20,628 research outputs found
Dissipative force on an external quark in heavy quark cloud
Within the finite temperature N = 4 strongly coupled super Yang- Mills, we
compute the dissipative force on an external quark in the presence of evenly
distributed heavy quark cloud. This is computed holographically by constructing
the corresponding gravity dual. We study the behaviour of this force as a
function of the cloud density. Along the way we also analyze the stability of
the gravity dual for vector and tensor perturbations.Comment: 14 pages, 2 figures, significant improvements, to be appeared in
Physics Letters
Comments on Takahashi-Tanimoto's scalar solution
We study the identity-based solution of Witten's cubic bosonic open string
field theory constructed by Takahashi and Tanimoto, which is claimed to
describe the tachyon vacuum. We argue that the observables of the solution
coincide with those of the tachyon vacuum using the method proposed by
Kishimoto and Takahashi. We also discuss how to treat the kinetic term of the
string field theory expanded around it.Comment: 25 pages, 4 figures, v3: several clarifications added, corrected
statements about operator U, v4: comments on operator U_k are adde
On a Nielsen-Thurston classification theory for cluster modular groups
We classify elements of a cluster modular group into three types. We
characterize them in terms of fixed point property of the action on the
tropical compactifications associated with the corresponding cluster ensemble.
The characterization gives an analogue of the Nielsen-Thurston classification
theory on the mapping class group of a surface.Comment: 33 pages, 9 figures. To appear in Annales de l'Institut Fourie
Higher Dimensional Bondi Energy with a Globally Specified Background Structure
A higher (even spacetime) dimensional generalization of the Bondi energy has
recently been proposed by gr-qc/0304054 within the framework of conformal
infinity and Hamiltonian formalizm. The gauge condition employed in
gr-qc/0304054 to derive the Bondi energy expression is, however, peculiar in
the sense that cross-sections of null infinity specified by that gauge are
anisotropic and in fact non-compact. For this reason, that gauge is difficult
to use for explicit computations of the Bondi energy in general, asymptotically
flat radiative spacetimes. Also it is not clear, under that gauge condition,
whether apparent difference between the expressions of higher dimensional Bondi
energy and the 4-dimensional one is due to the choice of gauges or
qualitatively different nature of higher dimensional gravity from 4-dimensional
gravity. In this paper, we consider instead, Gaussian null conformal gauge as
one of more natural gauge conditions that admit a global specification of
background structure with compact, spherical cross-sections of null infinity.
Accordingly, we modify the previous definition of higher dimensional news
tensor so that it becomes well-defined in the Gaussian null conformal gauge and
derive, for vacuum solutions, the expression for the Bondi energy-momentum in
the new gauge choice, which takes a universal form in arbitrary (even
spacetime) dimensions greater than or equal to four.Comment: 23 pages, no figures, references are added, minor corrections,
numerous minor revisions, to appear in Class. Quantum Gra
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