Gravitational waves from cosmological phase transitions

TL;DR

This paper analyzes the gravitational wave signals produced by first order cosmological phase transitions, especially electroweak, and predicts their detectability by LISA based on the dynamics of bubble collisions, turbulence, and magnetic fields.

Contribution

It provides a detailed theoretical prediction of the gravitational wave spectrum from first order phase transitions, linking spectral features to source characteristics and assessing detectability by LISA.

Findings

Gravitational wave signals are detectable if phase transition lasts about 1% of the Hubble time.

Turbulent motions with about 20% of total energy density produce observable signals.

Spectrum features depend on bubble collision dynamics, turbulence, and magnetic field amplification.

Abstract

First order phase transitions in the early universe can give rise to a stochastic background of gravitational waves. A hypothetical first order electroweak phase transition is particularly interesting in this respect, since the signal is in the good frequency range to be detectable by the space interferometer LISA. Three main processes lead to the production of the gravitational wave signal: the collision of the broken phase bubbles, the magnetohydrodynamical turbulence in the plasma stirred by the bubble collisions, and the magnetic fields amplified by the magnetohydrodynamical turbulence. The main features of the gravitational wave spectrum, such as the peak frequency, the amplitude, and the slopes both at low and high wave-number can be predicted by general arguments based on the characteristics of the source: in particular, the structure of its space and time correlation. We find that the gravitational wave signal from a first order phase transition occurring at electroweak symmetry breaking falls into the LISA sensitivity range if the phase transition lasts for about one hundredth of the Hubble time and the energy density of the turbulent motions is about twenty percent of the total energy density in the universe at the phase transition time.