WSP : Gravitational wave astronomy and the expansion history of the Universe

Giovannini, Massimo

doi:10.1142/S0217751X25300030

Article
Report number	arXiv:2412.13968
Title	Gravitational wave astronomy and the expansion history of the Universe
Author(s)	Giovannini, Massimo (CERN ; INFN, Milan Bicocca)
Publication	2025-03-17
Imprint	2024-12-18
Number of pages	119
Note	91 pages, 18 figures; corrected typos to match the version to be published
In:	Int. J. Mod. Phys. A 40 (2025) 2530003
DOI	10.1142/S0217751X25300030
Subject category	hep-th ; Particle Physics - Theory ; hep-ph ; Particle Physics - Phenomenology ; hep-ex ; Particle Physics - Experiment ; astro-ph.CO ; Astrophysics and Astronomy ; gr-qc ; General Relativity and Cosmology
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.
Copyright/License	publication: © 2025-2026 The Author(s) (License: CC BY 4.0) preprint: (License: CC BY 4.0)

$On the vertical axis the profile of $H^{-1}$ is illustrated in Planck units as a function of the logarithm of the scale factor. In this cartoon (where, for the sake of simplicity, the slow-roll corrections have been neglected) the full thick line describes the standard inflationary evolution followed by a radiation-dominated stage. The dashed and dot-dashed curves correspond instead to a post-inflationary expansion rate that is either faster or slower than radiation, at least for some time before radiation dominance. In subsection \ref{subsec22} the early expansion rate is estimated from the large-scale curvature inhomogeneities whereas in subsection \ref{subsec23} we are going to present a series of quantitative estimates of $N$, $N_{slower}$ and $N_{faster}$.$ $The common logarithm of $a\, H$ is illustrated as a function of the common logarithm of the scale factor. The two ellipses account for the indetermination of the post-inflationary evolution that can have different durations depending on the differences in the timeline of the expansion rate. In the lower part of the cartoon the CMB scales $k = {\mathcal O}(k_{p})$ approximately cross $a\, H$ (see the two filled squares).$ $The conventional radiation-dominated epoch (taking place for $a> a_{r}$) is preceded by an intermediate phase parametrized by the value of $\delta$. If $\delta\to 1$ we recover the case of a single post-inflationary radiation epoch. When $\delta < 1$ the expansion rate is slower than radiation; conversely if $\delta > 1$ the expansion rate is faster than radiation.$ Show more plots

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Record created 2025-02-11, last modified 2026-02-10

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