arXiv subjects

Robert Penna

Publications and source records attributed to Robert Penna.

4 recordsLinked to original sources

arXiv2025

Semiclassical cosmic string evaporation

We describe a new tunneling solution for the decay of a cosmic string into a burst of gravitational waves. We find the relevant instanton and compute the tunneling rate. Locally, our solution is just an analytic continuation of the Kerr metric (but there is no black hole in our solution). An interesting feature of our result is that there is a conical singularity in the initial state but there is no singularity in the final state. This demonstrates that singularities can disappear in quantum gravity in ways that are impossible in classical gravity.

hep-th↗

arXiv2025

$\mathrm{SL}(2,\mathbb{R})$ families of Kerr black holes

The stationary, axisymmetric sector of vacuum general relativity (with zero cosmological constant) enjoys an $\mathrm{SL}(2,\mathbb{R})$ symmetry called the Matzner-Misner group. We study the action of the Matzner-Misner group on the Kerr black hole. We show that the group acts naturally on a three parameter generalization of the usual two parameter Kerr solution. The new parameter represents a large diffeomorphism which gives the spacetime an asymptotic angular velocity. We explain how the $\mathrm{SL}(2,\mathbb{R})$ symmetry organizes the space of three parameter Kerr solutions into the classical analogue of principal series representations. We show that the $\mathrm{SL}(2,\mathbb{R})$ Casimir operator is the Bekenstein-Hawking entropy. The Matzner-Misner group sits inside a much larger Kac-Moody symmetry called the Geroch group. We show that the Kac-Moody level of the Kerr black hole is the Bekenstein-Hawking entropy.

hep-th↗

arXiv2024

Soft zero for cylindrical gravitational waves

The graviton S-matrix has a famous soft pole. We show that the S-matrix for cylindrical gravitational waves has a soft zero. The soft pole for ordinary gravitons comes from a Ward identity for supertranslation symmetry at asymptotic infinity. We show that the soft zero for cylindrical gravitational waves comes from a Ward identity for Geroch symmetry at asymptotic infinity. Because it is a zero rather than a pole, there is no memory effect. Overall, this soft zero is a manifestation of Geroch symmetry and of the extraordinary simplicity of cylindrical gravitational waves.

hep-th↗

arXiv2023

Scattering amplitudes for cylindrical gravitational waves

Cylindrical gravitational waves are interesting because they enjoy an infinite dimensional symmetry called Geroch symmetry. In this paper, we compute the 2-particle tree-level S-matrix for cylindrical gravitational waves. The model we use is a dimensional reduction of general relativity to two spacetime dimensions. The reduced theory is a nonlinear sigma model. We discuss an SO(2) symmetry of the amplitudes which is a special case of Geroch symmetry.

hep-th↗