Relic gravitational waves from colliding bubbles and cosmic turbulence

TL;DR

This paper compares gravitational wave production mechanisms during a cosmological first order phase transition, highlighting the conditions under which turbulence dominates and the potential detectability by LISA.

Contribution

It provides a detailed analysis of the relative importance of bubble collisions and turbulence in generating gravitational waves during phase transitions, with implications for LISA detection.

Findings

Turbulence dominates over bubble collisions in gravitational wave signals for strong first order transitions.

A strong first order electroweak phase transition (alpha > 0.2) can produce detectable gravitational waves.

The gravitational wave signal peaks within LISA's sensitivity band for certain parameter ranges.

Abstract

A stochastic background of gravitational waves can be generated during a cosmological first order phase transition, at least by two distinct mechanisms: collisions of true vacuum bubbles and turbulence in the cosmic fluid. I compare these two contributions, analyzing their relative importance for a generic phase transition. In particular, a first order electroweak phase transition is expected to generate a gravitational wave signal peaked at a frequency which today falls just within the band of the planned space interferometer LISA. For this transition, I find constraints for the relevant parameters in order to produce a signal within the reach of the sensitivity of LISA. The result is that the transition must be strongly first order, alpha > 0.2. In this regime the signal coming from turbulence dominates over that from colliding bubbles.