605 research outputs found
Interference between a large number of independent Bose-Einstein condensates
We study theoretically the interference patterns produced by the overlap of
an array of Bose-Einstein condensates that have no phase coherence among them.
We show that density-density correlations at different quasimomenta, which play
an important role in two-condensate interference, become negligible for large
, where is the number of overlapping condensates. In order to understand
the physics of this phenomenon, it is sufficient to consider the periodicity of
the lattice and the statistical probability distribution of a random-walk
problem. The average visibility of such interference patterns decreases as
for large .Comment: 9 pages, 2 figure
Cosmological Models with Shear and Rotation
Cosmological models involving shear and rotation are considered, first in the
General Relat ivistic and then in the Newtonian framework with the aim of
investigating singularities in them by using numerical and analytical
techniques. The dynamics of these rotating models ar e studied. It is shown
that singularities are unavoidable in such models and that the centr ifugal
force arising due to rotation can never overcome the gravitational and shearing
forc e over a length of time.Comment: 17 pages, 6 figures Journal Ref: J. Astrophys. Astr. (1999) 20, 79-8
The Clusters AgeS Experiment (CASE). II. The Eclipsing Blue Straggler OGLEGC-228 in the Globular Cluster 47 Tuc
We use photometric and spectroscopic observations of the eclipsing binary
OGLEGC-228 (V228) to derive the masses, radii, and luminosities of the
component stars. Based on measured systemic velocity, proper motion and
distance, the system is a blue straggler member of the globular cluster 47 Tuc.
Our analysis shows that V228 is a semi-detached Algol. We obtain M=1.512 +/-
0.022 Msun, R=1.357 +/- 0.019 Rsun, L=7.02 +/- 0.050 Lsun for the hotter and
more luminous primary component and M=0.200 +/- 0.007 Msun, R=1.238 +/- 0.013
Rsun, L=1.57 +/- 0.09 Lsun for the Roche lobe filling secondary.Comment: 19 pages, 5 figures, AJ, in pres
Exact solution for random walks on the triangular lattice with absorbing boundaries
The problem of a random walk on a finite triangular lattice with a single
interior source point and zig-zag absorbing boundaries is solved exactly. This
problem has been previously considered intractable.Comment: 10 pages, Latex, IOP macro
A cosmological model in Weyl-Cartan spacetime: I. Field equations and solutions
In this first article of a series on alternative cosmological models we
present an extended version of a cosmological model in Weyl-Cartan spacetime.
The new model can be viewed as a generalization of a model developed earlier
jointly with Tresguerres. Within this model the non-Riemannian quantities, i.e.
torsion and nonmetricity , are proportional to
the Weyl 1-form. The hypermomentum depends on our
ansatz for the nonmetricity and vice versa. We derive the explicit form of the
field equations for different cases and provide solutions for a broad class of
parameters. We demonstrate that it is possible to construct models in which the
non-Riemannian quantities die out with time. We show how our model fits into
the more general framework of metric-affine gravity (MAG).Comment: 22 pages, 2 figures, uses IOP preprint styl
New Path Equations in Absolute Parallelism Geometry
The Bazanski approach, for deriving the geodesic equations in Riemannian
geometry, is generalized in the absolute parallelism geometry. As a consequence
of this generalization three path equations are obtained. A striking feature in
the derived equations is the appearance of a torsion term with a numerical
coefficients that jumps by a step of one half from equation to another. This is
tempting to speculate that the paths in absolute parallelism geometry might
admit a quantum feature.Comment: 4 pages Latex file Journal Reference: Astrophysics and space science
228, 273, (1995
Two-Dimensional Axisymmetric Collapse of Thermally Unstable Primordial Clouds
We have performed two-dimensional hydrodynamic simulations of the collapse of
isolated axisymmetric clouds condensing via radiative cooling in a primordial
background gas. In order to study the development of the so-called
``shape-instability'', we have considered two types of axisymmetric clouds,
oblate and prolate clouds of various sizes and with axial ratios of . We find that the degree of oblateness or
prolateness is enhanced during the initial cooling phase. But it can be
reversed later, if the initial contrast in cooling times between the cloud gas
and the background gas is much greater than one. In such cases an oblate cloud
collapses to a structure composed of an outer thin disk and a central prolate
component. A prolate cloud, on the other hand, becomes a thin cigar-shape
structure with a central dense oblate component. The reversal of shape in the
central part of the cooled clouds is due to supersonic motions either along the
disk plane in the case of oblate clouds or along the symmetry axis in the case
of prolate clouds. For a background gas of K and n_h=0.1
\cm3 in a protogalactic halo environment, the mean density of the cloud gas
that has cooled to K increases to or so, in our simulations
where nonequilibrium cooling is adopted and the background gas cools too. The
spherical Jeans mass of such gas is estimated to be about M_J \sim
5\times10^{7}\Msun. In order for cloud mass to exceed the Jeans mass and at
the same time in order for the thermal instability to operate, the initial
cloud size should be around where is the
cooling length.Comment: 31 pages including 12 figures (reduced resolution), to appear in The
Astrophysical Journal (v584 n2 ApJ February 20, 2003 issue). Pdf with full
resolution figures can be downloaded from
ftp://canopus.chungnam.ac.kr/ryu/ryu.pd
Let's talk about varying G
It is possible that fundamental constants may not be constant at all. There
is a generally accepted view that one can only talk about variations of
dimensionless quantities, such as the fine structure constant . However, constraints on the strength of
gravity tend to focus on G itself, which is problematic. We stress that G needs
to be multiplied by the square of a mass, and hence, for example, one should be
constraining , where is
the proton mass. Failure to focus on such dimensionless quantities makes it
difficult to interpret the physical dependence of constraints on the variation
of G in many published studies. A thought experiment involving talking to
observers in another universe about the values of physical constants may be
useful for distinguishing what is genuinely measurable from what is merely part
of our particular system of units.Comment: 6 pages, Gravity Research Foundation essa
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