arXiv subjects

Abhishek Hegade K R

Publications and source records attributed to Abhishek Hegade K R.

3 recordsLinked to original sources

arXiv2023

How Do Axisymmetric Black Holes Grow Monopole and Dipole Hair?

We study the dynamical formation of scalar monopole and dipole hair in scalar Gauss-Bonnet theory and dynamical Chern-Simons theory. We prove that the spherically-symmetric mode of the dipole hair is completely determined by the product of the mass of the spacetime and the value of the monopole hair. We then show that the dynamics of the $\ell=1$ mode of the dipole hair is intimately tied to the appearance of the event horizon during axisymmetric collapse, which results in the radiation of certain modes that could have been divergent in the future of the collapse. We confirm these analytical predictions by simulating the gravitational collapse of a rapidly rotating neutron star in the decoupling limit, both in scalar Gauss-Bonnet and dynamical Chern-Simons theory. Our results, combined with those of Ref.~\cite{R:2022cwe}, provide a clear physical picture of the dynamics of scalar monopole and dipole radiation in axisymmetric and spherical gravitational collapse in these theories.

gr-qc↗

arXiv2023

The non-relativistic limit of first-order relativistic viscous fluids

Out-of-equilibrium effects may play an important role in the dynamics of neutron star mergers and in heavy-ion collisions. Bemfica, Disconzi, Noronha and Kovtun (BDNK) recently derived a causal, locally well-posed, and modally stable relativistic fluid model that incorporates the effects of viscosity and heat diffusion. We study the non-relativistic limit of this fluid model and show that causality for relativistic motion restricts the transport coefficients in the non-relativistic limit. This restriction provides an upper bound on the ratio of the shear viscosity to the entropy density for fluids that can be described as relativistic within the BDNK model and can be exactly modeled using the Navier-Stokes equation in the non-relativistic limit. Furthermore, we show that the Fourier law of heat conduction must be modified by higher gradient corrections for such fluids. We also show that the non-relativistic limit of the BDNK equations of motion are never hyperbolic, in contrast to the non-relativistic limit of extended variable models, whose non-relativistic equations of motion can be hyperbolic or not depending on the scaling of the transport coefficients present in the auxiliary equations for viscous degrees of freedom.

gr-qc↗

arXiv2023

Where and why does Einstein-Scalar-Gauss-Bonnet theory break down?

We present a systematic exploration of the loss of predictivity in Einstein-scalar-Gauss-Bonnet (ESGB) gravity. We first formulate a gauge covariant method of characterizing the breakdown of the hyperbolicity of the equations of motion in the theory. With this formalism, we show that strong geodesic focusing leads to the breakdown of hyperbolicity, and the latter is unrelated to the violation of the null convergence condition. We then numerically study the hyperbolicity of the equations during gravitational collapse for two specific ESGB gravity theories: "shift symmetric Gauss-Bonnet gravity" and a version of the theory that admits "spontaneously scalarized" black holes. We devise a "phase space" model to describe the end states for a given class of initial data. Using our phase space picture, we demonstrate that the two theories we consider remain predictive (hyperbolic) for a range of GB couplings. The range of couplings, however, is small, and thus, the presence of "spontaneously scalarized" solutions requires fine-tuning of initial data. Our results, therefore, cast doubt as to whether scalarized black hole solutions can be realistically realized in Nature even if ESGB gravity happened to be the correct gravitational description.

gr-qc↗