7,501 research outputs found
Quasi-Normal Modes in Topologically Massive Gravity
We determine the black hole quasi-normal mode spectrum for tensor
perturbations in topologically massive AdS-gravity. In the special case of
chiral gravity quasi-normal modes are absent despite of the presence of a
horizon. In the process we uncover a simple algebraic structure in the quasi
normal modes spectrum: the tower of QNM's consists of descendents of a "chiral
highest weight'' QNM which in turn satisfies a first order equation.Comment: 13 pages, typos correcte
Non-Abelian Born-Infeld Action and Solitons for Critical Non-BPS Branes
The non-abelian flat directions in the tachyon potential of stable non-BPS
branes recently found are shown to persist to all orders in alpha' at tree
level in the string coupling. We also obtain the non-abelian Born-Infeld action
including the tachyon potential for a stack of stable non-BPS branes on a
critical orbifold. Finally we discuss stable soliton states on the non-BPS
brane.Comment: 12+1 pages, Latex. Typos, a reference and the discussion of the
monopole solution are correcte
Smoothness of time functions and the metric splitting of globally hyperbolic spacetimes
The folk questions in Lorentzian Geometry, which concerns the smoothness of
time functions and slicings by Cauchy hypersurfaces, are solved by giving
simple proofs of: (a) any globally hyperbolic spacetime admits a smooth
time function whose levels are spacelike Cauchy hyperfurfaces and, thus,
also a smooth global splitting , , (b) if a spacetime admits a (continuous) time
function (i.e., it is stably causal) then it admits a smooth (time)
function with timelike gradient on all .Comment: 9 pages, Latex, to appear in Commun. Math. Phys. Some comments on
time functions and stably causal spacetimes are incorporated, and referred to
gr-qc/0411143 for further detail
How Do Nonlinear Voids Affect Light Propagation ?
Propagation of light in a clumpy universe is examined. As an inhomogeneous
matter distribution, we take a spherical void surrounded by a dust shell where
the ``lost mass'' in the void is compensated by the shell. We study how the
angular-diameter distance behaves when such a structure exists. The
angular-diameter distance is calculated by integrating the Raychaudhuri
equation including the shear. An explicit expression for the junction condition
for the massive thin shell is calculated. We apply these results to a dust
shell embedded in a Friedmann universe and determine how the distance-redshift
relation is modified compared with that in the purely Friedmann universe. We
also study the distribution of distances in a universe filled with voids. We
show that the void-filled universe gives a larger distance than the FRW
universe by at if the size of the void is of the
Horizon radius.Comment: To appear in Prog. Theor. Phys. 10
String Loop Corrections to Stable Non-BPS Branes
We calculate the string loop corrections to the tachyon potential for stable
non-BPS Dp-branes on the orbifold T^4/Z_2. We find a non-trivial phase
structure and we show that, after tachyon condensation, the non-BPS Dp-branes
are attracted to each other for p=0,1,2. We then identify the corresponding
closed string boundary states together with the massless long range fields they
excite. For p=3,4 the string loop correction diverge. We identify the massless
closed string fields responsible for these divergencies and regularise the
partition function using a Fischler-Susskind mechanism.Comment: References added and some minor correction
The Gauge Fields and Ghosts in Rindler Space
We consider 2d Maxwell system defined on the Rindler space with metric
ds^2=\exp(2a\xi)\cdot(d\eta^2-d\xi^2) with the goal to study the dynamics of
the ghosts. We find an extra contribution to the vacuum energy in comparison
with Minkowski space time with metric ds^2= dt^2-dx^2. This extra contribution
can be traced to the unphysical degrees of freedom (in Minkowski space). The
technical reason for this effect to occur is the property of Bogolubov's
coefficients which mix the positive and negative frequencies modes. The
corresponding mixture can not be avoided because the projections to positive
-frequency modes with respect to Minkowski time t and positive -frequency modes
with respect to the Rindler observer's proper time \eta are not equivalent. The
exact cancellation of unphysical degrees of freedom which is maintained in
Minkowski space can not hold in the Rindler space. In BRST approach this effect
manifests itself as the presence of BRST charge density in L and R parts. An
inertial observer in Minkowski vacuum |0> observes a universe with no net BRST
charge only as a result of cancellation between the two. However, the Rindler
observers who do not ever have access to the entire space time would see a net
BRST charge. In this respect the effect resembles the Unruh effect. The effect
is infrared (IR) in nature, and sensitive to the horizon and/or boundaries. We
interpret the extra energy as the formation of the "ghost condensate" when the
ghost degrees of freedom can not propagate, but nevertheless do contribute to
the vacuum energy. Exact computations in this simple 2d model support the claim
made in [1] that the ghost contribution might be responsible for the observed
dark energy in 4d FLRW universe.Comment: Final version to appear in Phys. Rev. D. Comments on relation with
energy momentum computations and few new refs are adde
On Higher Derivative Terms in Tachyon Effective Actions
We reconstruct the tachyon effective action for unstable D-branes in
superstring theory by examining its behaviour near exactly marginal
deformations, where the ambigous higher derivative terms can be eliminated. We
then compare this action with that obtained in boundary string field theory and
find remarkable agreement. In particular, the tension for lower dimensional
branes and the BI-action for the centre of mass motion are reprodued exactly.
We also comment on the action for tachyons on the kink in a
D-brane/anti-D-brane system and on bosonic string theory.Comment: 18 pages, Latex, 2 figures, references added and eqn (2.23) correcte
α-Synuclein Reduces Tension and Increases Undulations in Simulations of Small Unilamellar Vesicles
AbstractUsing coarse-grained molecular dynamics simulations we have explored the effect of α-Synuclein (αSyn) on the structural and mechanical properties of small unilamellar vesicles in the fluid-phase. The study is motivated by observations that a high density of membrane-bound αSyn inhibits the fusion of synthetic small unilamellar vesicles. By combining three-dimensional pressure tensor calculations with our recently developed spherical harmonics fluctuation analysis approach, we show a reduction in membrane surface tension and increased membrane undulations when αSyn is bound to the vesicle’s outer leaflet at a 200:1 L/P. The protein effects these changes by decreasing the negative pressure in the headgroup region of the outer leaflet and increasing the positive pressure throughout the hydrocarbon core
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