Coloured gravitational instantons, the strong CP problem and the companion axion solution

Understand

Quantum gravity introduces a new source of the combined parity (CP) violation in gauge theories.

• We argue that this new CP violation gets bundled with the strong CP violation through the coloured gravitational instantons.
• Consequently, the standard axion solution to the strong CP problem is compromised.
• Further, we argue that the ultimate solution to the strong CP problem must involve at least one additional axion particle.