113 research outputs found
The Local Velocity Anomaly
There is a velocity discontinuity at about 7 Mpc between the galaxies of the
Local Sheet that are moving together with low internal velocity dispersion and
the adjacent structures. The Local Sheet bounds the Local Void. The Local Sheet
is determined to have a peculiar velocity of 260 km/s away from the center of
the void. In order for this large velocity to be generated by an absence of
gravity, the Local Void must be at least 45 Mpc in diameter and be very empty.Comment: Invited review, "Galaxies in the Local Volume", Sydney, 8-13 July,
2007. eds. B. Koribalski & H. Jerjen, Astrophys. & Space Sci. Proceed. 10
pages with 7 figure
De Finetti theorem on the CAR algebra
The symmetric states on a quasi local C*-algebra on the infinite set of
indices J are those invariant under the action of the group of the permutations
moving only a finite, but arbitrary, number of elements of J. The celebrated De
Finetti Theorem describes the structure of the symmetric states (i.e.
exchangeable probability measures) in classical probability. In the present
paper we extend De Finetti Theorem to the case of the CAR algebra, that is for
physical systems describing Fermions. Namely, after showing that a symmetric
state is automatically even under the natural action of the parity
automorphism, we prove that the compact convex set of such states is a Choquet
simplex, whose extremal (i.e. ergodic w.r.t. the action of the group of
permutations previously described) are precisely the product states in the
sense of Araki-Moriya. In order to do that, we also prove some ergodic
properties naturally enjoyed by the symmetric states which have a
self--containing interest.Comment: 23 pages, juornal reference: Communications in Mathematical Physics,
to appea
Perturbative QCD and factorization of coherent pion photoproduction on the deuteron
We analyze the predictions of perturbative QCD for pion photoproduction on
the deuteron, gamma D -> pi^0 D, at large momentum transfer using the reduced
amplitude formalism. The cluster decomposition of the deuteron wave function at
small binding only allows the nuclear coherent process to proceed if each
nucleon absorbs an equal fraction of the overall momentum transfer.
Furthermore, each nucleon must scatter while remaining close to its mass shell.
Thus the nuclear photoproduction amplitude, M_{gamma D -> pi^0 D}(u,t),
factorizes as a product of three factors: (1) the nucleon photoproduction
amplitude, M_{gamma N_1 -> pi^0 N_1}(u/4,t/4), at half of the overall momentum
transfer, (2) a nucleon form factor, F_{N_2}(t/4), at half the overall momentum
transfer, and (3) the reduced deuteron form factor, f_d(t), which according to
perturbative QCD, has the same monopole falloff as a meson form factor. A
comparison with the recent JLAB data for gamma D -> pi^0 D of Meekins et al.
[Phys. Rev. C 60, 052201 (1999)] and the available gamma p -> pi^0 p data shows
good agreement between the perturbative QCD prediction and experiment over a
large range of momentum transfers and center of mass angles. The reduced
amplitude prediction is consistent with the constituent counting rule, p^11_T
M_{gamma D -> pi^0 D} -> F(theta_cm), at large momentum transfer. This is found
to be consistent with measurements for photon lab energies E_gamma > 3 GeV at
theta_cm=90 degrees and \elab > 10 GeV at 136 degrees.Comment: RevTeX 3.1, 17 pages, 6 figures; v2: incorporates minor changes as
version accepted by Phys Rev
Drag and jet quenching of heavy quarks in a strongly coupled N=2* plasma
The drag of a heavy quark and the jet quenching parameter are studied in the
strongly coupled N=2* plasma using the AdS/CFT correspondence. Both increase in
units of the spatial string tension as the theory departs from conformal
invariance. The description of heavy quark dynamics using a Langevin equation
is also considered. It is found that the difference between the velocity
dependent factors of the transverse and longitudinal momentum broadening of the
quark admit an interpretation in terms of relativistic effects, so the
distribution is spherical in the quark rest frame. When conformal invariance is
broken there is a broadening of the longitudinal momentum distribution. This
effect may be useful in understanding the jet distribution observed in
experiments.Comment: 30 pages, 5 figures, references added, minor corrections. To be
published in JHE
Comments on D-brane Interactions in PP-wave Backgrounds
We calculate the interaction potential between widely separated D-branes in
PP-wave backgrounds in string theory as well as in low-energy supergravity.
Timelike and spacelike orientations are qualitatively different but in both
cases the effective brane tensions and RR charges take the same values as in
Minkowski space in accordance with the expectations from the sigma model
perturbation theory.Comment: Latex, 22 pages. Typos corrected and a reference added, final versio
Cosmological evolution of interacting dark energy in Lorentz violation
The cosmological evolution of an interacting scalar field model in which the
scalar field interacts with dark matter, radiation, and baryon via Lorentz
violation is investigated. We propose a model of interaction through the
effective coupling . Using dynamical system analysis, we study the
linear dynamics of an interacting model and show that the dynamics of critical
points are completely controlled by two parameters. Some results can be
mentioned as follows. Firstly, the sequence of radiation, the dark matter, and
the scalar field dark energy exist and baryons are sub dominant. Secondly, the
model also allows the possibility of having a universe in the phantom phase
with constant potential. Thirdly, the effective gravitational constant varies
with respect to time through . In particular, we consider a simple
case where has a quadratic form and has a good agreement with the
modified CDM and quintessence models. Finally, we also calculate the
first post--Newtonian parameters for our model.Comment: 14 pages, published versio
Young and Intermediate-age Distance Indicators
Distance measurements beyond geometrical and semi-geometrical methods, rely
mainly on standard candles. As the name suggests, these objects have known
luminosities by virtue of their intrinsic proprieties and play a major role in
our understanding of modern cosmology. The main caveats associated with
standard candles are their absolute calibration, contamination of the sample
from other sources and systematic uncertainties. The absolute calibration
mainly depends on their chemical composition and age. To understand the impact
of these effects on the distance scale, it is essential to develop methods
based on different sample of standard candles. Here we review the fundamental
properties of young and intermediate-age distance indicators such as Cepheids,
Mira variables and Red Clump stars and the recent developments in their
application as distance indicators.Comment: Review article, 63 pages (28 figures), Accepted for publication in
Space Science Reviews (Chapter 3 of a special collection resulting from the
May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space
Age
Screening of cosmological constant for De Sitter Universe in non-local gravity, phantom-divide crossing and finite-time future singularities
We investigate de Sitter solutions in non-local gravity as well as in
non-local gravity with Lagrange constraint multiplier. We examine a condition
to avoid a ghost and discuss a screening scenario for a cosmological constant
in de Sitter solutions. Furthermore, we explicitly demonstrate that three types
of the finite-time future singularities can occur in non-local gravity and
explore their properties. In addition, we evaluate the effective equation of
state for the universe and show that the late-time accelerating universe may be
effectively the quintessence, cosmological constant or phantom-like phases. In
particular, it is found that there is a case in which a crossing of the phantom
divide from the non-phantom (quintessence) phase to the phantom one can be
realized when a finite-time future singularity occurs. Moreover, it is
demonstrated that the addition of an term can cure the finite-time future
singularities in non-local gravity. It is also suggested that in the framework
of non-local gravity, adding an term leads to possible unification of the
early-time inflation with the late-time cosmic acceleration.Comment: 42 pages, no figure, version accepted for publication in General
Relativity and Gravitatio
Categorizing Different Approaches to the Cosmological Constant Problem
We have found that proposals addressing the old cosmological constant problem
come in various categories. The aim of this paper is to identify as many
different, credible mechanisms as possible and to provide them with a code for
future reference. We find that they all can be classified into five different
schemes of which we indicate the advantages and drawbacks.
Besides, we add a new approach based on a symmetry principle mapping real to
imaginary spacetime.Comment: updated version, accepted for publicatio
EXTL3 mutations cause skeletal dysplasia, immune deficiency, and developmental delay.
We studied three patients with severe skeletal dysplasia, T cell immunodeficiency, and developmental delay. Whole-exome sequencing revealed homozygous missense mutations affecting exostosin-like 3 (EXTL3), a glycosyltransferase involved in heparan sulfate (HS) biosynthesis. Patient-derived fibroblasts showed abnormal HS composition and altered fibroblast growth factor 2 signaling, which was rescued by overexpression of wild-type EXTL3 cDNA. Interleukin-2-mediated STAT5 phosphorylation in patients' lymphocytes was markedly reduced. Interbreeding of the extl3-mutant zebrafish (box) with Tg(rag2:green fluorescent protein) transgenic zebrafish revealed defective thymopoiesis, which was rescued by injection of wild-type human EXTL3 RNA. Targeted differentiation of patient-derived induced pluripotent stem cells showed a reduced expansion of lymphohematopoietic progenitor cells and defects of thymic epithelial progenitor cell differentiation. These data identify EXTL3 mutations as a novel cause of severe immune deficiency with skeletal dysplasia and developmental delay and underline a crucial role of HS in thymopoiesis and skeletal and brain development
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