Gravitational waves from first order phase transitions

Abstract

Gravitational waves can constitute a unique probe of the very early universe. We discuss here the gravitational wave signal from a primordial first order phase transition occurring at high energy scales. The gravitational waves are produced by the collision of broken phase bubbles and by the magnetohydrodynamic turbulence in the primordial plasma stirred by the bubble collision. We show how the shape and amplitude of the gravitational wave power spectrum can be predicted by general arguments based on the source properties, such as its time evolution and space structure. The parametersdescribing the characteristics of the phasetransition (for instance, its duration and strength), which enter in the determination of the gravitational wave spectrum, can be evaluated in the context of specific models of the first order phase transition. We present here two examples of first order phase transitions which lead to a gravitational wave signal falling in the frequency range of detection of the future space-based interferometer eLISA.