1,108 research outputs found
Cosmological constant influence on cosmic string spacetime
We investigate the line element of spacetime around a linear cosmic string in
the presence of a cosmological constant. We obtain the metric and argue that it
should be discarded because of asymptotic considerations. Then a time dependent
and consistent form of the metric is obtained and its properties are discussed.Comment: 3 page
Varying c cosmology and Planck value constraints
It has been suggested that by increasing the speed of light during the early
universe various cosmological problems of standard big bang cosmology can be
overcome, without requiring an inflationary phase. However, we find that as the
Planck length and Planck time are then made correspondingly smaller, and
together with the need that the universe should not re-enter a Planck epoch,
the higher models have very limited ability to resolve such problems. For a
constantly decreasing the universe will quickly becomes quantum
gravitationally dominated as time increases: the opposite to standard cosmology
where quantum behaviour is only ascribed to early times.Comment: extended versio
Galaxy Clustering Topology in the Sloan Digital Sky Survey Main Galaxy Sample: a Test for Galaxy Formation Models
We measure the topology of the main galaxy distribution using the Seventh
Data Release of the Sloan Digital Sky Survey, examining the dependence of
galaxy clustering topology on galaxy properties. The observational results are
used to test galaxy formation models. A volume-limited sample defined by
enables us to measure the genus curve with amplitude of at
Mpc smoothing scale, with 4.8\% uncertainty including all systematics
and cosmic variance. The clustering topology over the smoothing length interval
from 6 to Mpc reveals a mild scale-dependence for the shift
() and void abundance () parameters of the genus curve. We find
substantial bias in the topology of galaxy clustering with respect to the
predicted topology of the matter distribution, which varies with luminosity,
morphology, color, and the smoothing scale of the density field. The
distribution of relatively brighter galaxies shows a greater prevalence of
isolated clusters and more percolated voids. Even though early (late)-type
galaxies show topology similar to that of red (blue) galaxies, the morphology
dependence of topology is not identical to the color dependence. In particular,
the void abundance parameter depends on morphology more strongly than on
color. We test five galaxy assignment schemes applied to cosmological N-body
simulations of a CDM universe to generate mock galaxies: the
Halo-Galaxy one-to-one Correspondence model, the Halo Occupation Distribution
model, and three implementations of Semi-Analytic Models (SAMs). None of the
models reproduces all aspects of the observed clustering topology; the
deviations vary from one model to another but include statistically significant
discrepancies in the abundance of isolated voids or isolated clusters and the
amplitude and overall shift of the genus curve. (Abridged)Comment: 24 pages, 19 figures, 10 tables, submitted to ApJS. Version with full
resolution images is available at
http://astro.kias.re.kr/~cbp/doc/dr7Topo.pd
Dark matter and non-Newtonian gravity from General Relativity coupled to a fluid of strings
An exact solution of Einstein's field equations for a point mass surrounded
by a static, spherically symmetric fluid of strings is presented. The solution
is singular at the origin. Near the string cloud limit there is a
correction to Newton's force law. It is noted that at large distances and small
accelerations, this law coincides with the phenomenological force law invented
by Milgrom in order to explain the flat rotation curves of galaxies without
introducing dark matter. When interpreted in the context of a cosmological
model with a string fluid, the new solution naturally explains why the critical
acceleration of Milgrom is of the same order of magnitude as the Hubble
parameter.Comment: 12 pages, REVTeX, no figure
Topology from the Simulated Sloan Digital Sky Survey
We measure the topology (genus curve) of the galaxy distribution in a mock
redshift catalog designed to resemble the upcoming Sloan Digital Sky Survey
(SDSS). The catalog, drawn from a large N-body simulation of a Lambda-CDM cos-
mological model, mimics the anticipated spectroscopic selection procedures of
the SDSS in some detail. Sky maps, redshift slices, and 3-D contour maps of the
mock survey reveal a rich and complex structure, including networks of voids
and superclusters that resemble the patterns seen in the CfA redshift survey
and the Las Campanas Redshift Survey (LCRS). The 3-D genus curve can be
measured from the simulated catalog with superb precision; this curve has the
general shape predicted for Gaussian, random phase initial conditions, but the
error bars are small enough to demonstrate with high significance the subtle
departures from this shape caused by non-linear gravitational evolution. These
distortions have the form predicted by Matsubara's (1994) perturbative anal-
ysis, but they are much smaller in amplitude. We also measure the 3-D genus
curve of the radial peculiar velocity field measured by applying distance-
indicator relations (with realistic errors) to the mock catalog. This genus
curve is consistent with the Gaussian random phase prediction, though it is of
relatively low precision because of the large smoothing length required to
overcome noise in the measured velocity field. Finally, we measure the 2-D
topology in redshift slices, similar to early slices from the SDSS and to
slices already observed in the LCRS. The genus curves of these slices are
consistent with the observed genus curves of the LCRS, providing further
evidence in favor of the inflationary CDM model with Omega_M~0.4. The catalog
is publicly available at http://www.astronomy.ohio-state.edu/~dhw/sdss.html.Comment: ASTeX 4.0 Preprint Style, 5 GIF figures (Figs 1, 2, 3a, 3b, 6; see
http://cfa-www.harvard.edu/~wcolley/SDSS_Top/ for PostScript versions), 7
PostScript figures. Figure 5 and Table 1 have minor corrections since
publicatio
Self-Dual Chern-Simons Solitons in (2+1)-Dimensional Einstein Gravity
We consider here a generalization of the Abelian Higgs model in curved space,
by adding a Chern--Simons term. The static equations are self-dual provided we
choose a suitable potential. The solutions give a self-dual
Maxwell--Chern--Simons soliton that possesses a mass and a spin
A prescription for probabilities in eternal inflation
Some of the parameters we call ``constants of Nature'' may in fact be
variables related to the local values of some dynamical fields. During
inflation, these variables are randomized by quantum fluctuations. In cases
when the variable in question (call it ) takes values in a continuous
range, all thermalized regions in the universe are statistically equivalent,
and a gauge invariant procedure for calculating the probability distribution
for is known. This is the so-called ``spherical cutoff method''. In
order to find the probability distribution for it suffices to consider a
large spherical patch in a single thermalized region. Here, we generalize this
method to the case when the range of is discontinuous and there are
several different types of thermalized region. We first formulate a set of
requirements that any such generalization should satisfy, and then introduce a
prescription that meets all the requirements. We finally apply this
prescription to calculate the relative probability for different bubble
universes in the open inflation scenario.Comment: 15 pages, 5 figure
Natural Wormholes as Gravitational Lenses
Visser has suggested traversable 3-dimensional wormholes that could plausibly
form naturally during Big Bang inflation. A wormhole mouth embedded in high
mass density might accrete mass, giving the other mouth a net *negative* mass
of unusual gravitational properties. The lensing of such a gravitationally
negative anomalous compact halo object (GNACHO) will enhance background stars
with a time profile that is observable and qualitatively different from that
recently observed for massive compact halo objects (MACHOs) of positive mass.
We recommend that MACHO search data be analyzed for GNACHOs.Comment: 4 pages; plus 4 figures; ReV_TeX 3.0; DOE/ER/40537-001/NPL94-07-01
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