25,457 research outputs found
Local freedom in the gravitational field
In a cosmological context, the electric and magnetic parts of the Weyl
tensor, E_{ab} and H_{ab}, represent the locally free curvature - i.e. they are
not pointwise determined by the matter fields. By performing a complete
covariant decomposition of the derivatives of E_{ab} and H_{ab}, we show that
the parts of the derivative of the curvature which are locally free (i.e. not
pointwise determined by the matter via the Bianchi identities) are exactly the
symmetrised trace-free spatial derivatives of E_{ab} and H_{ab} together with
their spatial curls. These parts of the derivatives are shown to be crucial for
the existence of gravitational waves.Comment: New results on gravitational waves included; new references added;
revised version (IOP style) to appear Class. Quantum Gra
Note on Signature Change and Colombeau Theory
Recent work alludes to various `controversies' associated with signature
change in general relativity. As we have argued previously, these are in fact
disagreements about the (often unstated) assumptions underlying various
possible approaches. The choice between approaches remains open.Comment: REVTex, 3 pages; to appear in GR
Gravity and Signature Change
The use of proper ``time'' to describe classical ``spacetimes'' which contain
both Euclidean and Lorentzian regions permits the introduction of smooth
(generalized) orthonormal frames. This remarkable fact permits one to describe
both a variational treatment of Einstein's equations and distribution theory
using straightforward generalizations of the standard treatments for constant
signature.Comment: Plain TeX, 6 pages; to appear in GR
The Venus Balloon Project
On June 11 and 15, 1985, two instrumental balloons were released from the Soviet VEGA 1 and VEGA 2 spacecraft and deployed in the atmosphere of Venus. The VEGA probes flew by the planet on their way to a rendezvous with comet Halley in March 1986. Drifting with the wind at altitudes of 54 km, the balloons traveled one-third of the way around the planet during their 46-hour lifetimes. Sensors on-board the gondolas made periodic measurements of pressure, temperature, vertical wind velocity, cloud particle density, ambient light level, and frequency of lightning. The data were transmitted to Earth and received at the Deep Space Network (DSN) 64-m stations and at several large antennas in the USSR. Approximately 95 percent of the telemetry data were successfully decoded at the DSN complexes and in the Soviet Union, and were provided to the international science team for analysis. Very Long Baseline Interferometry (VLBI) data were acquired by 20 radio observatories around the world for the purpose of monitoring the Venus winds. The DSN 64-m subnet was part of a 15-station VLBI network organized by the Centre National d'Etudes Spatiales (CNES) of France. In addition, five antennas of the Soviet network participated. VLBI data from the CNES network are currently being processed at the Jet Propulsion Laboratory
Dynamics of Inflationary Universes with Positive Spatial Curvature
If the spatial curvature of the universe is positive, then the curvature term
will always dominate at early enough times in a slow-rolling inflationary
epoch. This enhances inflationary effects and hence puts limits on the possible
number of e-foldings that can have occurred, independently of what happened
before inflation began and in particular without regard for what may have
happened in the Planck era. We use a simple multi-stage model to examine this
limit as a function of the present density parameter and the epoch
when inflation ends.Comment: 9 Pages RevTex4. Revised and update
Electroweak Precision Data and Gravitino Dark Matter
Electroweak precision measurements can provide indirect information about the
possible scale of supersymmetry already at the present level of accuracy. We
review present day sensitivities of precision data in mSUGRA-type models with
the gravitino as the lightest supersymmetric particle (LSP). The chi^2 fit is
based on M_W, sin^2 theta_eff, (g-2)_mu, BR(b -> s gamma) and the lightest MSSM
Higgs boson mass, M_h. We find indications for relatively light soft
supersymmetry-breaking masses, offering good prospects for the LHC and the ILC,
and in some cases also for the Tevatron.Comment: 4 pages, 1 figure. Talk given at the LCWS06 March 2006, Bangalore,
India. References adde
A fully covariant description of CMB anisotropies
Starting from the exact non-linear description of matter and radiation, a
fully covariant and gauge-invariant formula for the observed temperature
anisotropy of the cosmic microwave background (CBR) radiation, expressed in
terms of the electric () and magnetic () parts of the Weyl
tensor, is obtained by integrating photon geodesics from last scattering to the
point of observation today. This improves and extends earlier work by Russ et
al where a similar formula was obtained by taking first order variations of the
redshift. In the case of scalar (density) perturbations, is related to
the harmonic components of the gravitational potential and the usual
dominant Sachs-Wolfe contribution to the
temperature anisotropy is recovered, together with contributions due to the
time variation of the potential (Rees-Sciama effect), entropy and velocity
perturbations at last scattering and a pressure suppression term important in
low density universes. We also explicitly demonstrate the validity of assuming
that the perturbations are adiabatic at decoupling and show that if the surface
of last scattering is correctly placed and the background universe model is
taken to be a flat dust dominated Friedmann-Robertson-Walker model (FRW), then
the large scale temperature anisotropy can be interpreted as being due to the
motion of the matter relative to the surface of constant temperature which
defines the surface of last scattering on those scales.Comment: 18 pages LaTeX, 1 figure. Submitted to Classical and Quantum Gravity.
Also available at http://shiva.mth.uct.ac.za/preprints/9705.htm
Full one-loop amplitudes from tree amplitudes
We establish an efficient polynomial-complexity algorithm for one-loop
calculations, based on generalized -dimensional unitarity. It allows
automated computations of both cut-constructible {\it and} rational parts of
one-loop scattering amplitudes from on-shell tree amplitudes. We illustrate the
method by (re)-computing all four-, five- and six-gluon scattering amplitudes
in QCD at one-loop.Comment: 27 pages, revte
Locally extracting scalar, vector and tensor modes in cosmological perturbation theory
Cosmological perturbation theory relies on the decomposition of perturbations
into so-called scalar, vector and tensor modes. This decomposition is non-local
and depends on unknowable boundary conditions. The non-locality is particularly
important at second- and higher-order because perturbative modes are sourced by
products of lower-oder modes, which must be integrated over all space in order
to isolate each mode. However, given a trace-free rank-2 tensor, a locally
defined scalar mode may be trivially derived by taking two divergences, which
knocks out the vector and tensor degrees of freedom. A similar local
differential operation will return a pure vector mode. This means that scalar
and vector degrees of freedom have local descriptions. The corresponding local
extraction of the tensor mode is unknown however. We give it here. The
operators we define are useful for defining gauge-invariant quantities at
second-order. We perform much of our analysis using an index-free
`vector-calculus' approach which makes manipulating tensor equations
considerably simpler.Comment: 13 pages. Final version to appear in CQ
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