and energy
spectra. The character of the waves in the stagnant motion is quite different
from that either in a regular motion or in a more strongly chaotic motion. This
suggests that we may make a distinction between different chaotic behaviours of
the orbit via the gravitational waves.Comment: Published in Phys.Rev.D76:024018,200
Universality of power law correlations in gravitational clustering
We present an analysis of different sets of gravitational N-body simulations,
all describing the dynamics of discrete particles with a small initial velocity
dispersion. They encompass very different initial particle configurations,
different numerical algorithms for the computation of the force, with or
without the space expansion of cosmological models. Despite these differences
we find in all cases that the non-linear clustering which results is
essentially the same, with a well-defined simple power-law behaviour in the
two-point correlations in the range from a few times the lower cut-off in the
gravitational force to the scale at which fluctuations are of order one. We
argue, presenting quantitative evidence, that this apparently universal
behaviour can be understood by the domination of the small scale contribution
to the gravitational force, coming initially from nearest neighbor particles.Comment: 7 pages, latex, 3 postscript figures. Revised version to be published
in Europhysics Letters. Contains additional analysis showing more directly
the central role of nearest neighbour interactions in the development of
power-law correlation
A new algorithm for anisotropic solutions
We establish a new algorithm that generates a new solution to the Einstein
field equations, with an anisotropic matter distribution, from a seed isotropic
solution. The new solution is expressed in terms of integrals of an isotropic
gravitational potential; and the integration can be completed exactly for
particular isotropic seed metrics. A good feature of our approach is that the
anisotropic solutions necessarily have an isotropic limit. We find two examples
of anisotropic solutions which generalise the isothermal sphere and the
Schwarzschild interior sphere. Both examples are expressed in closed form
involving elementary functions only.Comment: 16 pages, to appear in Pramana - J. Phy
Gravitational dynamics of an infinite shuffled lattice: early time evolution and universality of non-linear correlations
In two recent articles a detailed study has been presented of the out of
equilibrium dynamics of an infinite system of self-gravitating points initially
located on a randomly perturbed lattice. In this article we extend the
treatment of the early time phase during which strong non-linear correlations
first develop, prior to the onset of ``self-similar'' scaling in the two point
correlation function. We establish more directly, using appropriate
modifications of the numerical integration, that the development of these
correlations can be well described by an approximation of the evolution in two
phases: a first perturbative phase in which particles' displacements are small
compared to the lattice spacing, and a subsequent phase in which particles
interact only with their nearest neighbor. For the range of initial amplitudes
considered we show that the first phase can be well approximated as a
transformation of the perturbed lattice configuration into a Poisson
distribution at the relevant scales. This appears to explain the
``universality'' of the spatial dependence of the asymptotic non-linear
clustering observed from both shuffled lattice and Poisson initial conditions.Comment: 11 pages, 11 figures, shortened introductory sections and other minor
modifications, version to appear in Phys. Rev.
Clustering in gravitating N-body systems
We study gravitational clustering of mass points in three dimensions with
random initial positions and periodic boundary conditions (no expansion) by
numerical simulations. Correlation properties are well defined in the system
and a sort of thermodynamic limit can be defined for the transient regime of
cluste ring. Structure formation proceeds along two paths: (i) fluid-like
evolution of density perturbations at large scales and (ii) shift of the
granular (non fluid) properties from small to large scales. The latter
mechanism finally dominates at all scales and it is responsible for the
self-similar characteristics of the clustering.Comment: 7 pages, 3 figures. Accepted for publication in Europhys. Let
Anisotropic static solutions in modelling highly compact bodies
Einstein field equations for anisotropic spheres are solved and exact
interior solutions obtained. This paper extends earlier treatments to include
anisotropic models which accommodate a wider variety of physically viable
energy densities. Two classes of solutions are possible. The first class
contains the limiting case for the energy density which
arises in many astrophysical applications. In the second class the singularity
at the center of the star is not present in the energy density. The models
presented in this paper allow for increasing and decreasing profiles in the
behavior of the energy density.Comment: 9 pages, to appear in Pramana - J. Phy
Clustering of Primordial Black Holes. II. Evolution of Bound Systems
Primordial Black Holes (PBHs) that form from the collapse of density
perturbations are more clustered than the underlying density field. In a
previous paper, we showed the constraints that this has on the prospects of PBH
dark matter. In this paper we examine another consequence of this clustering:
the formation of bound systems of PBHs in the early universe. These would
hypothetically be the earliest gravitationally collapsed structures, forming
when the universe is still radiation dominated. Depending upon the size and
occupation of the clusters, PBH merging occurs before they would have otherwise
evaporated due to Hawking evaporation.Comment: 23 pages, 1 figure. Submitted to PR
Free streaming in mixed dark matter
Free streaming in a \emph{mixture} of collisionless non-relativistic dark
matter (DM) particles is studied by implementing methods from the theory of
multicomponent plasmas. The mixture includes Fermionic, condensed and non
condensed Bosonic particles decoupling in equilibrium while relativistic, heavy
non-relativistic thermal relics (WIMPs), and sterile neutrinos that decouple
\emph{out of equilibrium} when they are relativistic. The free-streaming length
is obtained from the marginal zero of the gravitational
polarization function, which separates short wavelength Landau-damped from long
wavelength Jeans-unstable \emph{collective} modes. At redshift we find ,where are the \emph{fractions} of the respective DM components of mass
that decouple when the effective number of ultrarelativistic degrees of
freedom is , and only depend on the distribution functions at
decoupling, given explicitly in all cases. If sterile neutrinos produced either
resonantly or non-resonantly that decouple near the QCD scale are the
\emph{only} DM component,we find (non-resonant), (resonant).If WIMPs with
decoupling at are present in the mixture with
, is \emph{dominated} by CDM. If a Bose Einstein condensate is a DM
component its free streaming length is consistent with CDM because of the
infrared enhancement of the distribution function.Comment: 19 pages, 2 figures. More discussions same conclusions and results.
Version to appear in Phys. Rev.
Inhomogeneous imperfect fluid spherical models without Big-Bang singularity
So far all known singularity-free cosmological models are cylindrically
symmetric. Here we present a new family of spherically symmetric non-singular
models filled with imperfect fluid and radial heat flow, and satisfying the
weak and strong energy conditions. For large anisotropy in pressure and
heat flux tend to vanish leading to a perfect fluid. There is a free function
of time in the model, which can be suitably chosen for non-singular behaviour
and there exist multiplicity of such choices.Comment: 8 pages, LaTeX versio
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