300 research outputs found
The non-Abelian gauge theory of matrix big bangs
We study at the classical and quantum mechanical level the time-dependent
Yang-Mills theory that one obtains via the generalisation of discrete
light-cone quantisation to singular homogeneous plane waves. The non-Abelian
nature of this theory is known to be important for physics near the
singularity, at least as far as the number of degrees of freedom is concerned.
We will show that the quartic interaction is always subleading as one
approaches the singularity and that close enough to t=0 the evolution is driven
by the diverging tachyonic mass term. The evolution towards asymptotically flat
space-time also reveals some surprising features.Comment: 29 pages, 8 eps figures, v2: minor changes, references added: v3
small typographical changes
p-wave Holographic Superconductors and five-dimensional gauged Supergravity
We explore five-dimensional and
SO(6) gauged supergravities as frameworks for condensed matter applications.
These theories contain charged (dilatonic) black holes and 2-forms which have
non-trivial quantum numbers with respect to U(1) subgroups of SO(6). A question
of interest is whether they also contain black holes with two-form hair with
the required asymptotic to give rise to holographic superconductivity. We first
consider the case, which contains a complex two-form potential
which has U(1) charge . We find that a slight
generalization, where the two-form potential has an arbitrary charge , leads
to a five-dimensional model that exhibits second-order superconducting
transitions of p-wave type where the role of order parameter is played by
, provided . We identify the operator that condenses
in the dual CFT, which is closely related to Super Yang-Mills
theory with chemical potentials. Similar phase transitions between R-charged
black holes and black holes with 2-form hair are found in a generalized version
of the gauged supergravity Lagrangian where the two-forms have
charge .Comment: 35 pages, 14 figure
Probe Branes, Time-dependent Couplings and Thermalization in AdS/CFT
We present holographic descriptions of thermalization in conformal field
theories using probe D-branes in AdS X S space-times. We find that the induced
metrics on Dp-brane worldvolumes which are rotating in an internal sphere
direction have horizons with characteristic Hawking temperatures even if there
is no black hole in the bulk AdS. The AdS/CFT correspondence applied to such
systems indeed reveals thermal properties such as Brownian motions and AC
conductivities in the dual conformal field theories. We also use this framework
to holographically analyze time-dependent systems undergoing a quantum quench,
where parameters in quantum field theories, such as a mass or a coupling
constant, are suddenly changed. We confirm that this leads to thermal behavior
by demonstrating the formation of apparent horizons in the induced metric after
a certain time.Comment: LaTeX, 47 pages, 14 figures; Typos corrected and references added
(v2); minor corrections, references added(v3
Stochastic Gravity: Theory and Applications
Whereas semiclassical gravity is based on the semiclassical Einstein equation
with sources given by the expectation value of the stress-energy tensor of
quantum fields, stochastic semiclassical gravity is based on the
Einstein-Langevin equation, which has in addition sources due to the noise
kernel. In the first part, we describe the fundamentals of this new theory via
two approaches: the axiomatic and the functional. In the second part, we
describe three applications of stochastic gravity theory. First, we consider
metric perturbations in a Minkowski spacetime, compute the two-point
correlation functions of these perturbations and prove that Minkowski spacetime
is a stable solution of semiclassical gravity. Second, we discuss structure
formation from the stochastic gravity viewpoint. Third, we discuss the
backreaction of Hawking radiation in the gravitational background of a black
hole and describe the metric fluctuations near the event horizon of an
evaporating black holeComment: 100 pages, no figures; an update of the 2003 review in Living Reviews
in Relativity gr-qc/0307032 ; it includes new sections on the Validity of
Semiclassical Gravity, the Stability of Minkowski Spacetime, and the Metric
Fluctuations of an Evaporating Black Hol
Pair Production of small Black Holes in Heterotic String Theories
We study pair production of small BPS BH's in heterotic strings compactified
on tori and in the FHSV model. After recalling the identification of small BH's
in the perturbative BPS spectrum, we compute the tree-level amplitudes for
processes initiated by massless vector bosons or gravitons. We then analyze the
resulting cross sections in terms of energy and angular distributions. Finally,
we briefly comment on scenari with large extra dimensions and on
generalizations of our results to non-BPS, non-extremal and rotating BH's.Comment: 33 page
Vitamin B12 deficiency in metformin-treated type-2 diabetes patients, prevalence and association with peripheral neuropathy
BACKGROUND : The association between long-term metformin use and low vitamin B12 levels has been proven.
However, the prevalence estimates of metformin-induced vitamin B12 deficiency showed considerable variation
among the studies. The potential of the deficiency to cause or worsen peripheral neuropathy in type-2 diabetes
mellitus (T2DM) patients has been investigated with conflicting results. The aim of the study was to investigate:
1) the prevalence of vitamin B12 deficiency in T2DM patients on metformin; 2) the association between vitamin B12
and peripheral neuropathy; 3) and the risk factors for vitamin B12 deficiency in these patients.
METHODS : In this cross-sectional study, consecutive metformin-treated T2DM patients attending diabetes clinics of
two public hospitals in South Africa were approached for participation. Participation included measuring vitamin
B12 levels and assessing peripheral neuropathy using Neuropathy Total Symptom Score-6 (NTSS-6) questionnaire.
The prevalence of vitamin B12 deficiency (defined by concentrations <150 pmol/L) was determined. Those with
NTSS-6 scores >6 were considered to have peripheral neuropathy. The relationship between vitamin B12 and
peripheral neuropathy was investigated when the two variables were in the binary and continuous forms. Multiple
logistic regression was used to determine risk factors for vitamin B12 deficiency.
RESULTS : Among 121 participants, the prevalence of vitamin B12 deficiency was 28.1 %. There was no difference in
presence of neuropathy between those with normal and deficient vitamin levels (36.8 % vs. 32.3 %, P = 0.209).
Vitamin B12 levels and NTSS-6 scores were not correlated (Spearman’s rho =0.056, P = 0.54). HbA1c (mmol/mol)
(OR = 0.97, 95 % CI: 0.95 to 0.99, P = 0.003) and black race (OR = 0.34, 95 % CI: 0.13 to 0.92, P = 0.033) were risk
factors significantly associated with vitamin B12 deficiency. Metformin daily dose (gram) showed borderline
significance (OR = 1.96, 95 % CI: 0.99 to 3.88, P = 0.053).
CONCLUSIONS : Close to third of metformin-treated T2DM patients had vitamin B12 deficiency. The deficiency was
not associated with peripheral neuropathy. Black race was a protective factor for vitamin B12 deficiency.The Department of Pharmacology, University of Pretoriahttp://bmcpharmacoltoxicol.biomedcentral.comam2017Internal MedicinePharmacolog
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