Cosmological Gravitational Wave Backgrounds

TL;DR

The paper surveys cosmological sources that could generate stochastic gravitational-wave backgrounds within LISA's 0.1–100 mHz band, including inflationary fluctuations, first-order phase transitions, Goldstone-mode self-ordering, and cosmic strings. It introduces the redshifted Hubble frequency as a reference scale, discusses broad-band energy considerations and current observational constraints, and provides order-of-magnitude spectra and detectability estimates for each source. The work highlights that a LISA detection would illuminate mesoscopic early-universe phenomena and potentially connect to baryogenesis, dark matter, and Planck-scale physics, while also acknowledging the possibility that no cosmological background is detectable. Overall, it frames how spectral shape and amplitude across the LISA band can distinguish between inflationary, phase-transition, Goldstone, and string-generated backgrounds and measure fundamental high-energy processes from the early universe.

Abstract

An overview is presented of possible cosmologically distant sources of gravitational wave backgrounds, especially those which might produce detectable backgrounds in the LISA band between 0.1 and 100 mHz. Examples considered here include inflation-amplified vacuum fluctuations in inflaton and graviton fields, bubble collisions in first-order phase transitions, Goldstone modes of classical self-ordering scalars, and cosmic strings and other gauge defects. Characteristic scales and basic mechanisms are reviewed and spectra are estimated for each of these sources. The unique impact of a LISA detection on fundamental physics and cosmology is discussed.