Papers
Oh, he seems like an okay person, except for being a little strange in some ways. All day he sits at his desk and scribbles, scribbles, scribbles. Then, at the end of the day, he takes the sheets of paper he’s scribbled on, scrunges them all up, and throws them in the trash can.
A list of scientific papers I have written. They mainly concern black holes, spacetimes, and applications of geometric concepts to understand physics.

Physical stress, mass, and energy for nonrelativistic spinful matter
Michael Geracie, Kartik Prabhu, Matthew M. RobertsFor theories of relativistic matter fields with spin there exist two possible definitions of the stress‑energy tensor, one defined by a variation of the action with the coframes at fixed connection, and the other at fixed torsion. These two stress‑energy tensors do not necessarily coincide and it is the latter that corresponds to the Cauchy stress measured in the lab. In this note we discuss the corresponding issue for non‑relativistic matter theories. We point out that while the physical nonrelativistic stress, momentum, and mass currents are defined by a variation of the action at fixed torsion, the energy current does not admit such a description and is naturally defined at fixed connection. Any attempt to define an energy current at fixed torsion results in an ambiguity which cannot be resolved from the background spacetime data or conservation laws. We also provide computations of these quantities for some simple nonrelativistic actions. 
A Variational Principle for the Axisymmetric Stability of Rotating Relativistic Stars
Kartik Prabhu, Joshua S. Schiffrin, Robert M. WaldIt is well known that all rotating perfect fluid stars in general relativity are unstable to certain non‑axisymmetric perturbations via the Chandrasekhar‑Friedman‑Schutz (CFS) instability. However, the mechanism of the CFS instability requires, in an essential way, the loss of angular momentum by gravitational radiation and, in many instances, it acts on too long a timescale to be physically/astrophysically relevant. It is therefore of interest to examine the stability of rotating, relativistic stars to axisymmetric perturbations, where the CFS instability does not occur. In this paper, we provide a RayleighRitz type variational principle for testing the stability of perfect fluid stars to axisymmetric perturbations, which generalizes to axisymmetric perturbations of rotating stars a variational principle given by Chandrasekhar for spherical perturbations of static, spherical stars. Our variational principle provides a lower bound to the rate of exponential growth in the case of instability. The derivation closely parallels the derivation of a recently obtained variational principle for analyzing the axisymmetric stability of black holes. 
Covariant effective action for a Galilean invariant quantum Hall system
Michael Geracie, Kartik Prabhu, Matthew M. RobertsWe construct effective field theories for gapped quantum Hall systems coupled to background geometries with local Galilean invariance i.e. Bargmann spacetimes. Along with an electromagnetic field, these backgrounds include the effects of curved Galilean spacetimes, including torsion and a gravitational field, allowing us to study charge, energy, stress and mass currents within a unified framework. A shift symmetry specific to single constituent theories constraints the effective action to couple to an effective background gauge field and spin connection that is solved for by a selfconsistent equation, providing a manifestly covariant extension of Hoyos and Son’s improvement terms to arbitrary order in \(m\). 
The First Law of Black Hole Mechanics for Fields with Internal Gauge Freedom
Kartik PrabhuWe derive the first law of black hole mechanics for physical theories based on a local, covariant and gauge‑invariant Lagrangian where the dynamical fields transform non‑trivially under the action of some internal gauge transformations. The theories of interest include General Relativity formulated in terms of tetrads, Einstein‑Yang‑Mills theory and Einstein‑Dirac theory. Since the dynamical fields of these theories have some internal gauge freedom, we argue that there is no natural group action of diffeomorphisms of spacetime on such dynamical fields. In general, such fields cannot even be represented as smooth, globally well‑defined tensor fields on spacetime. Consequently the derivation of the first law by Iyer and Wald cannot be used directly. Nevertheless, we show how such theories can be formulated on a principal bundle and that there is a natural action of automorphisms of the bundle on the fields. These bundle automorphisms encode both spacetime diffeomorphisms and internal gauge transformations. Using this reformulation we define the Noether charge associated to an infinitesimal automorphism and the corresponding notion of stationarity and axisymmetry of the dynamical fields. We first show that we can define certain potentials and charges at the horizon of a black hole so that the potentials are constant on the bifurcate Killing horizon, giving a generalised zeroth law for bifurcate Killing horizons. We further identify the gravitational potential and perturbed charge as the temperature and perturbed entropy of the black hole which gives an explicit formula for the perturbed entropy analogous to the Wald entropy formula. We then obtain a general first law of black hole mechanics for such theories. The first law relates the perturbed Hamiltonians at spatial infinity and the horizon, and the horizon contributions take the form of a “potential times perturbed charge” term. We also comment on the ambiguities in defining a prescription for the total entropy for black holes.

Fields and fluids on curved nonrelativistic spacetimes
Michael Geracie, Kartik Prabhu, Matthew M. RobertsWe consider nonrelativistic curved geometries and argue that the background structure should be generalized from that considered in previous works. In this approach the derivative operator is defined by a Galilean spin connection valued in the Lie algebra of the Galilean group. This includes the usual spin connection plus an additional “boost connection” which parameterizes the freedom in the derivative operator not fixed by torsion or metric compatibility. As an example of this approach we develop the theory of nonrelativistic dissipative fluids and find significant differences in both equations of motion and allowed transport coefficients from those found previously. Our approach also immediately generalizes to systems with independent mass and charge currents as would arise in multicomponent fluids. Along the way we also discuss how to write general locally Galilean invariant nonrelativistic actions for multiple particle species at any order in derivatives. A detailed review of the geometry and its relation to nonrelativistic limits may be found in a companion paper [arXiv:1503.02682].

Curved nonrelativistic spacetimes, Newtonian gravitation and massive matter
Michael Geracie, Kartik Prabhu, Matthew M. RobertsThere is significant recent work on coupling matter to NewtonCartan spacetimes with the aim of investigating certain condensed matter phenomena. To this end, one needs to have a completely general spacetime consistent with local nonrelativisitic symmetries which supports massive matter fields. In particular, one can not impose a priori restrictions on the geometric data if one wants to analyze matter response to a perturbed geometry. In this paper we construct such a Bargmann spacetime in complete generality without any prior restrictions on the fields specifying the geometry. The resulting spacetime structure includes the familiar NewtonCartan structure with an additional gauge field which couples to mass. We illustrate the matter coupling with a few examples. The general spacetime we construct also includes as a special case the covariant description of Newtonian gravity, which has been thoroughly investigated in previous works. We also show how our Bargmann spacetimes arise from a suitable nonrelativistic limit of Lorentzian spacetimes. In a companion paper [arXiv:1503.02680] we use this Bargmann spacetime structure to investigate the details of matter couplings, including the NoetherWard identities, and transport phenomena and thermodynamics of nonrelativistic fluids.

Black Hole Instabilities and Exponential Growth
Kartik Prabhu, Robert M. WaldRecently, a general analysis has been given of the stability with respect to axisymmetric perturbations of stationaryaxisymmetric black holes and black branes in vacuum general relativity in arbitrary dimensions. It was shown that positivity of canonical energy on an appropriate space of perturbations is necessary and sufficient for stability. However, the notions of both “stability” and “instability” in this result are significantly weaker than one would like to obtain. In this paper, we prove that if a perturbation of the form \(£_t \delta g\)—with \(\delta g\) a solution to the linearized Einstein equation—has negative canonical energy, then that perturbation must, in fact, grow exponentially in time. The key idea is to make use of the \(t\) or (\(t\)\(\phi\))reflection isometry, \(i\), of the background spacetime and decompose the initial data for perturbations into their odd and even parts under \(i\). We then write the canonical energy as \(\mathscr E = \mathscr K + \mathscr U\), where \(\mathscr K\) and \(\mathscr U\), respectively, denote the canonical energy of the odd part (kinetic energy) and even part (potential energy). One of the main results of this paper is the proof that \(\mathscr K\) is positive definite for any black hole background. We use \(\mathscr K\) to construct a Hilbert space \(\mathscr H\) on which time evolution is given in terms of a selfadjoint operator \(\tilde {\mathcal A}\), whose spectrum includes negative values if and only if \(\mathscr U\) fails to be positive. Negative spectrum of \(\tilde{\mathcal A}\) implies exponential growth of the perturbations in \(\mathscr H\) that have nontrivial projection into the negative spectral subspace. This includes all perturbations of the form \(£_t \delta g\) with negative canonical energy. A “RayleighRitz” type of variational principle is derived, which can be used to obtain lower bounds on the rate of exponential growth.

On the static Lovelock black holes
Naresh Dadhich, Josep M. Pons, Kartik PrabhuWe consider static spherically symmetric Lovelock black holes and generalize the dimensionally continued black holes in such a way that they asymptotically for large r go over to the ddimensional Schwarzschild black hole in dS/AdS spacetime. This means that the master algebraic polynomial is not degenerate but instead its derivative is degenerate. This family of solutions contains an interesting class of pure Lovelock black holes which are the Nth order Lovelock \(\Lambda\)vacuum solutions having the remarkable property that their thermodynamical parameters have the universal character in terms of the event horizon radius. This is in fact a characterizing property of pure Lovelock theories. We also demonstrate the universality of the asymptotic Einstein limit for the Lovelock black holes in general.

Higher order geometric flows on three dimensional locally homogeneous spaces
Sanjit Das, Kartik Prabhu, Sayan KarWe analyse second order (in Riemann curvature) geometric flows (unnormalised) on locally homogeneous three manifolds and look for specific features through the solutions (analytic whereever possible, otherwise numerical) of the evolution equations. Several novelties appear in the context of scale factor evolution, fixed curves, phase portraits, approaches to singular metrics, isotropisation and curvature scalar evolution. The distinguishing features linked to the presence of the second order term in the flow equation are pointed out. Throughout the article, we compare the results obtained, with the corresponding results for unnormalized Ricci flows.

Thermodynamical universality of the Lovelock black holes
Naresh Dadhich, Josep M. Pons, Kartik PrabhuThe necessary and sufficient condition for the thermodynamical universality of the static spherically symmetric Lovelock black hole is that it is the pure Lovelock \(\Lambda\)vacuum solution. By universality we mean the thermodynamical parameters: temperature and entropy always bear the same relationship to the horizon radius irrespective of the Lovelock order and the spacetime dimension. For instance, the entropy always goes in terms of the horizon radius as \(r_h\) and \(r_h^2\) respectively for odd and even dimensions. This universality uniquely identifies the pure Lovelock black hole with \(\Lambda\).

On higher order geometric and renormalisation group flows
Kartik Prabhu, Sanjit Das, Sayan KarRenormalisation group flows of the bosonic nonlinear \(\sigma\)model are governed, perturbatively, at different orders of \(\alpha'\), by the perturbatively evaluated \(\beta\)functions. In regions where \(\frac{\alpha'}{R_c^2} << 1\) the flow equations at various orders in \(\alpha'\) can be thought of as approximating the full, nonperturbative RG flow. On the other hand, taking a different viewpoint, we may consider the abovementioned RG flow equations as viable geometric flows in their own right and without any reference to the RG aspect. Looked at as purely geometric flows where higher order terms appear, we no longer have the perturbative restrictions. In this paper, we perform our analysis from both these perspectives using specific target manifolds such as \(S^2\), \(H^2\), unwarped \(S^2 \times H^2\) and simple warped products. We analyze and solve the higher order RG flow equations within the appropriate perturbative domains and find the corrections arising due to the inclusion of higher order terms. Such corrections, within the perturbative regime, are shown to be small and they provide an estimate of the error which arises when higher orders are ignored.
We also investigate the higher order geometric flows on the same manifolds and figure out generic features of geometric evolution, the appearance of singularities and solitons. The aim, in this context, is to demonstrate the role of the higher order terms in modifying the flow. One interesting aspect of our analysis is that, separable solutions of the higher order flow equations for simple warped spacetimes, correspond to constant curvature Antide Sitter (AdS) spacetime, modulo an overall flowparameter dependent scale factor. The functional form of this scale factor (which we obtain) changes on the inclusion of successive higher order terms in the flow.

Energetics of a rotating charged black hole in 5dimensional supergravity
Kartik Prabhu, Naresh DadhichWe investigate the properties of the event horizon and static limit for a charged rotating black hole solution of minimal supergravity theory in (1 + 4) dimension. Unlike the fourdimensional case, there are in general two rotations, and they couple to both mass and charge. This gives rise to much richer structure to ergosphere leading to energy extraction even for axial fall. Another interesting feature is that the metric in this case is sensitive to the sign of the Maxwell charge.

Ricci flow of unwarped and warped product manifolds
Sanjit Das, Kartik Prabhu, Sayan KarWe analyse Ricci flow (normalised/unnormalised) of product manifolds — unwarped as well as warped, through a study of generic examples. First, we investigate such flows for the unwarped scenario with manifolds of the type \(\mathbb S^n\times \mathbb S^m\), \(\mathbb S^n\times \mathbb H^m\), \(\mathbb H^m\times \mathbb H^n\) and also, similar multiple products. We are able to single out generic features such as singularity formation, isotropisation at particular values of the flow parameter and evolution characteristics. Subsequently, motivated by warped braneworlds and extra dimensions, we look at Ricci flows of warped spacetimes. Here, we are able to find analytic solutions for a special case by variable separation. For others we numerically solve the equations (for both the forward and backward flow) and draw certain useful inferences about the evolution of the warp factor, the scalar curvature as well the occurence of singularities at finite values of the flow parameter. We also investigate the dependence of the singularities of the flow on the inital conditions. We expect our results to be useful in any physical/mathematical context where such product manifolds may arise.