350 research outputs found
Generalized Uncertainty Principle, Extra-dimensions and Holography
We consider Uncertainty Principles which take into account the role of
gravity and the possible existence of extra spatial dimensions. Explicit
expressions for such Generalized Uncertainty Principles in 4+n dimensions are
given and their holographic properties investigated. In particular, we show
that the predicted number of degrees of freedom enclosed in a given spatial
volume matches the holographic counting only for one of the available
generalizations and without extra dimensions.Comment: LaTeX, 13 pages, accepted for publication in Class. Quantum Gra
The Quantum Hall Effect of Interacting Electrons in a Periodic Potential
We consider the influence of an external periodic potential on the fractional
quantum Hall effect of two-dimensional interacting electron systems. For many
electrons on a torus, we find that the splitting of incompressible ground state
degeneracies by a weak external potential diminishes as at
large system size . We present numerical results consistent with a scenario
in which diverges at continuous phase transitions from fractional to
integer quantum Hall states which occur with increasing external potential
strength.Comment: 4 pages, REVTeX, 3 epsf-embedded color postscript figures, submitted
to PRB (Rapid Comm.), added reference in revised versio
Instabilities of Black Strings and Branes
We review recent progress on the instabilities of black strings and branes
both for pure Einstein gravity as well as supergravity theories which are
relevant for string theory. We focus mainly on Gregory-Laflamme instabilities.
In the first part of the review we provide a detailed discussion of the
classical gravitational instability of the neutral uniform black string in
higher dimensional gravity. The uniform black string is part of a larger phase
diagram of Kaluza-Klein black holes which will be discussed thoroughly. This
phase diagram exhibits many interesting features including new phases,
non-uniqueness and horizon-topology changing transitions. In the second part,
we turn to charged black branes in supergravity and show how the
Gregory-Laflamme instability of the neutral black string implies via a
boost/U-duality map similar instabilities for non- and near-extremal smeared
branes in string theory. We also comment on instabilities of D-brane bound
states. The connection between classical and thermodynamic stability, known as
the correlated stability conjecture, is also reviewed and illustrated with
examples. Finally, we examine the holographic implications of the
Gregory-Laflamme instability for a number of non-gravitational theories
including Yang-Mills theories and Little String Theory.Comment: 119 pages, 16 figures. Invited review for Classical and Quantum
Gravit
Compactification, topology change and surgery theory
We study the process of compactification as a topology change. It is shown
how the mediating spacetime topology, or cobordism, may be simplified through
surgery. Within the causal Lorentzian approach to quantum gravity, it is shown
that any topology change in dimensions may be achieved via a causally
continuous cobordism. This extends the known result for 4 dimensions.
Therefore, there is no selection rule for compactification at the level of
causal continuity. Theorems from surgery theory and handle theory are seen to
be very relevant for understanding topology change in higher dimensions.
Compactification via parallelisable cobordisms is particularly amenable to
study with these tools.Comment: 1+19 pages. LaTeX. 9 associated eps files. Discussion of disconnected
case adde
Analytic approximations, perturbation methods, and their applications
The paper summarizes the parallel session B3 {\em Analytic approximations,
perturbation methods, and their applications} of the GR18 conference. The talks
in the session reported notably recent advances in black hole perturbations and
post-Newtonian approximations as applied to sources of gravitational waves.Comment: Summary of the B3 parallel session of the GR18 conferenc
CHY representations for gauge theory and gravity amplitudes with up to three massive particles
We show that a wide class of tree-level scattering amplitudes involving
scalars, gauge bosons, and gravitons, up to three of which may be massive, can
be expressed in terms of a Cachazo-He-Yuan representation as a sum over
solutions of the scattering equations. These amplitudes, when expressed in
terms of the appropriate kinematic invariants, are independent of the masses
and therefore identical to the corresponding massless amplitudes.Comment: 20 pages, 1 figure; v2: minor typos corrected, published versio
Molecular Imaging of PD-L1 Expression and Dynamics with the Adnectin-Based PET Tracer F-18-BMS-986192
F-18-BMS-986192, an adnectin-based human programmed cell death ligand 1 (PD-L1) tracer, was developed to noninvasively determine whole-body PD-L1 expression by PET. We evaluated the usability of F-18-BMS-986192 PET to detect different PD-L1 expression levels and therapy-induced changes in PD-L1 expression in tumors. Methods: In vitro binding assays with F-18-BMS-986192 were performed on human tumor cell lines with different total cellular and membrane PD-L1 protein expression levels. Subsequently, PET imaging was performed on immunodeficient mice xenografted with these cell lines. The mice were treated with interferon gamma (IFN gamma) intraperitoneally for 3 d or with the mitogen-activated protein kinase kinase inhibitor selumetinib by oral gavage for 24 h. Afterward, F-18-BMS-986192 was administered intravenously, followed by a 60-min dynamic PET scan. Tracer uptake was expressed as percentage injected dose per gram of tissue. Tissues were collected to evaluate ex vivo tracer biodistribution and to perform flow cytometric, Western blot, and immunohistochemical tumor analyses. Results: F-18-BMS-986192 uptake reflected PD-L1 membrane levels in tumor cell lines, and tumor tracer uptake in mice was associated with PD-L1 expression measured immunohistochemically. In vitro IFN gamma treatment increased PD-L1 expression in the tumor cell lines and caused up to a 12-fold increase in tracer binding. In vivo, IFN gamma affected neither PD-L1 tumor expression measured immunohistochemically nor F-18-BMS-986192 tumor uptake. In vitro, selumetinib downregulated cellular and membrane levels of PD-L1 in tumor cells by 50% as measured by Western blotting and flow cytometry. In mice, selumetinib lowered cellular, but not membrane, PD-L1 levels of tumors, and consequently, no treatment-induced change in F-18-BMS-986192 tumor uptake was observed. Conclusion: F-18-BMS-986192 PET imaging allows detection of membrane-expressed PD-L1 as soon as 60 min after tracer injection. The tracer can discriminate a range of tumor cell PD-L1 membrane expression levels
Rotating Circular Strings, and Infinite Non-Uniqueness of Black Rings
We present new self-gravitating solutions in five dimensions that describe
circular strings, i.e., rings, electrically coupled to a two-form potential (as
e.g., fundamental strings do), or to a dual magnetic one-form. The rings are
prevented from collapsing by rotation, and they create a field analogous to a
dipole, with no net charge measured at infinity. They can have a regular
horizon, and we show that this implies the existence of an infinite number of
black rings, labeled by a continuous parameter, with the same mass and angular
momentum as neutral black rings and black holes. We also discuss the solution
for a rotating loop of fundamental string. We show how more general rings arise
from intersections of branes with a regular horizon (even at extremality),
closely related to the configurations that yield the four-dimensional black
hole with four charges. We reproduce the Bekenstein-Hawking entropy of a large
extremal ring through a microscopic calculation. Finally, we discuss some
qualitative ideas for a microscopic understanding of neutral and dipole black
rings.Comment: 31 pages, 7 figures. v2: minor changes, added reference. v3:
erroneous values of T_{ww} (eq.(3.39)) and n_p (eq.(5.20)) corrected, and
accompanying discussion amended. In the journal version these corrections
appear as an appended erratum. No major changes involve
The a-theorem and conformal symmetry breaking in holographic RG flows
We study holographic models describing an RG flow between two fixed points
driven by a relevant scalar operator. We show how to introduce a spurion field
to restore Weyl invariance and compute the anomalous contribution to the
generating functional in even dimensional theories. We find that the
coefficient of the anomalous term is proportional to the difference of the
conformal anomalies of the UV and IR fixed points, as expected from anomaly
matching arguments in field theory. For any even dimensions the coefficient is
positive as implied by the holographic a-theorem. For flows corresponding to
spontaneous breaking of conformal invariance, we also compute the two-point
functions of the energy-momentum tensor and the scalar operator and identify
the dilaton mode. Surprisingly we find that in the simplest models with just
one scalar field there is no dilaton pole in the two-point function of the
scalar operator but a stronger singularity. We discuss the possible
implications.Comment: 50 pages. v2: minor changes, added references, extended discussion.
v3: we have clarified some of the calculations and assumptions, results
unchanged. v4: published version in JHE
Equilibrium configurations of fluids and their stability in higher dimensions
We study equilibrium shapes, stability and possible bifurcation diagrams of
fluids in higher dimensions, held together by either surface tension or
self-gravity. We consider the equilibrium shape and stability problem of
self-gravitating spheroids, establishing the formalism to generalize the
MacLaurin sequence to higher dimensions. We show that such simple models, of
interest on their own, also provide accurate descriptions of their general
relativistic relatives with event horizons. The examples worked out here hint
at some model-independent dynamics, and thus at some universality: smooth
objects seem always to be well described by both ``replicas'' (either
self-gravity or surface tension). As an example, we exhibit an instability
afflicting self-gravitating (Newtonian) fluid cylinders. This instability is
the exact analogue, within Newtonian gravity, of the Gregory-Laflamme
instability in general relativity. Another example considered is a
self-gravitating Newtonian torus made of a homogeneous incompressible fluid. We
recover the features of the black ring in general relativity.Comment: 42 pages, 11 Figures, RevTeX4. Accepted for publication in Classical
and Quantum Gravity. v2: Minor corrections and references adde
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