String instabilities in black hole spacetimes

Abstract

We study the emergence of string instabilities in $D$-dimensional black hole spacetimes (Schwarzschild and Reissner-Nordström), and de Sitter space (in static coordinates to allow a better comparison with the black hole case). We solve the first-order string fluctuations around the center-of-mass motion at spatial infinity, near the horizon, and at the spacetime singularity. We find that the time components are always well behaved in the three regions and in the three backgrounds. The radial components are unstable: imaginary frequencies develop in the oscillatory modes near the horizon, and the evolution is like ${(\tau -{\tau }_0)}^{-P}$ ($P>\mathrm{}0\mathrm{}$) near the spacetime singularity $r\to 0$, where the world-sheet time $(\tau -{\tau }_0)\to 0$ and the proper string length grows infinitely. In the Schwarzschild black hole, the angular components are always well behaved, while in the Reissner-Nordström case they develop instabilities inside the horizon near $r\to 0$ where the repulsive effects of the charge dominate over those attractive of the mass. In general, whenever large enough repulsive effects in the gravitational background are present, string instabilities develop. In de Sitter space, all the spatial components exhibit instability. The infalling of the string to the black hole singularity is like the motion of a particle in a potential $\gamma {(\tau -{\tau }_0)}^{-2\mathrm{}}$ where $\gamma$ depends on the $D$ spacetime dimensions and string angular momentum, with $\gamma >0\mathrm{}$ for Schwarzschild and $\gamma <0\mathrm{}$ for Reissner-Nordström black holes. For $(\tau -{\tau }_0)\to 0$ the string ends trapped by the black hole singularity.