3,842 research outputs found
The Cosmic Censor Forbids Naked Topology
For any asymptotically flat spacetime with a suitable causal structure
obeying (a weak form of) Penrose's cosmic censorship conjecture and satisfying
conditions guaranteeing focusing of complete null geodesics, we prove that
active topological censorship holds. We do not assume global hyperbolicity, and
therefore make no use of Cauchy surfaces and their topology. Instead, we
replace this with two underlying assumptions concerning the causal structure:
that no compact set can signal to arbitrarily small neighbourhoods of spatial
infinity (``-avoidance''), and that no future incomplete null geodesic is
visible from future null infinity. We show that these and the focusing
condition together imply that the domain of outer communications is simply
connected. Furthermore, we prove lemmas which have as a consequence that if a
future incomplete null geodesic were visible from infinity, then given our
-avoidance assumption, it would also be visible from points of spacetime
that can communicate with infinity, and so would signify a true naked
singularity.Comment: To appear in CQG, this improved version contains minor revisions to
incorporate referee's suggestions. Two revised references. Plain TeX, 12
page
On the topology of stationary black holes
We prove that the domain of outer communication of a stationary, globally
hyperbolic spacetime satisfying the null energy condition must be simply
connected. Under suitable additional hypotheses, this implies, in particular,
that each connected component of a cross-section of the event horizon of a
stationary black hole must have spherical topology.Comment: 7 pages, Late
Mitigating effect of organic matter on the in vivo toxicity of metal oxide nanoparticles in the marine environment
This is the final version of the article. Available from the Royal Society of Chemistry via the DOI in this recordMajor constituents of seawater, i.e. ions and natural organic matter (NOM), can influence the environmental and toxicological behaviour of nanoparticles (NPs) in aquatic systems. By adsorbing-ligating-reacting to NP surface reactive sites, they can modify the NP surface structure and overall physico-chemical proprieties. This study explored the fate and in vivo toxicity of ZnO and MnO2NPs under artificial seawater conditions. These two nanomaterials are representative of metal oxide NPs inducing harm via dissolution and bandgap mechanisms, respectively. To gain a comprehensive understanding of the overall toxicological outcome, we traced the behaviour of NPs in the test systems (i.e. aggregation, sedimentation, dissolution, sorption), their fate in the model organism (i.e. ingestion and cellular internalization by oyster larvae), and the induction of a toxicological pathway (i.e. oxidative stress) up to pathogenesis. We found that ZnO NPs induced harm to oyster larvae under seawater conditions, but NOM mitigated its intensity. In contrast, MnO2NPs were not toxic at the tested concentrations (up to 200 μM), and their toxicological stasis was not modified by the presence of organic matter. We propose that strong ion sorption on the MnO2NP surface blocked redox-active sites thus preventing their bandgap mode of action.his project was funded by the European Union Horizon 2020 research and innovation programme under Marie Sklodowska-Curie grant agreement No 655134 (SOS-Nano project, Structure – Oxidative Stress relationships of metal oxide nanoparticles in the aquatic environment) and NERC FENAC access grant No PR120021. TG was further supported through NERC NE/N006178
Precision Ephemerides For Gravitational Wave Searches: II. Cyg X-2
Accreting neutron stars in low-mass X-ray binaries (LMXBs) are candidate
high-frequency persistent gravitational wave sources. These may be detectable
with next generation interferometers such as Advanced LIGO/VIRGO within this
decade. However, the search sensitivity is expected to be limited principally
by the uncertainty in the binary system parameters. We combine new optical
spectroscopy of Cyg X-2 obtained with the Liverpool Telescope (LT) with
available historical radial velocity data, which gives us improved orbital
parameter uncertainties based on a 44-year baseline. We obtained an improvement
of a factor of 2.6 in the orbital period precision and a factor of 2 in the
epoch of inferior conjunction T_0. The updated orbital parameters imply a mass
function of 0.65 +/- 0.01 M_sun, leading to a primary mass (M_1) of 1.67 +/-
0.22 M_sun (for i=62.5 +/- 4 deg). In addition, we estimate the likely orbital
parameter precision through to the expected Advanced LIGO and VIRGO detector
observing period and quantify the corresponding improvement in sensitivity via
the required number of templates.Comment: 7 pages, 3 figures, accepted for publication by Ap
Evidence for a parsec scale X-ray jet from the accreting neutron star Circinus X-1
We analyzed the zero-order image of a 50 ks Chandra gratings observation of
Circinus X-1, taken in 2005 during the source's low-flux state. Circinus X-1 is
an accreting neutron star that exhibits ultra-relativistic arcsecond-scale
radio jets and diffuse arcminute-scale radio jets and lobes. The image shows a
clear excess along the general direction of the north-western counter-jet,
coincident with the radio emission, suggesting that it originates either in the
jet itself or in the shock the jet is driving into its environment. This makes
Circinus X-1 the first neutron star for which an extended X-ray jet has been
detected. The kinetic jet power we infer is significantly larger than the
minimum power required for the jet to inflate the large scale radio nebula.Comment: Added journal reference, corrected on reference and typo in labels
for Fig. 1; 5 pages, 3 figures, ApJ Letter, in pres
Non-Existence of Black Holes in Certain Spacetimes
Assuming certain asymptotic conditions, we prove a general theorem on the
non-existence of static regular (i.e., nondegenerate) black holes in spacetimes
with a negative cosmological constant, given that the fundamental group of
space is infinite. We use this to rule out the existence of regular negative
mass AdS black holes with Ricci flat scri. For any mass, we also rule out a
class of conformally compactifiable static black holes whose conformal infinity
has positive scalar curvature and infinite fundamental group, subject to our
asymptotic conditions. In a limited, but important, special case our result
adds new support to the AdS/CFT inspired positive mass conjecture of Horowitz
and Myers.Comment: 17 pages, Latex. Typos corrected, minor changes to the text. Accepted
for publication in Classical and Quantum Gravit
Dissolution and bandgap paradigms for predicting the toxicity of metal oxide nanoparticles in the marine environment: an in vivo study with oyster embryos
This is the author accepted manuscript. The final version is available from Taylor & Francis via the DOI in this recordDissolution and bandgap paradigms have been proposed for predicting the ability of metal oxide nanoparticles (NPs) to induce oxidative stress in different in vitro and in vivo models. Here, we addressed the effectiveness of these paradigms in vivo and under conditions typical of the marine environment, a final sink for many NPs released through aquatic systems. We used ZnO and MnO2 NPs as models for dissolution and bandgap paradigms, respectively, and CeO2 NPs to assess reactive oxygen radical (ROS) production via Fenton-like reactions in vivo. Oyster embryos were exposed to 0.5-500 μM of each test NP over 24 h and oxidative stress was determined as a primary toxicity pathway across successive levels of biological complexity, with arrested development as the main pathological outcome. NPs were actively ingested by oyster larvae and entered cells. Dissolution was a viable paradigm for predicting the toxicity of NPs in the marine environment, whereas the surface reactivity based paradigms (i.e. bandgap and ROS generation via Fenton-like reaction) were not supported under seawater conditions. Bio-imaging identified potential cellular storage-disposal sites of solid particles that could ameliorate the toxicological behavior of non-dissolving NPs, whilst abiotic screening of surface reactivity suggested that the adsorption-complexation of surface active sites by seawater ions could provide a valuable hypothesis to explain the quenching of the intrinsic oxidation potential of MnO2 NPs in seawater.This project was funded by the European Union Horizon 2020 research and innovation program under Marie Sklodowska-Curie grant agreement No 655134 and NERC FENAC access grant No PR120021. TG acknowledges support from NERC grant NE/N006178/1
A uniqueness theorem for the adS soliton
The stability of physical systems depends on the existence of a state of
least energy. In gravity, this is guaranteed by the positive energy theorem.
For topological reasons this fails for nonsupersymmetric Kaluza-Klein
compactifications, which can decay to arbitrarily negative energy. For related
reasons, this also fails for the AdS soliton, a globally static, asymptotically
toroidal spacetime with negative mass. Nonetheless, arguing from
the AdS/CFT correspondence, Horowitz and Myers (hep-th/9808079) proposed a new
positive energy conjecture, which asserts that the AdS soliton is the unique
state of least energy in its asymptotic class. We give a new structure theorem
for static spacetimes and use it to prove uniqueness of the AdS
soliton. Our results offer significant support for the new positive energy
conjecture and add to the body of rigorous results inspired by the AdS/CFT
correspondence.Comment: Revtex, 4 pages; Matches published version. More detail in Abstract
and one equation corrected. For details of proofs and further results, see
hep-th/020408
On non-existence of static vacuum black holes with degenerate components of the event horizon
We present a simple proof of the non-existence of degenerate components of
the event horizon in static, vacuum, regular, four-dimensional black hole
spacetimes. We discuss the generalisation to higher dimensions and the
inclusion of a cosmological constant.Comment: latex2e, 9 pages in A
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