Noether Current, Horizon Virasoro Algebra and Entropy

TL;DR

This paper introduces a method to derive the entropy density of null surfaces using a Virasoro algebra constructed from the off-shell Noether current, applicable to various gravitational models and independent of field equations.

Contribution

It presents a novel off-shell approach to define a Virasoro algebra from the Noether current, enabling entropy calculation for null surfaces in general gravitational theories.

Findings

Reproduces Wald entropy for Lanczos-Lovelock models

Works off-shell without field equations

Defines entropy density for any null surface acting as a local Rindler horizon

Abstract

We provide a simple and straightforward procedure for defining a Virasoro algebra based on the diffeomorphisms near a null surface in a spacetime and obtain the entropy density of the null surface from its central charge. We use the off-shell Noether current corresponding to the diffeomorphism invariance of a gravitational Lagrangian $L(g_{ab},R_{abcd})$ and define the Virasoro algebra from its variation. This allows us to identify the central charge and the zero mode eigenvalue using which we obtain the entropy density of the Killing horizon. Our approach works for all Lanczos-Lovelock models and reproduces the correct Wald entropy. The entire analysis is done off-shell without using the field equations and allows us to define an entropy density for any null surface which acts as a local Rindler horizon for a particular class of observers.