Amitabh Virmani

Dynamics and stability of black rings

We examine the dynamics of neutral black rings, and identify and analyze a selection of possible instabilities. We find the dominating forces of very thin black rings to be a Newtonian competition between a string-like tension and a centrifugal force. We study in detail the radial balance of forces in black rings, and find evidence that all fat black rings are unstable to radial perturbations, while thin black rings are radially stable. Most thin black rings, if not all of them, also likely suffer from Gregory-Laflamme instabilities. We also study simple models for stability against emission/absorption of massless particles. Our results point to the conclusion that most neutral black rings suffer from classical dynamical instabilities, but there may still exist a small range of parameters where thin black rings are stable. We also discuss the absence of regular real Euclidean sections of black rings, and thermodynamics in the grand-canonical ensemble.

Covariant counter-terms and conserved charges in asymptotically flat spacetimes

Recent work has shown that the addition of an appropriate covariant boundary term to the gravitational action yields a well-defined variational principle for asymptotically flat spacetimes and thus leads to a natural definition of conserved quantities at spatial infinity. Here we connect such results to other formalisms by showing explicitly (i) that for spacetime dimension d ≥ 4 the canonical form of the above-mentioned covariant action is precisely the ADM action, with the familiar ADM boundary terms and (ii) that for d = 4 the conserved quantities defined by counter-term methods agree precisely with the Ashtekar-Hansen conserved charges at spatial infinity.

G2 dualities in D = 5 supergravity and black strings

Five-dimensional minimal supergravity dimensionally reduced on two commuting Killing directions gives rise to a G2 coset model. The symmetry group of the coset model can be used to generate new solutions by applying group transformations on a seed solution. We show that on a general solution the generators belonging to the Cartan and nilpotent subalgebras of G2 act as scaling and gauge transformations, respectively. The remaining generators of G2 form a subalgebra that can be used to generate non-trivial charges. We use these generators to generalize the five-dimensional Kerr string in a number of ways. In particular, we construct the spinning electric and spinning magnetic black strings of five-dimensional minimal supergravity. We analyze physical properties of these black strings and study their thermodynamics. We also explore their relation to black rings.

Microscopics of extremal Kerr from spinning M5 branes

We show that the spinning magnetic one-brane in minimal five-dimensional supergravity admits a decoupling limit that interpolates smoothly between a self-dual null orbifold of AdS3 × S2 and the near-horizon limit of the extremal Kerr black hole times a circle. We use this interpolating solution to understand the field theory dual to spinning M5 branes as a deformation of the Discrete Light Cone Quantized (DLCQ) Maldacena-Stominger-Witten (MSW) CFT. In particular, the conformal weights of the operators dual to the deformation around AdS3 × S2 are calculated. We present pieces of evidence showing that a CFT dual to the four-dimensional extremal Kerr can be obtained from the deformed MSW CFT.

Subtracted geometry from Harrison transformations

A bstractWe consider the rotating non-extremal black hole of N=2 D=4 STU supergravity carrying three magnetic charges and one electric charge. We show that its subtracted geometry is obtained by applying a specific SO(4,4) Harrison transformation on the black hole. As previously noted, the resulting subtracted geometry is a solution of the N=2 S=T=U supergravity.

Extremal solutions of the S3 model and nilpotent orbits of G2(2)

We study extremal black hole solutions of the S3 model (obtained by setting S=T=U in the STU model) using group theoretical methods. Upon dimensional reduction over time, the S3 model exhibits the pseudo-Riemannian coset structure

Integrability of five-dimensional minimal supergravity and charged rotating black holes

{{G} \left/ {{\tilde{K}}} \right.}

Holographic description of non-supersymmetric orbifolded D1-D5-P solutions

with G = G2(2) and

Inverse Scattering and the Geroch Group

\tilde{K} = {\text{S}}{{\text{O}}_0}\left( {2,2} \right)

On the stress tensor for asymptotically flat gravity

. We study nilpotent