Enhancement of second-order gravitational waves at Q-ball decay

Understand

The recent observation of $^4$He favors a large lepton asymmetry at the big bang nucleosynthesis.

• If Q-balls with a lepton charge decay after the electroweak phase transition, such a large lepton asymmetry can be generated without producing too large baryon asymmetry.
• In this scenario, Q-balls dominate the universe before the decay and induces the sharp transition from the early matter-dominated era to the radiation-dominated era.
• In this transition, the gravitational waves (GWs) are enhanced through a second-order effect of the scalar perturbations.