1,051 research outputs found
On the Topology of Black Hole Event Horizons in Higher Dimensions
In four dimensions the topology of the event horizon of an asymptotically
flat stationary black hole is uniquely determined to be the two-sphere .
We consider the topology of event horizons in higher dimensions. First, we
reconsider Hawking's theorem and show that the integrated Ricci scalar
curvature with respect to the induced metric on the event horizon is positive
also in higher dimensions. Using this and Thurston's geometric types
classification of three-manifolds, we find that the only possible geometric
types of event horizons in five dimensions are and . In
six dimensions we use the requirement that the horizon is cobordant to a
four-sphere (topological censorship), Friedman's classification of topological
four-manifolds and Donaldson's results on smooth four-manifolds, and show that
simply connected event horizons are homeomorphic to or .
We find allowed non-simply connected event horizons and
, and event horizons with finite non-abelian first homotopy
group, whose universal cover is . Finally, following Smale's results we
discuss the classification in dimensions higher than six.Comment: 12 pages, minor edits 27/09/0
Superhydrophobic structures on 316L stainless steel surfaces machined by nanosecond pulsed laser
In this paper nanosecond laser machining process was developed to improve the hydrophobicity of AISI 316L stainless steel surface. A geometrical model of laser machined Gaussian micro hole, together with constrain conditions, was established for the first time to predict surface contact angle and optimize structure geometries for maximizing its hydrophobicity. The effects of processing laser power and pitch of microstructures on the topography of the machined surface were investigated through laser machining experiment. Subsequently, the water droplet contact angle was measured to evaluate the hydrophobicity of different specimens. Results show that under the laser power of 10 W and 14 W, with the increase of the pitch of microstructures, the contact angle increases until it reaches its peak value then drops gradually. Under the large pitch of microstructure, the contact angle will increase with the increase of the processing laser power. Under the same pitch of microstructure, the contact angle will increase with the increase of ten-point height of surface topography, Sz which is a better parameter than Sa (arithmetical mean height) to characterise hydrophobicity of surface with Gaussian holes. This study shows that large Sz is an essential condition to form the stable and robust Cassie–Baxter state, i.e. a condition to achieve superhydrophobicity. The comparison between the predicted and measured contact angles in experiments shows that the proposed model can accurately predict contact angle and optimize the geometries of the microstructure to achieve maximum hydrophobicity
State transfer in dissipative and dephasing environments
By diagonalization of a generalized superoperator for solving the master
equation, we investigated effects of dissipative and dephasing environments on
quantum state transfer, as well as entanglement distribution and creation in
spin networks. Our results revealed that under the condition of the same
decoherence rate , the detrimental effects of the dissipative
environment are more severe than that of the dephasing environment. Beside
this, the critical time at which the transfer fidelity and the
concurrence attain their maxima arrives at the asymptotic value
quickly as the spin chain length increases. The transfer
fidelity of an excitation at time is independent of when the system
subjects to dissipative environment, while it decreases as increases when
the system subjects to dephasing environment. The average fidelity displays
three different patterns corresponding to , and . For
each pattern, the average fidelity at time is independent of when the
system subjects to dissipative environment, and decreases as increases when
the system subjects to dephasing environment. The maximum concurrence also
decreases as increases, and when , it arrives at an
asymptotic value determined by the decoherence rate and the structure
of the spin network.Comment: 12 pages, 6 figure
Polycystic Ovary Syndrome Signs and Metabolic Syndrome in Premenopausal Hispanic/Latina Women: the HCHS/SOL Study
Context: Polycystic ovary syndrome (PCOS), a condition of androgen excess in women, is associated with cardiometabolic risk factors; however, this association is not fully characterized in a population-based sample of premenopausal women and high-risk groups such as Hispanics/Latinas. Objective: We examined the association of PCOS signs and metabolic syndrome (MetS) in premenopausal Hispanic/Latina women. Methods: This cross-sectional analysis includes 1427 women age 24 to 44 years from the Hispanic Community Health Study/Study of Latinos. PCOS signs included menstrual cycle greater than 35 days or irregular, self-reported PCOS, and oral contraceptive use to regulate periods or acne, and a composite of 1 or more PCOS signs. We calculated odds ratios (OR) and 95% CI for MetS, accounting for sociodemographic factors and the complex survey design; an additional model included body mass index (BMI). Results: The mean age was 34 years and 30% reported any PCOS sign. The odds of MetS were higher in women reporting cycles greater than 35 days or irregular (OR 1.63; CI: 1.07-2.49) vs cycles 24 to 35 days, self-reported PCOS (OR 2.49; CI: 1.38-4.50) vs no PCOS, and any PCOS sign (OR 1.58; CI: 1.10-2.26) vs none. We found no association between OC use to regulate periods or acne and MetS (OR 1.1; CI: 0.6-1.8). When adjusting for BMI, only the association of self-reported PCOS and MetS was attenuated (OR 1.78; CI: 0.92-3.44). Conclusions: In Hispanic/Latina women, irregular menstrual cycles, self-reported PCOS, and any PCOS sign were associated with MetS and could indicate women at metabolic disease risk
Stationary Einstein-Maxwell fields in arbitrary dimensions
The Einstein-Maxwell equations in D-dimensions admitting (D-3) commuting
Killing vector fields have been investigated. The existence of the electric,
magnetic and twist potentials have been proved. The system is formulated as the
harmonic map coupled to gravity on three-dimensional base space generalizing
the Ernst system in the four-dimensional stationary Einstein-Maxwell theory.
Some classes of the new exact solutions have been provided, which include the
electro-magnetic generalization of the Myers-Perry solution, which describes
the rotating black hole immersed in a magnetic universe, and the static charged
black ring solution.Comment: 26 page
Thermodynamics in gravity in the Palatini formalism
We investigate thermodynamics of the apparent horizon in gravity in
the Palatini formalism with non-equilibrium and equilibrium descriptions. We
demonstrate that it is more transparent to understand the horizon entropy in
the equilibrium framework than that in the non-equilibrium one. Furthermore, we
show that the second law of thermodynamics can be explicitly verified in both
phantom and non-phantom phases for the same temperature of the universe outside
and inside the apparent horizon.Comment: 20 pages, no figure, accepted in JCA
Observation of Two New N* Peaks in J/psi -> and Decays
The system in decays of is limited to be
isospin 1/2 by isospin conservation. This provides a big advantage in studying
compared with and experiments which mix
isospin 1/2 and 3/2 for the system. Using 58 million decays
collected with the Beijing Electron Positron Collider, more than 100 thousand
events are obtained. Besides two well known
peaks at 1500 MeV and 1670 MeV, there are two new, clear peaks in
the invariant mass spectrum around 1360 MeV and 2030 MeV. They are the
first direct observation of the peak and a long-sought "missing"
peak above 2 GeV in the invariant mass spectrum. A simple
Breit-Wigner fit gives the mass and width for the peak as MeV and MeV, and for the new peak above 2 GeV
as MeV and MeV, respectively
Statefinder and Om Diagnostics for Interacting New Holographic Dark Energy Model and Generalized Second Law of Thermodynamics
In this work, we have considered that the flat FRW universe is filled with
the mixture of dark matter and the new holographic dark energy. If there is an
interaction, we have investigated the natures of deceleration parameter,
statefinder and diagnostics. We have examined the validity of the first
and generalized second laws of thermodynamics under these interactions on the
event as well as apparent horizon. It has been observed that the first law is
violated on the event horizon. However, the generalized second law is valid
throughout the evolution of the universe enveloped by the apparent horizon.
When the event horizon is considered as the enveloping horizon, the generalized
second law is found to break down excepting at late stage of the universe.Comment: 9 pages, 13 figure
Quintessential Maldacena-Maoz Cosmologies
Maldacena and Maoz have proposed a new approach to holographic cosmology
based on Euclidean manifolds with disconnected boundaries. This approach
appears, however, to be in conflict with the known geometric results [the
Witten-Yau theorem and its extensions] on spaces with boundaries of
non-negative scalar curvature. We show precisely how the Maldacena-Maoz
approach evades these theorems. We also exhibit Maldacena-Maoz cosmologies with
[cosmologically] more natural matter content, namely quintessence instead of
Yang-Mills fields, thereby demonstrating that these cosmologies do not depend
on a special choice of matter to split the Euclidean boundary. We conclude that
if our Universe is fundamentally anti-de Sitter-like [with the current
acceleration being only temporary], then this may force us to confront the
holography of spaces with a connected bulk but a disconnected boundary.Comment: Much improved exposition, exponent in Cai-Galloway theorem fixed,
axionic interpretation of scalar explained, JHEP version. 33 pages, 3 eps
figure
Affleck-Dine dynamics and the dark sector of pangenesis
Pangenesis is the mechanism for jointly producing the visible and dark matter
asymmetries via Affleck-Dine dynamics in a baryon-symmetric universe. The
baryon-symmetric feature means that the dark asymmetry cancels the visible
baryon asymmetry and thus enforces a tight relationship between the visible and
dark matter number densities. The purpose of this paper is to analyse the
general dynamics of this scenario in more detail and to construct specific
models. After reviewing the simple symmetry structure that underpins all
baryon-symmetric models, we turn to a detailed analysis of the required
Affleck-Dine dynamics. Both gravity-mediated and gauge-mediated supersymmetry
breaking are considered, with the messenger scale left arbitrary in the latter,
and the viable regions of parameter space are determined. In the gauge-mediated
case where gravitinos are light and stable, the regime where they constitute a
small fraction of the dark matter density is identified. We discuss the
formation of Q-balls, and delineate various regimes in the parameter space of
the Affleck-Dine potential with respect to their stability or lifetime and
their decay modes. We outline the regions in which Q-ball formation and decay
is consistent with successful pangenesis. Examples of viable dark sectors are
presented, and constraints are derived from big bang nucleosynthesis, large
scale structure formation and the Bullet cluster. Collider signatures and
implications for direct dark matter detection experiments are briefly
discussed. The following would constitute evidence for pangenesis:
supersymmetry, GeV-scale dark matter mass(es) and a Z' boson with a significant
invisible width into the dark sector.Comment: 51 pages, 7 figures; v2: minor modifications, comments and references
added; v3: minor changes, matches published versio
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