3,201 research outputs found
Magnification relations for Kerr lensing and testing Cosmic Censorship
A Kerr black hole with mass parameter m and angular momentum parameter a
acting as a gravitational lens gives rise to two images in the weak field
limit. We study the corresponding magnification relations, namely the signed
and absolute magnification sums and the centroid up to post-Newtonian order. We
show that there are post-Newtonian corrections to the total absolute
magnification and centroid proportional to a/m, which is in contrast to the
spherically symmetric case where such corrections vanish. Hence we also propose
a new set of lensing observables for the two images involving these
corrections, which should allow measuring a/m with gravitational lensing. In
fact, the resolution capabilities needed to observe this for the Galactic black
hole should in principle be accessible to current and near-future
instrumentation. Since a/m >1 indicates a naked singularity, a most interesting
application would be a test of the Cosmic Censorship conjecture. The technique
used to derive the image properties is based on the degeneracy of the Kerr lens
and a suitably displaced Schwarzschild lens at post-Newtonian order. A simple
physical explanation for this degeneracy is also given.Comment: 13 pages, version 2: references added, minor changes. To appear in
Phys. Rev.
The Dirac system on the Anti-de Sitter Universe
We investigate the global solutions of the Dirac equation on the
Anti-de-Sitter Universe. Since this space is not globally hyperbolic, the
Cauchy problem is not, {\it a priori}, well-posed. Nevertheless we can prove
that there exists unitary dynamics, but its uniqueness crucially depends on the
ratio beween the mass of the field and the cosmological constant
: it appears a critical value, , which plays a role
similar to the Breitenlohner-Freedman bound for the scalar fields. When
there exists a unique unitary dynamics. In opposite, for
the light fermions satisfying , we construct several asymptotic
conditions at infinity, such that the problem becomes well-posed. In all the
cases, the spectrum of the hamiltonian is discrete. We also prove a result of
equipartition of the energy.Comment: 33 page
On paraquaternionic submersions between paraquaternionic K\"ahler manifolds
In this paper we deal with some properties of a class of semi-Riemannian
submersions between manifolds endowed with paraquaternionic structures, proving
a result of non-existence of paraquaternionic submersions between
paraquaternionic K\"ahler non locally hyper paraK\"ahler manifolds. Then we
examine, as an example, the canonical projection of the tangent bundle, endowed
with the Sasaki metric, of an almost paraquaternionic Hermitian manifold.Comment: 13 pages, no figure
Halo properties and secular evolution in barred galaxies
The halo plays a crucial role in the evolution of barred galaxies. Its
near-resonant material absorbs angular momentum emitted from some of the disc
particles and helps the bar become stronger. As a result, a bar (oval) forms in
the inner parts of the halo of strongly barred disc galaxies. It is thinner in
the inner parts (but still considerably fatter than the disc bar) and tends to
spherical at larger radii. Its length increases with time, while always staying
shorter than the disc bar. It is roughly aligned with the disc bar, which it
trails only slightly, and it turns with roughly the same pattern speed. The
bi-symmetric component of the halo density continues well outside the halo bar,
where it clearly trails behind the disc bar. The length and strength of the
disc and halo bars correlate; the former being always much stronger than the
latter. If the halo is composed of weakly interacting massive particles, then
the formation of the halo bar, by redistributing the matter in the halo and
changing its shape, could influence the expected annihilation signal. This is
indeed found to be the case if the halo has a core, but not if it has a steep
cusp. The formation and evolution of the bar strongly affect the halo orbits. A
fraction of them becomes near-resonant, similar to the disc near-resonant
orbits at the same resonance, while another fraction becomes chaotic. Finally,
a massive and responsive halo makes it harder for a central mass concentration
to destroy the disc bar.Comment: 6 pages, 3 figures, to appear in "Island Universes - Structure and
Evolution of Disk Galaxies" ed. R. S. de Jon
Mapping evaporative water loss in desert passerines reveals an expanding threat of lethal dehydration
Extreme high environmental temperatures produce a variety of consequences for wildlife, including mass die-offs. Heat waves are increasing in frequency, intensity, and extent, and are projected to increase further under climate change. However, the spatial and temporal dynamics of die-off risk are poorly understood. Here, we examine the effects of heat waves on evaporative water loss (EWL) and survival in five desert passerine birds across the southwestern United States using a combination of physiological data, mechanistically informed models, and hourly geospatial temperature data. We ask how rates of EWL vary with temperature across species; how frequently, over what areas, and how rapidly lethal dehydration occurs; how EWL and die-off risk vary with body mass; and how die-off risk is affected by climate warming. We find that smaller-bodied passerines are subject to higher rates of mass-specific EWL than larger-bodied counterparts and thus encounter potentially lethal conditions much more frequently, over shorter daily intervals, and over larger geographic areas. Warming by 4 °C greatly expands the extent, frequency, and intensity of dehydration risk, and introduces new threats for larger passerine birds, particularly those with limited geographic ranges. Our models reveal that increasing air temperatures and heat wave occurrence will potentially have important impacts on the water balance, daily activity, and geographic distribution of arid-zone birds. Impacts may be exacerbated by chronic effects and interactions with other environmental changes. This work underscores the importance of acute risks of high temperatures, particularly for small-bodied species, and suggests conservation of thermal refugia and water sources
An argument for the use of Aristotelian method in bioethics
The main claim of this paper is that the method outlined and used in Aristotle's Ethics is an appropriate and credible one to use in bioethics. Here “appropriate” means that the method is capable of establishing claims and developing concepts in bioethics and “credible” that the method has some plausibility, it is not open to obvious and immediate objection. It begins by suggesting why this claim matters and then gives a brief outline of Aristotle's method. The main argument is made in three stages. First, it is argued that Aristotelian method is credible because it compares favourably with alternatives. In this section it is shown that Aristotelian method is not vulnerable to criticisms that are made both of methods that give a primary place to moral theory (such as utilitarianism) and those that eschew moral theory (such as casuistry and social science approaches). As such, it compares favourably with these other approaches that are vulnerable to at least some of these criticisms. Second, the appropriateness of Aristotelian method is indicated through outlining how it would deal with a particular case. Finally, it is argued that the success of Aristotle's philosophy is suggestive of both the credibility and appropriateness of his method.</p
The scaling of X-ray variability with luminosity in Ultra-luminous X-ray sources
We investigated the relationship between the X-ray variability amplitude and
X-ray luminosity for a sample of 14 bright Ultra-luminous X-ray sources (ULXs)
with XMM-Newton/EPIC data, and compare it with the well established similar
relationship for Active Galactic Nuclei (AGN). We computed the normalised
excess variance in the 2-10 keV light curves of these objects and their 2-10
keV band intrinsic luminosity. We also determined model
"variability-luminosity" relationships for AGN, under several assumptions
regarding their power-spectral shape. We compared these model predictions at
low luminosities with the ULX data. The variability amplitude of the ULXs is
significantly smaller than that expected from a simple extrapolation of the AGN
"variability-luminosity" relationship at low luminosities. We also find
evidence for an anti-correlation between the variability amplitude and L(2-10
keV) for ULXs. The shape of this relationship is consistent with the AGN data
but only if the ULXs data are shifted by four orders of magnitudes in
luminosity. Most (but not all) of the ULXs could be "scaled-down" version of
AGN if we assume that: i) their black hole mass and accretion rate are of the
order of ~(2.5-30)x 10E+03 Msolar and ~ 1-80 % of the Eddington limit, and ii)
their Power Spectral Density has a doubly broken power-law shape. This PDS
shape and accretion rate is consistent with Galactic black hole systems
operating in their so-called "low-hard" and "very-high" states.Comment: 10 pages, 5 figures, 2 tables, accepted for publication in A&
Using the stated preference method for the calculation of social discount rate
The aim of this paper is to build the stated preference method into the social discount rate methodology. The first part of the paper presents the results of a survey about stated time preferences through pair-choice decision situations for various topics and time horizons. It is assumed that stated time preferences differ from calculated time preferences and that the extent of stated rates depends on the time period, and on how much respondents are financially and emotionally involved in the transactions. A significant question remains: how can the gap between the calculation and the results of surveys be resolved, and how can the real time preferences of individuals be interpreted using a social time preference rate. The second part of the paper estimates the social time preference rate for Hungary using the results of the survey, while paying special attention to the pure time preference component. The results suggest that the current method of calculation of the pure time preference rate does not reflect the real attitudes of individuals towards future generations
Chris Brown’s Liberal Conservatism, the Process of Moral Learning and Global Institutional Transformations
In the normative theory of International Relations, the major debate has been between the state-centric view and cosmopolitanism. A quarter of century ago I attempted to deconstruct one round of this debate – between Chris Brown and Mark Hoffman – by showing that the debate is both utopian and has an other-negating character. With the help of epistemological discussions and an analysis of two “case studies”, I built a case for a contextualist and dialogical morality, grounded on the concept of judgement. Interestingly, Brown’s 2010 collection of essays is entitled Practical Judgement in International Political Theory. In this paper I first explore whether similar or analogical deconstruction would anymore be possible. Second, I discuss Brown’s ideas about global civil society, democracy and justice, particularly in light of world-historical developments since the early 1990s and in relation to the development of my own thinking on the topic. While Brown has tried to overcome the dichotomy between the state-centric view and cosmopolitanism, I examine whether the idea of universal ethico-political learning and its cosmopolitan implications might explain the divergence in our practical judgements. I conclude by arguing any area of activities in international relations and world society, from property and contract to nuclear safety and global warming, can be subject to normative debates and potentially democratic politics. Good normative arguments often involve designs for better institutions, but they must be realizable by virtue of being connected to real causal processes.Peer reviewe
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