Gravitational waves from a very strong electroweak phase transition

TL;DR

The study investigates the stochastic gravitational wave background from very strong electroweak phase transitions in Standard Model extensions, modeling the full transition from nucleation to percolation with friction saturation and hydrodynamics. It computes GW spectra from bubble collisions, sound waves, and MHD turbulence, using a bag-EOS framework to relate transition parameters ($\alpha$, $\beta$, $v_w$) to observable signals for eLISA designs. The results indicate that sound waves and turbulence typically dominate the GW output, while bubble collisions become more relevant in runaway scenarios but often at lower frequencies, reducing detectability; heavier extra bosons generally produce stronger signals. The work identifies which new-physics scenarios maximize GW observability and stresses the importance of realistic wall dynamics and percolation in predicting the stochastic GW background relevant for space-based detectors.

Abstract

We investigate the production of a stochastic background of gravitational waves in the electroweak phase transition. We consider extensions of the Standard Model which can give very strongly first-order phase transitions, such that the transition fronts either propagate as detonations or run away. To compute the bubble wall velocity, we estimate the friction with the plasma and take into account the hydrodynamics. We track the development of the phase transition up to the percolation time, and we calculate the gravitational wave spectrum generated by bubble collisions, magnetohydrodynamic turbulence, and sound waves. For the kinds of models we consider, we find parameter regions for which the gravitational waves are potentially observable at the planned space-based interferometer eLISA. In such cases, the signal from sound waves is generally dominant, while that from bubble collisions is the least significant of them. Since the sound waves and turbulence mechanisms are diminished for runaway walls, the models with the best prospects of detection at eLISA are those which do not have such solutions. In particular, we find that heavy extra bosons provide stronger gravitational wave signals than tree-level terms.

Figures (12)

• Figure 1: Left: thermal instanton action as a function of temperature, for $g=2$ and several values of $h$. Center: the critical temperature $T_c$, the temperature $T_i$ at the onset of nucleation, and the temperature $T_p$ at percolation. Right: time intervals between the temperature $T_c$ and the temperature $T_i$ (dashed line) and between $T_c$ and $T_p$ (solid line).
• Figure 2: The wall velocity for the SM with $g=2$ extra bosons with mass $m=h\phi$.
• Figure 3: Left panel: peak amplitude of the three components of the GW spectrum for the SM with $g=2$ extra boson d.o.f., as a function of the coupling $h$. Right panel: peak amplitude vs. peak frequency. The stars indicate the values of $h$ given in Table \ref{['tabbos']}. The colored lines correspond to the sensitivity curves of eLISA discussed in Sec. \ref{['gwgen']}.
• Figure 4: The GW spectra for the benchmark points of Table \ref{['tabbos']}. The blue dashed line denotes the contribution from bubble collisions, the red dashed line the contribution from mhd turbulence, the green dashed line the contribution from sound waves, and the black line the sum of the three signals. The shaded areas represent the regions detectable by the different eLISA configurations.
• Figure 5: Wall velocity (left panel) and net force (right panel) for the model with a cubic term, as a function of the parameter $A$.