47,486 research outputs found
Recent results in Euclidean dynamical triangulations
We study a formulation of lattice gravity defined via Euclidean dynamical
triangulations (EDT). After fine-tuning a non-trivial local measure term we
find evidence that four-dimensional, semi-classical geometries are recovered at
long distance scales in the continuum limit. Furthermore, we find that the
spectral dimension at short distance scales is consistent with 3/2, a value
that is also observed in the causal dynamical triangulation (CDT) approach to
quantum gravity.Comment: 7 pages, 3 figures. Proceedings for the 3rd conference of the Polish
society on relativit
Finding the Origin of the Pioneer Anomaly
Analysis of radio-metric tracking data from the Pioneer 10/11 spacecraft at
distances between 20 - 70 astronomical units (AU) from the Sun has consistently
indicated the presence of an anomalous, small, constant Doppler frequency
drift. The drift can be interpreted as being due to a constant acceleration of
a_P= (8.74 \pm 1.33) x 10^{-8} cm/s^2 directed towards the Sun. Although it is
suspected that there is a systematic origin to the effect, none has been found.
As a result, the nature of this anomaly has become of growing interest. Here we
present a concept for a deep-space experiment that will reveal the origin of
the discovered anomaly and also will characterize its properties to an accuracy
of at least two orders of magnitude below the anomaly's size. The proposed
mission will not only provide a significant accuracy improvement in the search
for small anomalous accelerations, it will also determine if the anomaly is due
to some internal systematic or has an external origin. A number of critical
requirements and design considerations for the mission are outlined and
addressed. If only already existing technologies were used, the mission could
be flown as early as 2010.Comment: 21 SS pages, 4+1 figures. final changes for publicatio
Lattice Quantum Gravity and Asymptotic Safety
We study the nonperturbative formulation of quantum gravity defined via
Euclidean dynamical triangulations (EDT) in an attempt to make contact with
Weinberg's asymptotic safety scenario. We find that a fine-tuning is necessary
in order to recover semiclassical behavior. Such a fine-tuning is generally
associated with the breaking of a target symmetry by the lattice regulator; in
this case we argue that the target symmetry is the general coordinate
invariance of the theory. After introducing and fine-tuning a nontrivial local
measure term, we find no barrier to taking a continuum limit, and we find
evidence that four-dimensional, semiclassical geometries are recovered at long
distance scales in the continuum limit. We also find that the spectral
dimension at short distance scales is consistent with 3/2, a value that could
resolve the tension between asymptotic safety and the holographic entropy
scaling of black holes. We argue that the number of relevant couplings in the
continuum theory is one, once symmetry breaking by the lattice regulator is
accounted for. Such a theory is maximally predictive, with no adjustable
parameters. The cosmological constant in Planck units is the only relevant
parameter, which serves to set the lattice scale. The cosmological constant in
Planck units is of order 1 in the ultraviolet and undergoes renormalization
group running to small values in the infrared. If these findings hold up under
further scrutiny, the lattice may provide a nonperturbative definition of a
renormalizable quantum field theory of general relativity with no adjustable
parameters and a cosmological constant that is naturally small in the infrared.Comment: 69 pages, 25 figures. Revised discussion of target symmetry
throughout paper. Numerical results unchanged and main conclusions largely
unchanged. Added references and corrected typos. Conforms with version
published in Phys. Rev.
Cosmology with Varying Constants
The idea of possible time or space variations of the `fundamental' constants
of nature, although not new, is only now beginning to be actively considered by
large numbers of researchers in the particle physics, cosmology and
astrophysics communities. This revival is mostly due to the claims of possible
detection of such variations, in various different contexts and by several
groups. Here, I present the current theoretical motivations and expectations
for such variations, review the current observational status, and discuss the
impact of a possible confirmation of these results in our views of cosmology
and physics as a whole.Comment: 14 pages, no figures. Essay to appear in Phil. Trans. Roy. Soc. Lond.
A Triennial Series (Christmas 2002 Issue
Recommended from our members
An unusual cause of gynaecomastia in a male
Summary: Beta-human chorionic gonadotropin (βhCG) is normally produced by syncytiotrophoblasts of the placenta during pregnancy and aids embryo implantation. However, it is also secreted in varying amounts in non-pregnant conditions commonly heralding a neoplastic process. We present a case of 50-year-old man, who presented with bilateral gynaecomastia with elevated testosterone, oestradiol, suppressed gonadotropins with progressively increasing levels of human chorionic gonadotropin (hCG). Biochemical and radiological investigations including ultrasonography of testes, breast tissue, MRI pituitary and CT scan full body did not identify the source of hCG. FDG PET scan revealed a large mediastinal mass with lung metastasis. Immunostaining and histological analysis confirmed the diagnosis of primary choriocarcinoma of the mediastinum. It is highly aggressive and malignant tumor with poor prognosis. Early diagnosis and management are essential for the best outcome. Learning Points: High βhCG in a male patient or a non-pregnant female suggests a paraneoplastic syndrome. In the case of persistently positive serum hCG, exclude immunoassay interference by doing the urine hCG as heterophilic antibodies are not present in the urine. Non-gestational choriocarcinoma is an extremely rare trophoblastic tumor and should be considered in young men presenting with gynaecomastia and high concentration of hCG with normal gonads. A high index of suspicion and extensive investigations are required to establish an early diagnosis of extra-gonadal choriocarcinoma. Early diagnosis is crucial to formulate optimal management strategy and to minimize widespread metastasis for best clinical outcome
Nodeless superconductivity in the noncentrosymmetric MoRhN superconductor: a SR study
The noncentrosymmetric superconductor MoRhN, with K,
adopts a -Mn-type structure (space group 432), similar to that of
MoAlC. Its bulk superconductivity was characterized by magnetization
and heat-capacity measurements, while its microscopic electronic properties
were investigated by means of muon-spin rotation and relaxation (SR). The
low-temperature superfluid density, measured via transverse-field (TF)-SR,
evidences a fully-gapped superconducting state with , very close to 1.76 - the BCS gap value for
the weak coupling case, and a magnetic penetration depth nm.
The absence of spontaneous magnetic fields below the onset of
superconductivity, as determined by zero-field (ZF)-SR measurements, hints
at a preserved time-reversal symmetry in the superconducting state. Both TF-and
ZF-SR results evidence a spin-singlet pairing in MoRhN.Comment: 5 figures and 5 pages. Accepted for publication as a Rapid
Communication in Phys. Rev.
Working Within Constraints: Managing African Savannas for Animal Production and Biodiversity
The mean density of livestock biomass on African rangelands now greatly exceeds that of indigenous large herbivores, although livestock cannot fully substitute for wildlife with respect to co-evolved ecosystem processes involving herbivory. The dominance of livestock in semi-arid rangelands is largely due to water provision, which uncouples livestock population dynamics from the rainfall-driven trajectories followed by indigenous ungulate species in wildlife areas. Ecological sustainability cannot be achieved with a few exotic species maintained at unprecedented biomass densities in savanna ecosystems, which are evolutionarily adapted for species-rich communities of ungulates of a wise range of sizes. Integrating wildlife and livestock in multi-species animal production systems offers a partial solution. Community-based ecotourism can effectively augment pastoralism in semi-arid rangelands but, as rainfall increases, the opportunity costs become too high. Direct payment approaches show promise for offsetting opportunity costs, although major obstacles remain in the form of political corruption and obstructive practices by national governments
Reviews
J. Combrink en J. Spies. SARA. Sakboek van regte Afrikaans, 2de uitgawe,lste druk 1994, 214 pp. ISBN 0 624 03252 3. Kaapstad: TafelbergUitgewers.Prys R49,95
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