Black Holes and the Strong CP Problem

TL;DR

This paper proposes that the existence of at least one black hole in the universe can resolve the strong CP problem in QCD by eliminating the need for a small CP-violating term, drawing an analogy to charge quantization.

Contribution

It introduces a novel solution to the strong CP problem based on black hole existence, linking gravitational and quantum gauge theories in a new way.

Findings

Black holes can eliminate the strong CP problem

Analogy to Dirac's charge quantization argument

Proposes a universe-wide implication of black hole existence

Abstract

The strong CP problem is that SU(3) gauge field instantons naturally induce a CP violating term in the QCD Lagrangian which is constrained by experiment to be very small for no obvious reason. We show that this problem disappears if one assumes the existence of at least one black hole somewhere in the universe. The argument is reminiscent of Dirac's argument for the quantization of charge, in which the existence of one monople anywhere in the universe suffices to require the quantization of electric charge everywhere.