Gravitational waves from a first order electroweak phase transition: a brief review (1705.01783v3)

Abstract: We review the production of gravitational waves by an electroweak first order phase transition. The resulting signal is a good candidate for detection at next-generation gravitational wave detectors, such as LISA. Detection of such a source of gravitational waves could yield information about physics beyond the Standard Model that is complementary to that accessible to current and near-future collider experiments. We summarise efforts to simulate and model the phase transition and the resulting production of gravitational waves.

• The paper demonstrates that a first order electroweak phase transition, enabled by extended scalar sectors, produces gravitational waves that may signal physics beyond the Standard Model.
• The paper compares simulation methods, such as the envelope approximation and field-fluid model, to capture bubble collisions, sound shell dynamics, and turbulence effects.
• The paper emphasizes that space-based detectors like LISA could detect these gravitational waves, offering complementary insights to collider experiments and electroweak baryogenesis.

Gravitational Waves Produced by Electroweak Phase Transitions

The paper "Gravitational waves from a first order electroweak phase transition: a brief review" by David J. Weir provides a comprehensive examination of gravitational waves as potential signatures of physics beyond the Standard Model. Specifically, it addresses the gravitational waves produced by a first-order electroweak phase transition. The author argues that space-based gravitational wave detectors like LISA could be instrumental in detecting these waves, offering complementary insights to particle colliders by probing electroweak phase transitions in the early universe.

Electroweak Phase Transition and Model Extensions

A first-order phase transition in the electroweak sector offers an intriguing avenue for gravitational wave production under certain extensions to the Standard Model. Currently, the electroweak phase transition in the Standard Model is a crossover, not a first-order transition. However, the introduction of extra scalar fields, such as a singlet or a second Higgs doublet, can yield a first-order phase transition at the electroweak scale. This transition is critical not only for gravitational wave sourcing but also for scenarios of baryogenesis, particularly electroweak baryogenesis, which remains a viable pathway to address the observed baryon asymmetry in the universe.

Gravitational Wave Production Mechanisms

The paper discusses two primary motivations for studying gravitational waves from electroweak phase transitions: the potential for baryogenesis and the generation of detectable gravitational waves. The author outlines several factors essential for the transition's dynamics, including the motion of bubble walls and the associated energy budget. Key parameters influencing gravitational wave production are outlined, such as the phase transition strength $\alpha$ and the wall velocity $v_w$.

The work recognizes tension between the desired wall velocities for efficient baryogenesis and those for optimal gravitational wave production. Wall velocities lower than the speed of sound facilitate baryogenesis processes at the transition front, whereas higher velocities enhance gravitational wave energy density. This trade-off is scrutinized with respect to variant models where electroweak baryogenesis facilitates faster wall speeds through symmetry restoration behind the bubble wall.

Simulations and Modelling: Techniques and Challenges

The challenges of modelling gravitational wave production during such phase transitions are thoroughly addressed. Simulations of bubble collisions, the fluid dynamics of the sound shell, and modeling techniques such as the envelope approximation and the field-fluid model are discussed. The paper expounds on the envelope approximation, which simplifies the complex dynamics by considering infinitesimally thin bubble walls that disappear upon collision. This approach is contrasted with the more detailed field-fluid model, which accounts for the coupling between the scalar field and the relativistic fluid.

Simulation results show that the acoustic source, caused by expanding sound shells from fluid kinetic energy post-collision, is pivotal. However, turbulence resulting from these expansions remains less understood, necessitating further investigation and simulation to properly capture this phenomenon.

Implications and Future Prospects

The potential detection of gravitational waves from an electroweak phase transition holds significant implications for understanding physics beyond the Standard Model. Such detections could validate models positing new scalar fields and provide key insights into the processes behind electroweak baryogenesis.

The paper also acknowledges the need for precise simulation results to determine crucial transition parameters and quantifies the opportunity for refining observational strategies with LISA and similar detectors. As methodologies improve, especially regarding turbulence modeling and acoustic phase changes, these avenues could confirm or refute predictions stemming from theoretical model extensions.

In conclusion, the pursuit of gravitational wave evidence from electroweak phase transitions represents a promising frontier. Discovering such phenomena could elucidate the underlying dynamics of the early universe and guide future theoretical developments, providing paths to reconcile current model predictions with observed cosmic properties. The paper motivates further research to refine simulation techniques and enhance our understanding of fundamental physics processes that shaped the universe.