Scalar induced gravitational waves review

TL;DR

This review summarizes the current understanding of gravitational waves generated by primordial fluctuations, emphasizing analytical formulations and the impact of different early universe expansion histories.

Contribution

It unifies and simplifies the analytical framework for induced gravitational waves and highlights the significance of alternative cosmological scenarios.

Findings

Provides compact formulas for induced gravitational waves

Highlights the importance of early universe expansion history

Summarizes recent advances in the field

Abstract

We provide a review on the state-of-the-art of gravitational waves induced by primordial fluctuations, so-called induced gravitational waves. We present the intuitive physics behind induced gravitational waves and we revisit and unify the general analytical formulation. We then present general formulas in a compact form, ready to be applied. This review places emphasis on the open possibility that the primordial universe experienced a different expansion history than the often assumed radiation dominated cosmology. We hope that anyone interested in the topic will become aware of current advances in the cosmology of induced gravitational waves, as well as becoming familiar with the calculations behind.