Black holes are mysterious objects in space that physicists use as a tool to learn about gravity. On the experimental side, physicists use data from large telescopes to further understand how our universe works. On the theoretical side, the mathematics behind how black holes work have become our main guide towards a more fundamental interpretation of our universe.
Currently, physicists, including myself, are trying to explain the properties of black holes. This turns out to be quite a challenge! One way to tackle this is to use a principle that relates the gravitational theory of black holes to another physical theory called quantum field theory. This principle has become a dictionary of sorts, allowing us to translate ideas about black holes, such as entropy and temperature, between gravitational theory and quantum field theory.
Recently, a group of scientists has predicted the entropy of black holes on the quantum side using a unique method, called the Cardy limit. The Cardy limit is a limit on the angular momentum of the black hole and corresponds to highly spinning black holes. No one has yet confirmed this method on the gravity side.
My collaborators and I have taken on this challenge to verify that the Cardy limit is valid on the gravity side and reproduces the expected entropy. It turns out this method works best when we zoom towards the event horizon of a black hole — a region of spacetime that acts a bit strange. If we get too close to the event horizon, nothing — not even light — can escape and we are destined to fall into the black hole! Yet, this region of spacetime along with the Cardy limit lead us to the same entropy and other properties of the black hole that were computed on the quantum side.
Our paper is one of many recent results that aim to solve the riddles behind black holes. It is a step towards getting closer to a full understanding our of universe.