Large Gravitational Wave Background Signals in Electroweak Baryogenesis Scenarios

TL;DR

This work shows that, once bubble-wall hydrodynamics are properly included, the plasma velocity in front of the wall, $v_+$, is the crucial quantity for electroweak baryogenesis rather than the wall speed $V_w$. As phase transitions strengthen, $v_+$ can be much smaller than $V_w$, satisfying diffusion bounds while allowing large $V_w$ and potentially sizable gravitational-wave production from bubble collisions. Consequently, electroweak baryogenesis and a detectable stochastic gravitational-wave background are not mutually exclusive; moderate-strength transitions may yield observable signals at BBO, while extremely strong transitions required for LISA are less likely and may demand fine-tuning or stability of hybrid solutions. This has important implications for model-building and interpreting future gravitational-wave observations in the context of baryogenesis.

Abstract

The bubble wall velocity in an electroweak first order phase transition is a key quantity both for electroweak baryogenesis and for the production of a stochastic background of gravitational waves that may be probed in the future through gravitational wave experiments like LISA or BBO. We show that, contrary to the conclusion drawn from previous studies, it is actually possible to generate a potentially large gravitational wave signal while satisfying the requirements for viable electroweak baryogenesis, once the effects of the hydrodynamics of bubble growth are taken into account. Then, the observation of a large gravitational wave background from the electroweak phase transition would not necessarily rule out electroweak baryogenesis as the mechanism having generated the observed baryon asymmetry of the universe.

Figures (4)

• Figure 1: Movement of plasma volume elements with time.
• Figure 2: Relation $v_{+}(\alpha_{N}, V_{w})$.
• Figure 3: Regions in the ($V_{w}, \alpha_{N}$) plane compatible with $v_{+} < 0.15$, $v_{+} < 0.2$ and $v_{+} < 0.3$ (region above each line).
• Figure 4: $\Omega_{GW} h^2 (f)$ for various values of $\alpha_N$ and $V_w = 0.15$ (blue lines) or $v_{+} = 0.15$ (red lines).