arXiv subjects

Gautam Satishchandran

Publications and source records attributed to Gautam Satishchandran.

22 records · Page 2Linked to original sources

arXiv2021

Blázquez-Salcedo-Knoll-Radu Wormholes Are Not Solutions to the Einstein-Dirac-Maxwell Equations

Recently, Blázquez-Salcedo, Knoll, and Radu (BSKR) have given a class of static, spherically symmetric, traversable wormhole spacetimes with Dirac and Maxwell fields. The BSKR wormholes are obtained by joining a classical solution to the Einstein-Dirac-Maxwell (EDM) equations on the "up" side of the wormhole ($r \geq 0$) to a corresponding solution on the "down" side of the wormhole ($r \leq 0$). However, it can be seen that the BSKR metric fails to be $C^3$ on the wormhole throat at $r=0$. We prove that if the matching were done in such a way that the resulting spacetime metric, Dirac field, and Maxwell field composed a solution to the EDM equations in a neighborhood of $r=0$, then all of the fields would be smooth at $r=0$ in a suitable gauge. Thus, the BSKR wormholes cannot be solutions to the EDM equations. The failure of the BSKR wormholes to solve the EDM equations arises both from the failure of the Maxwell field to satisfy the required matching conditions (which implies the presence of an additional shell of charged matter at $r=0$) and, more significantly, from the failure of the Dirac field to satisfy required matching conditions (which implies the presence of a spurious source term for the Dirac field at $r=0$.

gr-qc↗

arXiv2019

The Asymptotic Behavior of Massless Fields and the Memory Effect

We investigate the behavior of massless scalar, electromagnetic, and linearized gravitational perturbations near null infinity in d \geq 4 dimensional Minkowski spacetime (of both even and odd dimension) under the assumption that these fields admit a suitable expansion in 1/r. We also investigate the behavior of asymptotically flat, nonlinear gravitational perturbations near null infinity in all dimensions d\geq 4. We then consider the memory effect in fully nonlinear general relativity. We show that in even dimensions, the memory effect first arises at Coulombic order--i.e., order 1/r^{d-3}--and can naturally be decomposed into `null memory' and `ordinary memory.' Null memory is associated with an energy flux to null infinity. We show that ordinary memory is associated with the metric failing to be stationary at one order faster fall-off than Coulombic in the past and/or future. In odd dimensions, we show that the total memory effect at Coulombic order and slower fall-off always vanishes. Null memory is always of `scalar type', but the ordinary memory can be of any (i.e., scalar, vector, or tensor) type. In 4-spacetime dimensions, we give an explicit example in linearized gravity which gives rise to a nontrivial vector (i.e., magnetic parity) ordinary memory effect at order 1/r. We show that scalar memory is described by a diffeomorphism; vector and tensor memory cannot be. In d=4 dimensions, we show that there is a close relationship between memory and the charge and flux expressions associated with supertranslations. We analyze the behavior of solutions that are stationary at Coulombic order and show how these suggest `antipodal matching' between future and past null infinity, which gives rise to conservation laws. The relationship between memory and infrared divergences of the `out' state in quantum gravity is analyzed, and the nature of the `soft theorems' is explained.

gr-qc↗

arXiv2018

Memory Effect for Particle Scattering in Odd Spacetime Dimensions

We investigate the gravitational memory effect for linearized perturbations off of Minkowski space in odd spacetime dimensions $d$ by examining the effects of gravitational radiation from classical point particle scattering. We also investigate analogous memory effects for electromagnetic and scalar radiation. We find that there is no gravitational memory effect in all odd dimensions. For scalar and electromagnetic fields, there is no memory effect for $d\geq 7$; for $d=3$ there is an infinite momentum memory effect, whereas for $d=5$ there is no momentum memory effect but the displacement of a test particle will grow unboundedly with time. Our results are further elucidated by analyzing the memory effect for any slowly moving source of compact spatial support in odd dimensions.

gr-qc↗

arXiv2015

Cosmography of KNdS Black Holes and Isentropic Phase Transitions

We present a new analysis of Kerr-Newman-deSitter black holes in terms of thermodynamic quantities that are defined in the observable portion of the universe; between the black hole and cosmological horizons. In particular, we replace the mass $m$ with a new 'area product' parameter $X$. The physical region of parameter space is found analytically and thermodynamic quantities are given by simple algebraic functions of these parameters. We find that different geometrical properties of the black holes are usefully distinguished by the sum of the black hole and cosmological entropies. The physical parameter space breaks into a region in which the total entropy, together with $Λ$, $a$ and $q$ uniquely specifies the black hole, and a region in which there is a two-fold degeneracy. In this latter region, there are isentropic pairs of black holes, having the same $Λ$, $a$, and $q$, but different $X$. The thermodynamic volumes and masses differ in such that there are high and low density branches. The partner spacetimes are related by a simple inversion of $X$, which has a fixed point at the state of maximal total entropy. We compute the compressibility at fixed total entropy and find that it diverges at the maximal entropy point. Hence a picture emerges of high and low density phases merging at this critical point.

hep-th↗