934 research outputs found
Conformal Tensors via Lovelock Gravity
Constructs from conformal geometry are important in low dimensional gravity
models, while in higher dimensions the higher curvature interactions of
Lovelock gravity are similarly prominent. Considering conformal invariance in
the context of Lovelock gravity leads to natural, higher-curvature
generalizations of the Weyl, Schouten, Cotton and Bach tensors, with properties
that straightforwardly extend those of their familiar counterparts. As a first
application, we introduce a new set of conformally invariant gravity theories
in D=4k dimensions, based on the squares of the higher curvature Weyl tensors.Comment: 16 pages; v2 - references adde
Magnetic Fields in an Expanding Universe
We find a solution to Einstein-Maxwell theory coupled to a massless
dilaton field describing a Melvin magnetic field in an expanding universe with
'stiff matter' equation of state parameter . As the universe expands,
magnetic flux becomes more concentrated around the symmetry axis for dilaton
coupling . An electric
field circulates around the symmetry axis in the direction determined by Lenz's
law. For the magnetic flux through a disk of fixed comoving radius is
proportional to the proper area of the disk. This result disagrees with the
usual expectation based on a test magnetic field that this flux should be
constant, and we show why this difference arises. We also find a Melvin
solution in an accelerating universe with for a dilaton field with a
certain exponential potential. Our main tools are simple manipulations in
Kaluza-Klein theory and related solution generating techniques. We also discuss
a number of directions for possible extensions of this work.Comment: 17 pages, 2 figures; v2 - references adde
Micromagnetic Simulations on the Dependence of Gilbert Damping on Domain Wall Velocities in Magnetic Nanowires
he dependence of damping on domain wall motion and velocity in Permalloy nanowires is presented. The domain wall motion in isolated two micron long Permalloy nanowires, with a rectangular cross-section 10 nm thick and 100 nm wide, is simulated using the Landau-Lifshitz Gilbert (LLG) simulation.Interpreting LLG dynamics can be difficult due to the dependence of the results on the Gilbert damping parameter alpha. The Walker model also predicts the critical field and domain wall velocity as a function of alpha. For these combined reasons the dependence of the domain wall speeds on the damping parameter is explored
On black strings & branes in Lovelock gravity
It is well known that black strings and branes may be constructed in pure
Einstein gravity simply by adding flat directions to a vacuum black hole
solution. A similar construction holds in the presence of a cosmological
constant. While these constructions fail in general Lovelock theories, we show
that they carry over straightforwardly within a class of Lovelock gravity
theories that have (locally) unique constant curvature vacua.Comment: 12 pages, Latex, references addde
The Thermodynamics of Kaluza-Klein Black Hole/Bubble Chains
A Killing bubble is a minimal surface that arises as the fixed surface of a
spacelike Killing field. We compute the bubble contributions to the Smarr
relations and the mass and tension first laws for spacetimes containing both
black holes and Killing bubbles. The resulting relations display an interesting
interchange symmetry between the properties of black hole horizons and those of
KK bubbles. This interchange symmetry reflects the underlying relation between
static bubbles and black holes under double analytic continuation of the time
and Kaluza-Klein directions. The thermodynamics of bubbles involve a
geometrical quantity that we call the bubble surface gravity, which we show has
several properties in common with the black hole surface gravity.Comment: 20 pages, 1 figur
Dynamics of localized Kaluza-Klein black holes in a collapsing universe
The Clayton Antitrust Act of 1914 prohibits corporate mergers that would
result in certain highly undesired end states. We study an exact solution of
the Einstein equations describing localized, charged Kaluza-Klein black holes
in a collapsing deSitter universe and seek to demonstrate that a similar effect
holds, preventing a potentially catastrophic black hole merger. As the collapse
proceeds, it is natural to expect that the black hole undergoes a topological
transition, wrapping around the shrinking compact dimension to merge with
itself and form a black string. However, the putative uniform charged black
string end state is singular and such a transition would violate (a reasonable
notion of) cosmic censorship. We present analytic and numerical evidence that
strongly suggests the absence of such a transition. Based on this evidence, we
expect that the Kaluza-Klein black hole horizon stays localized, despite the
increasingly constraining size of the compact dimension. On the other hand, the
deSitter horizon does change between spherical and cylindrical topologies in a
simple way.Comment: 25 pages, 6 figure
Smarr Formula and an Extended First Law for Lovelock Gravity
We study properties of static, asymptotically AdS black holes in Lovelock
gravity. Our main result is a Smarr formula that gives the mass in terms of
geometrical quantities together with the parameters of the Lovelock theory. As
in Einstein gravity, the Smarr formula follows from applying the first law to
an infinitesimal change in the overall length scale. However, because the
Lovelock couplings are dimensionful, we must first prove an extension of the
first law that includes their variations. Key ingredients in this construction
are the Killing-Lovelock potentials associated with each of the the higher
curvature Lovelock interactions. Geometric expressions are obtained for the new
thermodynamic potentials conjugate to variation of the Lovelock couplings.Comment: 20 pages; v2 - references added; v3 - includes important corrections
to result
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