3,784 research outputs found
Non-linear conformally invariant generalization of the Poisson equation to D>2 dimensions
I propound a non-linear generalization of the Poisson equation describing a
"medium" in D dimensions with a "dielectric constant" proportional to the field
strength to the power D-2. It is the only conformally invariant scalar theory
that is second order, and in which the scalar couples to the sources
via a contact term. The symmetry is used to generate
solutions for the field for some non-trivial configurations (e.g. for two
oppositely charged points). Systems comprising N point charges afford further
application of the symmetry. For these I derive e.g. exact expressions for the
following quantities: the general two-point-charge force; the energy function
and the forces in any three-body configuration with zero total charge; the
few-body force for some special configurations; the virial theorem for an
arbitrary, bound, many-particle system relating the time-average kinetic energy
to the particle charges. Possible connections with an underlying conformal
quantum field theory are mentioned.Comment: Revtex, 16 pages. To be published in Phys. Rev.
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
Children's tooth decay in a public health program to encourage low-income pregnant women to utilize dental care
<p>Abstract</p> <p>Background</p> <p>A community-based public health program to provide a dental home for women covered by the Oregon Health Plan (Medicaid) in Klamath County, Oregon USA was instituted with the long-term goal to promote preventive oral care for both mothers and their new infants provided by dental managed care companies.</p> <p>Methods</p> <p>As part of the evaluation of the program, children in Klamath and comparable non-program counties were examined in their 2<sup>nd </sup>year of life to begin to determine if benefits accrued to the offspring of the mothers in Klamath County.</p> <p>Results</p> <p>Eighty-five and 58.9% of the children were caries free in the Klamath and comparison county samples, respectively (RR = 1.48, 95% CI 1.13, 1.93). The mean (SD) number of teeth with any decay was .75 (2.5) in the test population and 1.6 (2.5) in the comparison population (t = 2.08, p = .04).</p> <p>Conclusions</p> <p>The assessment showed that children of mothers in the Klamath County program were about one and a half times more likely to be caries free than children in the comparison counties. Additional controlled studies are being undertaken.</p
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The probability of sale and price premiums in withdrawn auctioned properties
This paper examines the impact of the auction process of residential properties that whilst unsuccessful at auction sold subsequently. The empirical analysis considers both the probability of sale and the premium of the subsequent sale price over the guide price, reserve and opening bid. The findings highlight that the final achieved sale price is influenced by key price variables revealed both prior to and during the auction itself. Factors such as auction participation, the number of individual bidders and the number of bids are significant in a number of the alternative specifications
The Study of the Pioneer Anomaly: New Data and Objectives for New Investigation
Radiometric tracking data from Pioneer 10 and 11 spacecraft has consistently
indicated the presence of a small, anomalous, Doppler frequency drift,
uniformly changing with a rate of ~6 x 10^{-9} Hz/s; the drift can be
interpreted as a constant sunward acceleration of each particular spacecraft of
a_P = (8.74 \pm 1.33) x 10^{-10} m/s^2. This signal is known as the Pioneer
anomaly; the nature of this anomaly remains unexplained. We discuss the efforts
to retrieve the entire data sets of the Pioneer 10/11 radiometric Doppler data.
We also report on the recently recovered telemetry files that may be used to
reconstruct the engineering history of both spacecraft using original project
documentation and newly developed software tools. We discuss possible ways to
further investigate the discovered effect using these telemetry files in
conjunction with the analysis of the much extended Doppler data. We present the
main objectives of new upcoming study of the Pioneer anomaly, namely i)
analysis of the early data that could yield the direction of the anomaly, ii)
analysis of planetary encounters, that should tell more about the onset of the
anomaly, iii) analysis of the entire dataset, to better determine the anomaly's
temporal behavior, iv) comparative analysis of individual anomalous
accelerations for the two Pioneers, v) the detailed study of on-board
systematics, and vi) development of a thermal-electric-dynamical model using
on-board telemetry. The outlined strategy may allow for a higher accuracy
solution for a_P and, possibly, will lead to an unambiguous determination of
the origin of the Pioneer anomaly.Comment: 43 pages, 40 figures, 3 tables, minor changes before publicatio
Virial mass in DGP brane cosmology
We study the virial mass discrepancy in the context of a DPG brane-world
scenario and show that such a framework can offer viable explanations to
account for the mass discrepancy problem. This is done by defining a
geometrical mass that we prove to be proportional to the virial
mass. Estimating using observational data, we show that it
behaves linearly with and has a value of the order of , pointing
to a possible resolution of the virial mass discrepancy. We also obtain the
radial velocity dispersion of galaxy clusters and show that it is compatible
with the radial velocity dispersion profile of such clusters. This velocity
dispersion profile can be used to differentiate various models predicting the
virial mass.Comment: 12 pages, 1 figure, to appear in CQ
Stability of disk galaxies in the modified dynamics
General analytic arguments lead us to expect that in the modified dynamics
(MOND) self-gravitating disks are more stable than their like in Newtonian
dynamics. We study this question numerically, using a particle-mesh code based
on a multi-grid solver for the (nonlinear) MOND field equation. We start with
equilibrium distribution functions for MOND disk models having a smoothly
truncated, exponential surface-density profiles and a constant Toomre
parameter. We find that, indeed, disks of a given ``temperature'' are locally
more stable in MOND than in Newtonian dynamics. As regards global instability
to bar formation, we find that as the mean acceleration in the disk is lowered,
the stability of the disk is increased as we cross from the Newtonian to the
MOND regime. The degree of stability levels off deep in the MOND regime, as
expected from scaling laws in MOND. For the disk model we use, this maximum
degree of stability is similar to the one imparted to a Newtonian disk by a
halo three times as massive at five disk scale lengths.Comment: 20 pages, Latex, 8 embedded figures, version to be published in The
Astrophys.
Cosmological extrapolation of MOND
Regime of MOND, which is used in astronomy to describe the gravitating
systems of island type without the need to postulate the existence of a
hypothetical dark matter, is generalized to the case of homogeneous
distribution of usual matter by introducing a linear dependence of the critical
acceleration on the size of region under consideration. We show that such the
extrapolation of MOND in cosmology is consistent with both the observed
dependence of brightness on the redshift for type Ia supernovae and the
parameters of large-scale structure of Universe in the evolution, that is
determined by the presence of a cosmological constant, the ordinary matter of
baryons and electrons as well as the photon and neutrino radiation without any
dark matter.Comment: 20 pages, 5 figures, comments adde
A Nonlocal Metric Formulation of MOND
We study a class of nonlocal, but causal, covariant and conserved field
equations for the metric. Although nonlocal, these equations do not seem to
possess extra graviton solutions in weak field perturbation theory. Indeed, the
equations reduce to those of general relativity when the Ricci scalar vanishes
throughout spacetime. When a static matter source is present we show how these
equations can be adjusted to reproduce Milgrom's Modified Newtonian Dynamics in
the weak field regime, while reducing to general relativity for strong fields.
We compute the angular deflection of light in the weak field regime and
demonstrate that it is the same as for general relativity, resulting in far too
little lensing if no dark matter is present. We also study the field equations
for a general Robertson-Walker geometry. An interesting feature of our
equations is that they become conformally invariant in the MOND limit.Comment: 22 pages, LaTeX 2 epsilon, no figure
The Evolutionary Status of SS433
We consider possible evolutionary models for SS 433. We assume that
common-envelope evolution is avoided if radiation pressure is able to expel
most of a super-Eddington accretion flow from a region smaller than the
accretor's Roche lobe. This condition is satisfied, at least initially, for
largely radiative donors with masses in the range 4-12 solar masses. For donors
more massive than about 5 solar masses, moderate mass ratios q = M_2/M_1 > 1
are indicated, thus tending to favor black-hole accretors. For lower mass
donors, evolutionary considerations do not distinguish between a neutron star
or black hole accretor. In all cases the mass transfer (and mass loss) rates
are much larger than the likely mass-loss rate in the precessing jets. Almost
all of the transferred mass is expelled at radii considerably larger than the
jet acceleration region, producing the "stationary" H-alpha line, the infrared
luminosity, and accounting for the low X-ray luminosity.Comment: 13 pages, Astrophysical Journal Letters, accepte
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