We present in this paper a quantitative method for defining void size in
large-scale structure based on percolation threshold density. Beginning with
two-dimensional gravitational clustering simulations smoothed to the threshold
of nonlinearity, we perform percolation analysis to determine the large scale
structure. The resulting objective definition of voids has a natural scaling
property, is topologically interesting, and can be applied immediately to redshift
surveys.

We present in this paper a quantitative method for defining void size in
large-scale structure based on percolation threshold density. Beginning with
two-dimensional gravitational clustering simulations smoothed to the threshold
of nonlinearity, we perform percolation analysis to determine the large scale
structure. The resulting objective definition of voids has a natural scaling
property, is topologically interesting, and can be applied immediately to redshift
surveys.

A gravitational lens model of the radio quasar B1422+231 is presented which can account
for the image arrangement and approximately for the relative magnifications. The locations
of the principal lensing mass and a more distant secondary mass concentration were predicted
and subsequently luminous galaxies were found at these locations. This argues against the
existence of substantial numbers of “dark” galaxies. The model suggests that if the compact
radio source is intrinsically superluminal then the observed component motions may be as large
as
∼ 100
c with image B moving in the opposite direction to images A and C. The prospects for
a measuring the Hubble constant from a model incorporating lens galaxy locations, compact
radio source expansion speeds and radio time delays, if and when these are measured, are
briefly assessed.

We consider the production of gravitons in an inflationary cosmology by ap-
proximating each epoch of change in the equation of state as sudden, from which a
simple analytic graviton mode function has been derived. We use this mode func-
tion to compute the graviton spectral energy density and the tensor-induced cosmic
microwave background anisotropy.

Various theoretical uncertainties in the standard solar model and in the
Mikheyev-Smirnov-Wolfenstein (MSW) analysis are discussed. It is shown
that two methods of estimating the solar neutrino flux uncertainties are
equivalent:

In the non-critical string framework that we have proposed recently, the time t is identified
with a dynamical local renormalization group scale, the Liouville mode, and behaves as
a statistical evolution parameter, flowing irreversibly from an infrared fixed point - which
we conjecture to be a topological string phase - to an ultraviolet one - which corresponds
to a static critical string vacuum. When applied to a toy two-dimensional model of space-
time singularities, this formalism yields an apparent renormalization of the velocity of
light, and a t-dependent form of the uncertainty relation for position and momentum of a
test string. We speculate within this framework on a stringy alternative to conventional
field-theoretical inflation, and the decay towards zero of the cosmological constant in a
maximally-symmetric space.

Using an analytical model for the string network we show that the
kurtosis of cosmic microwave background (CMB) temperature gradi-
ent maps is a good statistic to distinguish between the cosmic string
model and inflationary models of structure formation.

We employ N{body/3D gas dynamic simulations of the formation of galaxy clusters
to determine whether cluster X{ray morphologies can be used as cosmological constraints.
Conrming the analytic expectations of Richstone, Loeb, & Turner, we demonstrate that
cluster evolution is sensitive to the cosmological model in which the clusters form. We
further show that evolutionary dierences are echoed in the gross morphological features of
the cluster X{ray emission.

We examine the possibility that gamma-ray bursts arise from sources in the Oort comet
cloud, basing most of our arguments on accepted models for the formation and spatial
distribution of the cloud

Good statistics for measuring large-scale structure in the Universe must be able
to distinguish between different models of structure formation. In this paper, two
and three dimensional “counts in cell” statistics and a new “discrete genus statis-
tic” are applied to toy versions of several popular theories of structure formation:
random phase cold dark matter model, cosmic string models, and global texture
scenario. All three statistics appear quite promising in terms of differentiating
between the models.

In this letter we propose a physical explanation for recently reported correla-
tions between pairs of close and antipodal gamma-ray bursts from publicly avail-
able BATSE catalogue. Our model is based on the cosmological scenario in which
bursters are located at cosmological distances of order of 0.5–2 Gpc. Observed dis-
tribution of gamma-ray bursts strongly suports this assumption. If so gamma-ray
bursts may provide a very good probe for investigating the topological structure
of the Universe. We notice that correlation between antipodal events may in fact
indicate that we live in the so called Ellis’ small universe which has Friedman-
Roberston-Walker metric structure and nontrivial topology.

Although Potent purports to use only radial velocities in retrieving the potential ve-
locity eld of galaxies, the derivation of transverse components is implicit in the smoothing
procedures. Thus the possibility of using nonradial line integrals to derive the velocity
eld arises. In the case of inhomogeneous distributions of galaxies, the optimal path for
integration need not be radial, and can be obtained by using max-
ow algorithms. In this
paper we present the results of using Dijkstra's algorithm to obtain this optimal path and
velocity eld.

We investigate the eect of using dierent distance estimators on the recovery of the
peculiar velocity eld of galaxies using Potent. An inappropriate choice of distance
estimator will give rise to spurious
ows. We discuss methods of minimising these biases
and the levels of accuracy required of distance estimators to retrieve velocity elds to a
given standard.

Methods for inferring the velocity eld from the peculiar velocity data are described and applied
to old and newer data. Inhomogeneous Malmquist bias and ways to avoid it are discussed and
utilized. We infer that these biases are probably important in interpreting the data.

Studies of inclined rings inside galaxy potentials have mostly considered the in-
uence of self{gravity and viscous dissipation separately. In this study, we construct
models of highly-inclined (\polar") rings in an external potential including both self{
gravity and dissipation due to a drag force.

We describe an analytic distribution function of a nite, oblate stellar system that is
useful for the practical modelling of dark halos. The function is determined by lowering
Evans's (1993) distribution function of a
attened, cored isothermal system in analogy
to the lowering of the singular, isothermal sphere in the denition of the King (1966)
model

A pseudo-Nambu-Goldstone boson, with a potential of the form V (φ) = Λ4
[1 + cos(φ/f)],
can naturally give rise to an epoch of inflation in the early universe (Freese, Frieman, and Olinto
1990). The potential is naturally flat (as required by microwave background limits on the
amplitude of density fluctuations), without any fine-tuning. Successful inflation can be achieved
if f ∼ mpl and Λ ∼ mGUT . Such mass scales arise in particle physics models with a large gauge
group that becomes strongly interacting at a scale ∼ Λ, e.g., as can happen in the hidden sector
of superstring theories.

We study nonlinear wave phenomena in self-gravitating fluid systems, with a partic-
ular emphasis on applications to molecular clouds. This paper presents analytical results
for one spatial dimension

We have performed a series of high resolution N-body experiments on a Connection
Machine CM-5 in order to simulate the formation of galaxy clusters gravitationally
dominated by a massive dark background.

The recent detection of microlensing of stars of LMC by compact objects in
the halo of our galaxy
1
−
2
suggests that our galaxy is surrounded by a non-
luminous halo made of compact objects with mass of about (0.03−0.5)M⊙.

We use high resolution dissipationless N-body simulations to examine the spatial dis-
tribution of galaxy clusters on large scales. The Standard CDM model and two of its
main competitors,

We use ensembles of high-resolution CDM simulations to investigate the shape and
amplitude of the two point correlation function of rich clusters. The standard scale-
invariant CDM model with
= 1 provides a poor description of the clustering mea-
sured from the APM rich cluster redshift survey, which is better tted by models with
more power at large scale

Most of the mass density in the Universe—and in the halo of our own
galaxy—exists in the form of dark matter. Overall, the contribution of lu-
minous matter (in stars) to the mass density of the Universe is less than
1%;

We examine the 10µm emission of the central regions of 281 spiral galaxies, after
having compiled all ground-based, small-aperture (∼5”) broad-band photometric
observations at λ∼10µm (N magnitudes) published in the literature. We evaluate
the compactness of the ∼10µm emission of galaxy nuclei by comparing these
small-beam measures with the large-beam IRAS 12µm fluxes. In the analysis
of different subsets of objects, we apply survival analysis techniques in order to
exploit the information contained in “censored” data (i.e., upper limits on the
fluxes)

The determined abundances of primordial 4He and 7Li provide a basis with which to test
the standard model of big bang nucleosynthesis in conjunction with the other two light element
isotopes D and 3He, also produced in the big bang.

The Lagrangian theory of gravitational instability of homogeneous-isotropic Friedman-Lema^tre
cosmogonies investigated and solved in the series of papers by Buchert (1989), (1992), Buchert &
Ehlers (1993), Buchert (1993a,b), Ehlers & Buchert (1993), is illustrated. The third-order solution
of this theory for generic initial conditions is presented and realized in a special case by employing
methods of high-spatial resolution of the density eld.

Several simple mergers between model galaxy clusters containing a
mixture of gas and dark matter are examined, testing the coupling of
the gas to the underlying collisionless material. The gas is shocked,
irreversibly dissipating the energy fed into it by the collisionless compo-
nent and forms a resolved constant-density core. For the dark matter,
however, admixture of phase space vacuum is not very efficient and a
constant-density core is not produced. In the final state the central gas
has little residual kinetic energy, indicating that streaming motions do
not help to support the gas.