10,763 research outputs found
Black Holes in Higher-Derivative Gravity
Extensions of Einstein gravity with higher-order derivative terms arise in
string theory and other effective theories, as well as being of interest in
their own right. In this paper we study static black-hole solutions in the
example of Einstein gravity with additional quadratic curvature terms. A
Lichnerowicz-type theorem simplifies the analysis by establishing that they
must have vanishing Ricci scalar curvature. By numerical methods we then
demonstrate the existence of further black-hole solutions over and above the
Schwarzschild solution. We discuss some of their thermodynamic properties, and
show that they obey the first law of thermodynamics.Comment: Typos corrected, discussion added, figure changed. 4 pages, 6 figure
Lichnerowicz Modes and Black Hole Families in Ricci Quadratic Gravity
A new branch of black hole solutions occurs along with the standard
Schwarzschild branch in -dimensional extensions of general relativity
including terms quadratic in the Ricci tensor. The standard and new branches
cross at a point determined by a static negative-eigenvalue eigenfunction of
the Lichnerowicz operator, analogous to the Gross-Perry-Yaffe eigenfunction for
the Schwarzschild solution in standard dimensional general relativity.
This static eigenfunction has two r\^oles: both as a perturbation away from
Schwarzschild along the new black-hole branch and also as a threshold unstable
mode lying at the edge of a domain of Gregory-Laflamme-type instability of the
Schwarzschild solution for small-radius black holes. A thermodynamic analogy
with the Gubser and Mitra conjecture on the relation between quantum
thermodynamic and classical dynamical instabilities leads to a suggestion that
there may be a switch of stability properties between the old and new
black-hole branches for small black holes with radii below the branch crossing
point.Comment: 33 pages, 8 figure
Spectrum-generating Symmetries for BPS Solitons
We show that there exist nonlinearly realised duality symmetries that are
independent of the standard supergravity global symmetries, and which provide
active spectrum-generating symmetries for the fundamental BPS solitons. The
additional ingredient, in any spacetime dimension, is a single scaling
transformation that allows one to map between BPS solitons with different
masses. Without the inclusion of this additional transformation, which is a
symmetry of the classical equations of motion, but not the action, it is not
possible to find a spectrum-generating symmetry. The necessity of including
this scaling transformation highlights the vulnerability of duality multiplets
to quantum anomalies. We argue that fundamental BPS solitons may be immune to
this threat.Comment: References added. Latex, 29 page
Spherically Symmetric Solutions in Higher-Derivative Gravity
Extensions of Einstein gravity with quadratic curvature terms in the action
arise in most effective theories of quantised gravity, including string theory.
This article explores the set of static, spherically symmetric and
asymptotically flat solutions of this class of theories. An important element
in the analysis is the careful treatment of a Lichnerowicz-type `no-hair'
theorem. From a Frobenius analysis of the asymptotic small-radius behaviour,
the solution space is found to split into three asymptotic families, one of
which contains the classic Schwarzschild solution. These three families are
carefully analysed to determine the corresponding numbers of free parameters in
each. One solution family is capable of arising from coupling to a
distributional shell of matter near the origin; this family can then match on
to an asymptotically flat solution at spatial infinity without encountering a
horizon. Another family, with horizons, contains the Schwarzschild solution but
includes also non-Schwarzschild black holes. The third family of solutions
obtained from the Frobenius analysis is nonsingular and corresponds to `vacuum'
solutions. In addition to the three families identified from near-origin
behaviour, there are solutions that may be identified as `wormholes', which can
match symmetrically on to another sheet of spacetime at finite radius.Comment: 57 pages, 6 figures; version appearing in journal; minor corrections
and clarifications to v
Dilatonic p-brane solitons
We find new 4-brane and 5-brane solitons in massive gauged , and
, supergravities. In each case, the solutions preserve half of the
original supersymmetry. These solutions make use of the metric and dilaton
fields only. We also present more general dilatonic -branes in
dimensions.Comment: 9 pages, Latex, no figure
The 3D structure of the Lagrangian acceleration in turbulent flows
We report experimental results on the three dimensional Lagrangian
acceleration in highly turbulent flows. Tracer particles are tracked optically
using four silicon strip detectors from high energy physics that provide high
temporal and spatial resolution. The components of the acceleration are shown
to be statistically dependent. The probability density function (PDF) of the
acceleration magnitude is comparable to a log-normal distribution. Assuming
isotropy, a log-normal distribution of the magnitude can account for the
observed dependency of the components. The time dynamics of the acceleration
components is found to be typical of the dissipation scales whereas the
magnitude evolves over longer times, possibly close to the integral time scale.Comment: accepted for publication in Physical Review Letter
The Low-level Spectrum of the String
We investigate the spectrum of physical states in the string theory, up
to level 2 for a multi-scalar string, and up to level 4 for the two-scalar
string. The (open) string has a photon as its only massless state. By
using screening charges to study the null physical states in the two-scalar
string, we are able to learn about the gauge symmetries of the states in
the multi-scalar string.Comment: 31 pages, Plain Tex, CTP TAMU-70/92, Goteborg ITP 92-43,
Imperial/TP/91-92/22, KCL-TH-92-
Euclidean-signature Supergravities, Dualities and Instantons
We study the Euclidean-signature supergravities that arise by compactifying
D=11 supergravity or type IIB supergravity on a torus that includes the time
direction. We show that the usual T-duality relation between type IIA and type
IIB supergravities compactified on a spatial circle no longer holds if the
reduction is performed on the time direction. Thus there are two inequivalent
Euclidean-signature nine-dimensional maximal supergravities. They become
equivalent upon further spatial compactification to D=8. We also show that
duality symmetries of Euclidean-signature supergravities allow the harmonic
functions of any single-charge or multi-charge instanton to be rescaled and
shifted by constant factors. Combined with the usual diagonal dimensional
reduction and oxidation procedures, this allows us to use the duality
symmetries to map any single-charge or multi-charge p-brane soliton, or any
intersection, into its near-horizon regime. Similar transformations can also be
made on non-extremal p-branes. We also study the structures of duality
multiplets of instanton and (D-3)-brane solutions.Comment: Latex, 50 pages, typos corrected and references adde
Polyfluorene as a model system for space-charge-limited conduction
Ethyl-hexyl substituted polyfluorene (PF) with its high level of molecular
disorder can be described very well by one-carrier space-charge-limited
conduction for a discrete set of trap levels with energy 0.5 eV above
the valence band edge. Sweeping the bias above the trap-filling limit in the
as-is polymer generates a new set of exponential traps, which is clearly seen
in the density of states calculations. The trapped charges in the new set of
traps have very long lifetimes and can be detrapped by photoexcitation. Thermal
cycling the PF film to a crystalline phase prevents creation of additional
traps at higher voltages.Comment: 13 pages, 4 figures. Physical Review B (accepted, 2007
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