270 research outputs found
On the Possibility of Quantum Gravity Effects at Astrophysical Scales
The nonperturbative renormalization group flow of Quantum Einstein Gravity
(QEG) is reviewed. It is argued that at large distances there could be strong
renormalization effects, including a scale dependence of Newton's constant,
which mimic the presence of dark matter at galactic and cosmological scales.Comment: LaTeX, 18 pages, 4 figures. Invited contribution to the Int. J. Mod.
Phys. D special issue on dark matter and dark energ
Indication, from Pioneer 10/11, Galileo, and Ulysses Data, of an Apparent Anomalous, Weak, Long-Range Acceleration
Radio metric data from the Pioneer 10/11, Galileo, and Ulysses spacecraft
indicate an apparent anomalous, constant, acceleration acting on the spacecraft
with a magnitude cm/s, directed towards the Sun.
Two independent codes and physical strategies have been used to analyze the
data. A number of potential causes have been ruled out. We discuss future
kinematic tests and possible origins of the signal.Comment: Revtex, 4 pages and 1 figure. Minor changes for publicatio
Substructure lensing in galaxy clusters as a constraint on low-mass sterile neutrinos in tensor-vector-scalar theory: The straight arc of Abell 2390
Certain covariant theories of the modified Newtonian dynamics paradigm seem
to require an additional hot dark matter (HDM) component - in the form of
either heavy ordinary neutrinos or more recently light sterile neutrinos (SNs)
with a mass around 11eV - to be relieved of problems ranging from cosmological
scales down to intermediate ones relevant for galaxy clusters. Here we suggest
using gravitational lensing by galaxy clusters to test such a marriage of
neutrino HDM and modified gravity, adopting the framework of
tensor-vector-scalar theory (TeVeS). Unlike conventional cold dark matter
(CDM), such HDM is subject to strong phase-space constraints, which allows one
to check cluster lens models inferred within the modified framework for
consistency. Since the considered HDM particles cannot collapse into
arbitrarily dense clumps and only form structures well above the galactic
scale, systems which indicate the need for dark substructure are of particular
interest. As a first example, we study the cluster lens Abell 2390 and its
impressive straight arc with the help of numerical simulations. Based on our
results, we outline a general and systematic approach to model cluster lenses
in TeVeS which significantly reduces the calculation complexity. We further
consider a simple bimodal lens configuration, capable of producing the straight
arc, to demonstrate our approach. We find that such a model is marginally
consistent with the hypothesis of 11eV SNs. Future work including more detailed
and realistic lens models may further constrain the necessary SN distribution
and help to conclusively assess this point. Cluster lenses could therefore
provide an interesting discriminator between CDM and such modified gravity
scenarios supplemented by SNs or other choices of HDM.Comment: 22 pages, 14 figures, 2 tables; minor changes to match accepted
versio
The nucleus of the Sagittarius dSph galaxy and M54: a window on the process of galaxy nucleation
We present the results of a thorough study of the nucleus of the Sgr dwarf
spheroidal galaxy and of the bright globular cluster M54 that resides within
the same nucleus (Sgr,N). We have obtained accurate radial velocities and
metallicity estimates for 1152 candidate Red Giant Branch stars of Sgr and M54
lying within ~ 9 arcmin from the center of the galaxy, from Keck/DEIMOS and
VLT/FLAMES spectra of the infrared Calcium II triplet. Using both velocity and
metallicity information we selected two samples of 425 and 321 very-likely
members of M54 and of Sgr,N, respectively. The two considered systems display
significantly different velocity dispersion profiles: M54 has a steeply
decreasing profile from r=0, where sigma= 14.2 km/s, to r=3.5 arcmin where it
reaches sigma=5.3 km/s, then it appears to rise again to sigma= 10 km/s at r=7
arcmin. In contrast Sgr,N has a uniformly flat profile at sigma=9.6 km/s over
the whole 0 < r < 9 arcmin range. Using data from the literature we show that
the velocity dispersion of Sgr remains constant at least out to r ~ 100 arcmin
and there is no sign of the transition between the outer
flat-luminosity-profile core and the inner nucleus in the velocity profile.
These results - together with a re-analysis of the surface brightness profile
of Sgr,N and a suite of dedicated N-body simulations - provide very strong
support for the hypothesis that the nucleus of Sgr formed independently of M54,
which probably plunged to its present position, coincident with Sgr,N, because
of significant decay of the original orbit due to dynamical friction.Comment: Accepted for publication by the Astronomical Journal. emulateapj.cls,
26 pag., 21 low resolution figures. A full-resolution color version of the
paper can be retrieved from http://www.bo.astro.it/SGR/Sgr_nucleus.ps.g
Maximum Likelihood Analysis of Clusters of Ultra-High Energy Cosmic Rays
We present a numerical code designed to conduct a likelihood analysis for
clusters of nucleons above 10**19 eV originating from discrete astrophysical
sources such as powerful radio galaxies, gamma-ray bursts or topological
defects. The code simulates the propagation of nucleons in a large-scale
magnetic field and constructs the likelihood of a given observed event cluster
as a function of the average time delay due to deflection in the magnetic
field, the source activity time scale, the total fluence of the source, and the
power law index of the particle injection spectrum. Other parameters such as
the coherence length and the power spectrum of the magnetic field are also
considered. We apply it to the three pairs of events above 4X10**19 eV recently
reported by the Akeno Giant Air Shower Array (AGASA) experiment, assuming that
these pairs were caused by nucleon primaries which originated from a common
source. Although current data are too sparse to fully constrain each of the
parameters considered, and/or to discriminate models of the origin of
ultra-high energy cosmic rays, several tendencies are indicated. If the
clustering suggested by AGASA is real, next generation experiments with their
increased exposure should detect more than 10 particles per source over a few
years and our method will put strong constraints on both the large-scale
magnetic field parameters and the nature of these sources.Comment: 11 latex pages, 8 postscript figures included, uses revtex.sty in
two-column format and epsf.sty. Submitted to Physical Review
Driving vascular endothelial cell fate of human multipotent Isl1+ heart progenitors with VEGF modified mRNA
Distinct families of multipotent heart progenitors play a central role in the generation of diverse cardiac, smooth muscle and endothelial cell lineages during mammalian cardiogenesis. The identification of precise paracrine signals that drive the cell-fate decision of these multipotent progenitors, and the development of novel approaches to deliver these signals in vivo, are critical steps towards unlocking their regenerative therapeutic potential. Herein, we have identified a family of human cardiac endothelial intermediates located in outflow tract of the early human fetal hearts (OFT-ECs), characterized by coexpression of Isl1 and CD144/vWF. By comparing angiocrine factors expressed by the human OFT-ECs and non-cardiac ECs, vascular endothelial growth factor (VEGF)-A was identified as the most abundantly expressed factor, and clonal assays documented its ability to drive endothelial specification of human embryonic stem cell (ESC)-derived Isl1+ progenitors in a VEGF receptor-dependent manner. Human Isl1-ECs (endothelial cells differentiated from hESC-derived ISL1+ progenitors) resemble OFT-ECs in terms of expression of the cardiac endothelial progenitor- and endocardial cell-specific genes, confirming their organ specificity. To determine whether VEGF-A might serve as an in vivo cell-fate switch for human ESC-derived Isl1-ECs, we established a novel approach using chemically modified mRNA as a platform for transient, yet highly efficient expression of paracrine factors in cardiovascular progenitors. Overexpression of VEGF-A promotes not only the endothelial specification but also engraftment, proliferation and survival (reduced apoptosis) of the human Isl1+ progenitors in vivo. The large-scale derivation of cardiac-specific human Isl1-ECs from human pluripotent stem cells, coupled with the ability to drive endothelial specification, engraftment, and survival following transplantation, suggest a novel strategy for vascular regeneration in the heart
Genetic randomization reveals functional relationships among morphologic and tissue-quality traits that contribute to bone strength and fragility
We examined femora from adult AXB/BXA recombinant inbred (RI) mouse strains to identify skeletal traits that are functionally related and to determine how functional interactions among these traits contribute to genetic variability in whole-bone stiffness, strength, and toughness. Randomization of A/J and C57BL/6J genomic regions resulted in each adult male and female RI strain building mechanically functional femora by assembling unique sets of morphologic and tissue-quality traits. A correlation analysis was conducted using the mean trait values for each RI strain. A third of the 66 correlations examined were significant, indicating that many bone traits covaried or were functionally related. Path analysis revealed important functional interactions among bone slenderness, cortical thickness, and tissue mineral density. The path coefficients describing these functional relations were similar for both sexes. The causal relationship among these three traits suggested that cellular processes during growth simultaneously regulate bone slenderness, cortical thickness, and tissue mineral density so that the combination of traits is sufficiently stiff and strong to satisfy daily loading demands. A disadvantage of these functional interactions was that increases in tissue mineral density also deleteriously affected tissue ductility. Consequently, slender bones with high mineral density may be stiff and strong but they are also brittle. Thus, genetically randomized mouse strains revealed a basic biological paradigm that allows for flexibility in building bones that are functional for daily activities but that creates preferred sets of traits under extreme loading conditions. Genetic or environmental perturbations that alter these functional interactions during growth would be expected to lead to loss of function and suboptimal adult bone quality
Physicians' social competence in the provision of care to persons living in poverty: research protocol
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