Production of Gravitational Waves in the Early Universe From turbulence triggered by first-order phase transitions

TL;DR

This paper investigates how first-order phase transitions in the early universe could generate primordial gravitational waves by modeling turbulence and analyzing their spectra using relativistic hydrodynamics and different turbulence models.

Contribution

It introduces a new model for gravitational wave production from early universe turbulence, incorporating a time-dependent de-coherence function within the freely decaying turbulence framework.

Findings

Both models produce gravitational wave spectra consistent with turbulence theory.

The new model shows differences in spectral amplitude and frequency compared to existing models.

Results suggest detectable signals for future gravitational wave observatories.

Abstract

This project is aimed at studying the first-order phase transitions, that is presumed to have ensued in the early universe, and its consequences on the primordial gravitational waves. The effects of bubble nucleation, growth, and coalescence are reviewed. The resulting first-order phase transition is taken as the source of the gravitational waves that were produced, in order to determine the energy density, amplitude, and frequency spectra of the relic gravitational wave background. This is accomplished by modelling the first-order phase transition as a turbulent fluid and employing relativistic hydrodynamic equations to estimate the required physical quantities. Two models are majorly studied for all the analysis done in this project. Both models compute the necessary gravitational wave spectra using the exponential Kraichnan function as the temporal decorrelation function. Also, both models contemplate the turbulence in the flow of plasma to be stationary, obeying the conditions dictated by the Kolmogorov turbulence. However, the first model uses a de-coherence function that depends on the wavenumber and time, while, the second model uses the top hat correlation, to compute the anisotropic stress. The new model introduced here adheres to the freely decaying turbulence model, but employs the time dependent de-coherence function in its computations.