968 research outputs found
Membranes on Calibrations
M2-branes can blow up into BPS funnels that end on calibrated intersections
of M5-branes. In this quick note, we make the observation that the constraints
required for the consistency of these solutions are automatic in
Bagger-Lambert-Gustavsson (BLG) theory, thanks to the fundamental identity and
the supersymmetry of the calibration. We use this to explain how the previous
ad hoc fuzzy funnel constructions emerge in this picture, and make some
comments about the role of the 3-algebra trace form in the derivation.Comment: 9 pages, no figures; references added, minor change
A Comment on Kerr-CFT and Wald Entropy
We point out that the entropies of black holes in general diffeomorphism
invariant theories, computed using the Kerr-CFT correspondence and the Wald
formula (as implemented in the entropy function formalism), need not always
agree. A simple way to illustrate this is to consider Einstein-Gauss-Bonnet
gravity in four dimensions, where the Gauss-Bonnet term is topological. This
means that the central charge of Kerr-CFT computed in the
Barnich-Brandt-Compere formalism remains the same as in Einstein gravity, while
the entropy computed using the entropy function gives a universal correction
proportional to the Gauss-Bonnet coupling. We argue that at least in this
example, the Kerr-CFT result is the physically reasonable one. The resolution
to this discrepancy might lie in a better understanding of boundary terms.Comment: 11 pages, v2,3: refs added, minor change
Higher Spin Resolution of a Toy Big Bang
Diffeomorphisms preserve spacetime singularities, whereas higher spin
symmetries need not. Since three dimensional de Sitter space has quotients that
have big-bang/big-crunch singularities and since dS_3-gravity can be written as
an SL(2,C) Chern-Simons theory, we investigate SL(3,C) Chern-Simons theory as a
higher-spin context in which these singularities might get resolved. As in the
case of higher spin black holes in AdS_3, the solutions are invariantly
characterized by their holonomies. We show that the dS_3 quotient singularity
can be de-singularized by an SL(3,C) gauge transformation that preserves the
holonomy: this is a higher spin resolution the cosmological singularity. Our
work deals exclusively with the bulk theory, and is independent of the
subtleties involved in defining a CFT_2 dual to dS_3 in the sense of dS/CFT.Comment: v2-v3: typos removed, refs added. v4: minor improvements, Phys Rev D
version, v5: one more typo fixed, footnote adde
A Neumann Boundary Term for Gravity
The Gibbons-Hawking-York (GHY) boundary term makes the Dirichlet problem for
gravity well defined, but no such general term seems to be known for Neumann
boundary conditions. In this paper, we view Neumann {\em not} as fixing the
normal derivative of the metric ("velocity") at the boundary, but as fixing the
functional derivative of the action with respect to the boundary metric
("momentum"). This leads directly to a new boundary term for gravity: the trace
of the extrinsic curvature with a specific dimension-dependent coefficient. In
three dimensions this boundary term reduces to a "one-half" GHY term noted in
the literature previously, and we observe that our action translates precisely
to the Chern-Simons action with no extra boundary terms. In four dimensions the
boundary term vanishes, giving a natural Neumann interpretation to the standard
Einstein-Hilbert action without boundary terms. We argue that in light of
AdS/CFT, ours is a natural approach for defining a "microcanonical" path
integral for gravity in the spirit of the (pre-AdS/CFT) work of Brown and York.Comment: v3: emphasized that a covariant notion of Neumann boundary condition
inevitably leads to our boundary term. v4: more ref
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