Gravitational wave astronomy and the expansion history of the Universe

TL;DR

This review explores how gravitational wave backgrounds across a broad spectrum can reveal the Universe's expansion history, connecting early cosmology with high-energy physics and quantum sensing technologies.

Contribution

It discusses the potential of relic gravitational waves to test the standard cosmological paradigm and emphasizes the importance of high-frequency signals in quantum sensing applications.

Findings

Relic gravitons could confirm or challenge the current post-inflationary paradigm.

High-frequency gravitational wave signals are crucial for probing early Universe physics.

The spectrum's maximal frequency relates to the quantum entanglement of relic gravitons.

Abstract

The timeline of the expansion rate ultimately defines the interplay between high energy physics, astrophysics and cosmology. The guiding theme of this topical review is provided by the scrutiny of the early history of the space-time curvature through the diffuse backgrounds of gravitational radiation that are sensitive to all the stages of the evolution of the plasma. Due to their broad spectrum (extending from the aHz region to the THz domain) they bridge the macroworld described by general relativity and the microworld of the fundamental constituents of matter. It is argued that during the next score year the analysis of the relic gravitons may infirm or confirm the current paradigm where a radiation plasma is assumed to dominate the whole post-inflationary epoch. The role of high frequency and ultra-high frequency signals between the MHz and the THz is emphasized in the perspective of quantum sensing. The multiparticle final state of the relic gravitons and its macroscopic quantumness is also discussed with particular attention to the interplay between the entanglement entropy and the maximal frequency of the spectrum.