1,660 research outputs found
What is General Relativity?
General relativity is a set of physical and geometric principles, which lead
to a set of (Einstein) field equations that determine the gravitational field,
and to the geodesic equations that describe light propagation and the motion of
particles on the background. But open questions remain, including: What is the
scale on which matter and geometry are dynamically coupled in the Einstein
equations? Are the field equations valid on small and large scales? What is the
largest scale on which matter can be coarse grained while following a geodesic
of a solution to Einstein's equations? We address these questions. If the field
equations are causal evolution equations, whose average on cosmological scales
is not an exact solution of the Einstein equations, then some simplifying
physical principle is required to explain the statistical homogeneity of the
late epoch Universe. Such a principle may have its origin in the dynamical
coupling between matter and geometry at the quantum level in the early
Universe. This possibility is hinted at by diverse approaches to quantum
gravity which find a dynamical reduction to two effective dimensions at high
energies on one hand, and by cosmological observations which are beginning to
strongly restrict the class of viable inflationary phenomenologies on the
other. We suggest that the foundational principles of general relativity will
play a central role in reformulating the theory of spacetime structure to meet
the challenges of cosmology in the 21st century.Comment: 18 pages. Invited article for Physica Scripta Focus issue on 21st
Century Frontiers. v2: Appendix amended, references added. v3: Small
corrections, references added, matches published versio
The role of forensic geoscience in wildlife crime detection
The increase in both automation and precision in the analysis of geological materials has had significant impact upon forensic investigations in the last 10 years. There is however, a fundamental philosophical difference between forensic and geological enquiry. This paper presents the results of forensic geoscientific investigations of three cases of wildlife crime. Two cases involve the analysis of soils recovered after incidents of illegal badger baiting in the United Kingdom. The third case involves the illegal importation of Eleonora's Falcon (Falco eleonorae) into the United Kingdom from the Mediterranean. All three cases utilise the analysis of soils by a variety of physical, chemical and biological techniques. These involve mineral and grain size analyses, cation and anion compositions, pH, organic content and pollen analysis.The independent analysis undertaken by specialists in each of these three main fields conclude firstly, that there is a significant similarity between sediments taken at the crime site at both badger setts and with sediments recovered from various spades, shovels and clothing belonging to suspects and secondly, that the soils analysed associated with the removal of the falcon eggs in the Mediterranean contained characteristics similar in many respects to the soils of the breeding areas of E eleonorae on the cliffs of Mallorca. The use of these independent techniques in wildlife crime detection has great potential given the ubiquitous nature of soils and sediments found in association with wildlife sites. (c) 2006 Elsevier Ireland Ltd. All rights reserved
The Determination of the Compatibility Between the Reasons for Attending and the Career Goals of Secondary Students at the Norfolk Technical-Vocational Center
The purpose of this study was to answer the following questions: 1. Why do students elect the programs at NTVC? 2. What do these students intend to do after high school graduation
Timescape cosmology with radiation fluid
The timescape cosmology represents a potentially viable alternative to the
standard homogeneous cosmology, without the need for dark energy. Although
average cosmic evolution in the timescape scenario only differs substantially
from that of Friedmann-Lemaitre model at relatively late epochs when the
contribution from the energy density of radiation is negligible, a full
solution of the Buchert equations to incorporate radiation is necessary to
smoothly match parameters to the epoch of photon decoupling and to obtain
constraints from cosmic microwave background data. Here we extend the
matter-dominated solution found in earlier work to include radiation, providing
series solutions at early times and an efficient numerical integration strategy
for generating the complete solution. The numerical solution is used to
directly calculate the scale of the sound horizon at decoupling, and at the
baryon drag epoch. The constraints on these scales from the Planck satellite
data yield bounds on the timescape cosmological parameters, which are found to
also agree with the best-fit values from a recent analysis of SDSS-II supernova
data, while avoiding the problem of a primordial lithium-7 abundance anomaly.Comment: 21 pages, 6 figures; v2: small additions, accepted by Class. Quantum
Gra
Twisted Electromagnetic Modes and Sagnac Ring-Lasers
A new approximation scheme, designed to solve the covariant Maxwell equations
inside a rotating hollow slender conducting cavity (modelling a ring-laser), is
constructed. It is shown that for well-defined conditions there exist TE and TM
modes with respect to the longitudinal axis of the cavity. A twisted mode
spectrum is found to depend on the integrated Frenet torsion of the cavity and
this in turn may affect the Sagnac beat frequency induced by a non-zero
rotation of the cavity. The analysis is motivated by attempts to use
ring-lasers to measure terrestrial gravito-magnetism or the Lense-Thirring
effect produced by the rotation of the Earth.Comment: LaTeX 31 pages, 3 Figure
Average observational quantities in the timescape cosmology
We examine the properties of a recently proposed observationally viable
alternative to homogeneous cosmology with smooth dark energy, the timescape
cosmology. In the timescape model cosmic acceleration is realized as an
apparent effect related to the calibration of clocks and rods of observers in
bound systems relative to volume-average observers in an inhomogeneous geometry
in ordinary general relativity. The model is based on an exact solution to a
Buchert average of the Einstein equations with backreaction. The present paper
examines a number of observational tests which will enable the timescape model
to be distinguished from homogeneous cosmologies with a cosmological constant
or other smooth dark energy, in current and future generations of dark energy
experiments. Predictions are presented for: comoving distance measures; H(z);
the equivalent of the dark energy equation of state, w(z); the Om(z) measure of
Sahni, Shafieloo and Starobinsky; the Alcock-Paczynski test; the baryon
acoustic oscillation measure, D_v; the inhomogeneity test of Clarkson, Bassett
and Lu; and the time drift of cosmological redshifts. Where possible, the
predictions are compared to recent independent studies of similar measures in
homogeneous cosmologies with dark energy. Three separate tests with indications
of results in possible tension with the Lambda CDM model are found to be
consistent with the expectations of the timescape cosmology.Comment: 22 pages, 12 figures; v2 discussion, references added, matches
published versio
Observational Challenges for the Standard FLRW Model
We summarise some of the main observational challenges for the standard
Friedmann-Lemaitre-Robertson-Walker cosmological model and describe how results
recently presented in the parallel session `Large--scale Structure and
Statistics' (DE3) at the `Fourteenth Marcel Grossman Meeting on General
Relativity' are related to these challenges.Comment: 17 pages; references added. Matches published version in Int. J. Mod.
Phys. D; Report on Parallel Session DE3 of MG1
Future deceleration due to cosmic backreaction in presence of the event horizon
The present acceleration of the universe leads to the formation of a
cosmological future event horizon. We explore the effects of the event horizon
on cosmological backreaction due to inhomogeneities in the universe. Beginning
from the onset of the present accelerated era, we show that backreaction in
presence of the event horizon causes acceleration to slow down in the
subsequent evolution. Transition to deceleration occurs eventually, ensuring
avoidance of a big rip.Comment: Latex, 5 pages, 2 figures. This version has small changes to match
with the version published in MNRAS: Letter
Robertson-Walker fluid sources endowed with rotation characterised by quadratic terms in angular velocity parameter
Einstein's equations for a Robertson-Walker fluid source endowed with
rotation Einstein's equations for a Robertson-Walker fluid source endowed with
rotation are presented upto and including quadratic terms in angular velocity
parameter. A family of analytic solutions are obtained for the case in which
the source angular velocity is purely time-dependent. A subclass of solutions
is presented which merge smoothly to homogeneous rotating and non-rotating
central sources. The particular solution for dust endowed with rotation is
presented. In all cases explicit expressions, depending sinusoidally on polar
angle, are given for the density and internal supporting pressure of the
rotating source. In addition to the non-zero axial velocity of the fluid
particles it is shown that there is also a radial component of velocity which
vanishes only at the poles. The velocity four-vector has a zero component
between poles
Solution Of Wheeler-De Witt Equation, Potential Well And Tunnel Effect
This paper uses the relation of the cosmic scale factor and scalar field to
solve Wheeler-DeWitt equation, gives the tunnel effect of the cosmic scale
factor a and quantum potential well of scalar field, and makes it fit with the
physics of cosmic quantum birth. By solving Wheeler-DeWitt equation we achieve
a general probability distribution of the cosmic birth, and give the analysis
of cosmic quantum birth.Comment: 12 page
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