Within the standard framework in which electroweak sphaleron processes relate lepton and baryon numbers, we derive an upper limit on the amplitude of a chiral gravitational-wave background produced prior to the electroweak epoch. This bound is independent of the production time of chiral GWs for superhorizon modes, while it becomes sensitive to the production time for subhorizon modes. For sufficiently high reheating temperatures, the bound becomes significantly more stringent than the conventional big bang nucleosynthesis constraints at frequencies above the MHz scale, thereby providing a powerful and model-independent probe of parity-violating physics in the early Universe.

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• Baryogenesis & leptogenesis
• Gravitational waves