801 research outputs found
New thought experiment to test the generalized second law of thermodynamics
We propose an extension of the original thought experiment proposed by
Geroch, which sparked much of the actual debate and interest on black hole
thermodynamics, and show that the generalized second law of thermodynamics is
in compliance with it.Comment: 4 pages (revtex), 3 figure
Black holes in asymptotically Lifshitz spacetimes with arbitrary critical exponent
Recently, a class of gravitational backgrounds in 3+1 dimensions have been
proposed as holographic duals to a Lifshitz theory describing critical
phenomena in 2+1 dimensions with critical exponent . We numerically
explore black holes in these backgrounds for a range of values of . We find
drastically different behavior for and
() the Lifshitz fixed point is repulsive (attractive) when going to larger
radial parameter . For the repulsive backgrounds, we find a continuous
family of black holes satisfying a finite energy condition. However, for
we find that the finite energy condition is more restrictive, and we expect
only a discrete set of black hole solutions, unless some unexpected
cancellations occur. For all black holes, we plot temperature as a function
of horizon radius . For we find that this curve
develops a negative slope for certain values of possibly indicating a
thermodynamic instability.Comment: 23 pages, 6 figures, references corrected, graphs made readable in
greyscal
The Einstein-Cartan-Elko system
The present paper analyses the Einstein-Cartan theory of gravitation with
Elko spinors as sources of curvature and torsion. After minimally coupling the
Elko spinors to torsion, the spin angular momentum tensor is derived and its
structure is discussed. It shows a much richer structure than the Dirac
analogue and hence it is demonstrated that spin one half particles do not
necessarily yield only an axial vector torsion component. Moreover, it is
argued that the presence of Elko spinors partially solves the problem of
minimally coupling Maxwell fields to Einstein-Cartan theory.Comment: 12 pages, no figure
On a Holographic Model for Confinement/Deconfinement
We study the thermodynamics of the hard wall model, which consists in the
introduction of an infrared cut-off in asymptotically AdS spaces. This is a toy
model for confining backgrounds in the context of the gauge/gravity
correspondence. We use holographic renormalization and reproduce the existence
of a Hawking Page phase transition recently discussed by Herzog. We also show
that the entropy jumps from to , which reinforces the interpretation
of this transition as the gravity dual of confinement/deconfinement. We also
show that similar results hold for the phenomenologically motivated soft wall
model, underlining the potential universality of our analysis.Comment: 14 pages. V2: We included a new section discussing the soft wall
model and new references. V3: We clarified some points and updated the
references. Results unchanged. Version published in PR
Experimental behaviour of a steel structure under natural fire
Current design codes for fire resistance of structures are based on isolated member tests subjected to standard fire conditions. Such tests do not reflect the behaviour of a complete building under either normal temperature or fire conditions. Many aspects of behaviour occur due to the interaction between members and cannot be predicted or observed in tests of isolated elements. Performance of real structures subject to real fires is often much better than that predicted from standard tests due to structural continuity and the provision of alternative load paths.http://www.sciencedirect.com/science/article/B6V37-4KN5C4D-1/1/8f781d0c96159d54029bef7c9ec451b
Experimental behaviour of a steel structure under natural fire
Current design codes for fire resistance of structures are based on isolated member tests subjected to standard fire conditions. Such tests do not reflect the behaviour of a complete building under either normal temperature or fire conditions. Many aspects of behaviour occur due to the interaction between members and cannot be predicted or observed in tests of isolated elements. Performance of real structures subject to real fires is often much better than that predicted from standard tests due to structural continuity and the provision of alternative load paths.http://www.sciencedirect.com/science/article/B6V37-4KN5C4D-1/1/8f781d0c96159d54029bef7c9ec451b
A Note on Thermodynamics of Black Holes in Lovelock Gravity
The Lovelock gravity consists of the dimensionally extended Euler densities.
The geometry and horizon structure of black hole solutions could be quite
complicated in this gravity, however, we find that some thermodynamic
quantities of the black holes like the mass, Hawking temperature and entropy,
have simple forms expressed in terms of horizon radius. The case with black
hole horizon being a Ricci flat hypersurface is particularly simple. In that
case the black holes are always thermodynamically stable with a positive heat
capacity and their entropy still obeys the area formula, which is no longer
valid for black holes with positive or negative constant curvature horizon
hypersurface. In addition, for black holes in the gravity theory of Ricci
scalar plus a -dimensional Euler density with a positive coefficient,
thermodynamically stable small black holes always exist in dimensions,
which are absent in the case without the Euler density term, while the
thermodynamic properties of the black hole solutions with the Euler density
term are qualitatively similar to those of black holes without the Euler
density term as .Comment: Latex, 10 pages, v2: typos corrected, references added, to appear in
PL
First-trimester or second-trimester screening, or both, for Down's syndrome
BACKGROUND: It is uncertain how best to screen pregnant women for the presence of fetal Down's syndrome: to perform first-trimester screening, to perform second-trimester screening, or to use strategies incorporating measurements in both trimesters.METHODS: Women with singleton pregnancies underwent first-trimester combined screening (measurement of nuchal translucency, pregnancy-associated plasma protein A [PAPP-A], and the free beta subunit of human chorionic gonadotropin at 10 weeks 3 days through 13 weeks 6 days of gestation) and second-trimester quadruple screening (measurement of alpha-fetoprotein, total human chorionic gonadotropin, unconjugated estriol, and inhibin A at 15 through 18 weeks of gestation). We compared the results of stepwise sequential screening (risk results provided after each test), fully integrated screening (single risk result provided), and serum integrated screening (identical to fully integrated screening, but without nuchal translucency).RESULTS: First-trimester screening was performed in 38,167 patients; 117 had a fetus with Down's syndrome. At a 5 percent false positive rate, the rates of detection of Down's syndrome were as follows: with first-trimester combined screening, 87 percent, 85 percent, and 82 percent for measurements performed at 11, 12, and 13 weeks, respectively; with second-trimester quadruple screening, 81 percent; with stepwise sequential screening, 95 percent; with serum integrated screening, 88 percent; and with fully integrated screening with first-trimester measurements performed at 11 weeks, 96 percent. Paired comparisons found significant differences between the tests, except for the comparison between serum integrated screening and combined screening.CONCLUSIONS: First-trimester combined screening at 11 weeks of gestation is better than second-trimester quadruple screening but at 13 weeks has results similar to second-trimester quadruple screening. Both stepwise sequential screening and fully integrated screening have high rates of detection of Down's syndrome, with low false positive rates
Static Observers in Curved Spaces and Non-inertial Frames in Minkowski Spacetime
Static observers in curved spacetimes may interpret their proper acceleration
as the opposite of a local gravitational field (in the Newtonian sense). Based
on this interpretation and motivated by the equivalence principle, we are led
to investigate congruences of timelike curves in Minkowski spacetime whose
acceleration field coincides with the acceleration field of static observers of
curved spaces. The congruences give rise to non-inertial frames that are
examined. Specifically we find, based on the locality principle, the embedding
of simultaneity hypersurfaces adapted to the non-inertial frame in an explicit
form for arbitrary acceleration fields. We also determine, from the Einstein
equations, a covariant field equation that regulates the behavior of the proper
acceleration of static observers in curved spacetimes. It corresponds to an
exact relativistic version of the Newtonian gravitational field equation. In
the specific case in which the level surfaces of the norm of the acceleration
field of the static observers are maximally symmetric two-dimensional spaces,
the energy-momentum tensor of the source is analyzed.Comment: 28 pages, 4 figures
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