1,502 research outputs found
Effective stress-energy tensors, self-force, and broken symmetry
Deriving the motion of a compact mass or charge can be complicated by the
presence of large self-fields. Simplifications are known to arise when these
fields are split into two parts in the so-called Detweiler-Whiting
decomposition. One component satisfies vacuum field equations, while the other
does not. The force and torque exerted by the (often ignored) inhomogeneous
"S-type" portion is analyzed here for extended scalar charges in curved
spacetimes. If the geometry is sufficiently smooth, it is found to introduce
effective shifts in all multipole moments of the body's stress-energy tensor.
This greatly expands the validity of statements that the homogeneous R field
determines the self-force and self-torque up to renormalization effects. The
forces and torques exerted by the S field directly measure the degree to which
a spacetime fails to admit Killing vectors inside the body. A number of
mathematical results related to the use of generalized Killing fields are
therefore derived, and may be of wider interest. As an example of their
application, the effective shift in the quadrupole moment of a charge's
stress-energy tensor is explicitly computed to lowest nontrivial order.Comment: 22 pages, fixed typos and simplified discussio
Mechanics of extended masses in general relativity
The "external" or "bulk" motion of extended bodies is studied in general
relativity. Compact material objects of essentially arbitrary shape, spin,
internal composition, and velocity are allowed as long as there is no direct
(non-gravitational) contact with other sources of stress-energy. Physically
reasonable linear and angular momenta are proposed for such bodies and exact
equations describing their evolution are derived. Changes in the momenta depend
on a certain "effective metric" that is closely related to a non-perturbative
generalization of the Detweiler-Whiting R-field originally introduced in the
self-force literature. If the effective metric inside a self-gravitating body
can be adequately approximated by an appropriate power series, the
instantaneous gravitational force and torque exerted on it is shown to be
identical to the force and torque exerted on an appropriate test body moving in
the effective metric. This result holds to all multipole orders. The only
instantaneous effect of a body's self-field is to finitely renormalize the
"bare" multipole moments of its stress-energy tensor. The MiSaTaQuWa expression
for the gravitational self-force is recovered as a simple application. A
gravitational self-torque is obtained as well. Lastly, it is shown that the
effective metric in which objects appear to move is approximately a solution to
the vacuum Einstein equation if the physical metric is an approximate solution
to Einstein's equation linearized about a vacuum background.Comment: 39 pages, 2 figures; fixed equation satisfied by the Green function
used to construct the effective metri
Self-forces from generalized Killing fields
A non-perturbative formalism is developed that simplifies the understanding
of self-forces and self-torques acting on extended scalar charges in curved
spacetimes. Laws of motion are locally derived using momenta generated by a set
of generalized Killing fields. Self-interactions that may be interpreted as
arising from the details of a body's internal structure are shown to have very
simple geometric and physical interpretations. Certain modifications to the
usual definition for a center-of-mass are identified that significantly
simplify the motions of charges with strong self-fields. A derivation is also
provided for a generalized form of the Detweiler-Whiting axiom that pointlike
charges should react only to the so-called regular component of their
self-field. Standard results are shown to be recovered for sufficiently small
charge distributions.Comment: 21 page
Multipole structure of current vectors in curved spacetime
A method is presented which allows the exact construction of conserved (i.e.
divergence-free) current vectors from appropriate sets of multipole moments.
Physically, such objects may be taken to represent the flux of particles or
electric charge inside some classical extended body. Several applications are
discussed. In particular, it is shown how to easily write down the class of all
smooth and spatially-bounded currents with a given total charge. This
implicitly provides restrictions on the moments arising from the smoothness of
physically reasonable vector fields. We also show that requiring all of the
moments to be constant in an appropriate sense is often impossible; likely
limiting the applicability of the Ehlers-Rudolph-Dixon notion of quasirigid
motion. A simple condition is also derived that allows currents to exist in two
different spacetimes with identical sets of multipole moments (in a natural
sense).Comment: 13 pages, minor changes, accepted to J. Math. Phy
Electromagnetic self-forces and generalized Killing fields
Building upon previous results in scalar field theory, a formalism is
developed that uses generalized Killing fields to understand the behavior of
extended charges interacting with their own electromagnetic fields. New notions
of effective linear and angular momenta are identified, and their evolution
equations are derived exactly in arbitrary (but fixed) curved spacetimes. A
slightly modified form of the Detweiler-Whiting axiom that a charge's motion
should only be influenced by the so-called "regular" component of its
self-field is shown to follow very easily. It is exact in some interesting
cases, and approximate in most others. Explicit equations describing the
center-of-mass motion, spin angular momentum, and changes in mass of a small
charge are also derived in a particular limit. The chosen approximations --
although standard -- incorporate dipole and spin forces that do not appear in
the traditional Abraham-Lorentz-Dirac or Dewitt-Brehme equations. They have,
however, been previously identified in the test body limit.Comment: 20 pages, minor typos correcte
Effects of exenatide and liraglutide on heart rate, blood pressure and body weight : systematic review and meta-analysis
Objectives: To synthesise current evidence for the effects of exenatide and liraglutide on heart rate, blood pressure and body weight.
Design: Meta-analysis of available data from randomised controlled trials comparing Glucagon-like peptide-1 (GLP-1) analogues with placebo, active antidiabetic drug therapy or lifestyle intervention.
Participants: Patients with type 2 diabetes.
Outcome measures: Weighted mean differences between trial arms for changes in heart rate, blood pressure and body weight, after a minimum of 12-week follow-up.
Results: 32 trials were included. Overall, GLP-1 agonists increased the heart rate by 1.86 beats/min (bpm) (95% CI 0.85 to 2.87) versus placebo and 1.90 bpm (1.30 to 2.50) versus active control. This effect was more evident for liraglutide and exenatide long-acting release than for exenatide twice daily. GLP-1 agonists decreased systolic blood pressure by −1.79 mm Hg (−2.94 to −0.64) and −2.39 mm Hg (−3.35 to −1.42) compared to placebo and active control, respectively. Reduction in diastolic blood pressure failed to reach statistical significance (−0.54 mm Hg (−1.15 to 0.07) vs placebo and −0.50 mm Hg (−1.24 to 0.24) vs active control). Body weight decreased by −3.31 kg (−4.05 to −2.57) compared to active control, but by only −1.22 kg (−1.51 to −0.93) compared to placebo.
Conclusions: GLP-1 analogues are associated with a small increase in heart rate and modest reductions in body weight and blood pressure. Mechanisms underlying the rise in heart rate require further investigation
The World of UCL
From its foundation in 1826, UCL embraced a progressive and pioneering spirit. It was the first university in England to admit students regardless of religion and made higher education affordable and accessible to a much broader section of society. It was also effectively the first university to welcome women on equal terms with men. From the outset UCL showed a commitment to innovative ideas and new methods of teaching and research.
This book charts the history of UCL from 1826 through to the present day, highlighting its many contributions to society in Britain and around the world. It covers the expansion of the university through the growth in student numbers and institutional mergers. It documents shifts in governance throughout the years and the changing social and economic context in which UCL operated, including challenging periods of reconstruction after two World Wars.
Today UCL is one of the powerhouses of research and teaching, and a truly global university. It is currently seventh in the QS World University Rankings. This completely revised and updated edition features a new chapter based on interviews with key individuals at UCL. It comes at a time of ambitious development for UCL with the establishment of an entirely new campus in East London, UCL East, and Provost Michael Arthur’s ‘UCL 2034’ strategy which aims to secure the university’s long-term future and commits UCL to delivering global impact
On the multifractal statistics of the local order parameter at random critical points : application to wetting transitions with disorder
Disordered systems present multifractal properties at criticality. In
particular, as discovered by Ludwig (A.W.W. Ludwig, Nucl. Phys. B 330, 639
(1990)) on the case of diluted two-dimensional Potts model, the moments
of the local order parameter scale with a set
of non-trivial exponents . In this paper, we revisit
these ideas to incorporate more recent findings: (i) whenever a multifractal
measure normalized over space occurs in a random
system, it is crucial to distinguish between the typical values and the
disorder averaged values of the generalized moments , since
they may scale with different generalized dimensions and
(ii) as discovered by Wiseman and Domany (S. Wiseman and E. Domany, Phys Rev E
{\bf 52}, 3469 (1995)), the presence of an infinite correlation length induces
a lack of self-averaging at critical points for thermodynamic observables, in
particular for the order parameter. After this general discussion valid for any
random critical point, we apply these ideas to random polymer models that can
be studied numerically for large sizes and good statistics over the samples. We
study the bidimensional wetting or the Poland-Scheraga DNA model with loop
exponent (marginal disorder) and (relevant disorder). Finally,
we argue that the presence of finite Griffiths ordered clusters at criticality
determines the asymptotic value and the minimal value of the typical multifractal spectrum
.Comment: 17 pages, 20 figure
Insulin mediated upregulation of the renin angiotensin system in human subcutaneous adipocytes is reduced by Rosiglitazone
Background: Obesity associated hypertension is likely to be due to multiple mechanisms. Identification of the renin-angiotensin system (RAS) within adipose tissue does, however, suggest a potential causal role for it in obesity-associated hypertension. Obese patients are often hyperinsulinaemic, but mechanisms underlying insulin upregulation of the RAS in adipose tissue are unclear. TNFα, an inducer of angiotensinogen in hepatocytes, is elevated in hyperinsulinaemic, obese individuals, and may provide a link in mediating insulin upregulation of the RAS in adipose tissue. Further, thiazolidinediones lower blood pressure in vivo and downregulation of the RAS in adipose tissue may contribute to this effect. We therefore examined the effect of rosiglitazone (RSG), on the insulin mediated upregulation of the RAS.
Methods and Results: Sera were obtained from the arterial circulation and from venous blood draining subcutaneous abdominal adipose tissue. Isolated human abdominal subcutaneous adipocytes (n=12) were treated with insulin (1-1000nM) and insulin in combination with RSG (10nM), and RSG (10nM) alone to determine angiotensinogen expression, angiotensin II, bradykinin and TNFα secretion. Subcutaneous adipocytes were also treated with TNFα (10-100ng/mL) to examine the direct effect on angiotensinogen expression and angiotensin II secretion. The findings showed that the arterio-venous difference in angiotensin II levels was significant (↑23%; p<0.001). Insulin increased TNFα secretion in a concentration-dependent manner (p<0.01) whilst RSG (10nM) significantly reduced the insulin mediated rise in TNFα (p<0.001), as well as AGT and angiotensin II. TNFα also increased angiotensinogen and angiotensin II in isolated adipocytes.
Conclusions: Our in vivo data suggest that human subcutaneous adipose tissue is a significant source of angiotensin II. This study also demonstrates a potential TNFα mediated
mechanism through which insulin may stimulate the RAS and may contribute to explain obesity associated hypertension. RSG downregulates the RAS in subcutaneous adipose tissue and this effect may contribute to the long-term effect of RSG on blood pressure
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