Proposal to Search for the CP Violating Electromagnetic Vacuum Angle at the Event Horizon Telescope

TL;DR

The paper investigates a CP-violating electromagnetic vacuum angle, $\theta_{QED}$, as imprinted on black-hole horizons via the Fischler-Kundu (FK) horizon Hall effect and proposes an observable ${\cal C}$ derived from EHT polarization maps. By formulating a parity-odd, time-persistent boundary condition for the horizon currents, the authors show how a nonzero ${\cal C}$ could signal horizon-scale CP violation, distinct from frequency-dependent Faraday plasma effects. They derive that ${\cal C}$ can be expressed as a net EVPA winding around the photon ring and argue that a time-averaged, frequency-independent ${\cal C}$ would favor the FK signal, while Faraday rotation requires persistent azimuthal RM gradients to mimic it. The work outlines strategies for separating FK from plasma physics via multi-frequency EHT observations (ngEHT) and time averaging, with cross-comparison between Sgr A* and M87* as a test of a universal horizon signal that could constrain $\theta_{QED}$. If successful, this approach would provide a rare probe of CP-violating physics at black hole horizons and offer a concrete observational handle on horizon boundary conditions in strong gravity.

Abstract

We examine the possibility that evidence for a non-zero value of the CP violating $ \frac{e^2 }{32π^2}θ_{EM} \int d^4 x {\vec E}\cdot {\vec B}$ coupling might be extracted from Event Horizon Telescope observations of the black holes SgA* and M87*. The Fischler-Kundu\cite{FK} effect predicts a universal Hall current in the relaxation of charge falling onto the black hole horizon. We argue that this leads to a non-zero value of a certain CP-violating observable ${\cal C}$, defined below. The effect can be masked by parity violating plasma currents. In particular, evidence for polarization flips \cite{flip} in the signals from M87* indicate strong plasma effects in the data. We suggest that time averaging the data over periods including the flip might leave over a residual that would be an indicator of the FK signal. In addition, similarities in the polarization patterns between the two very different black holes, and a part of the signal that is uniform in frequency, might enable us to distinguish the universal topological signal from source and frequency dependent plasma effects. Current data does not appear to be sufficient to perform such a test.