Nanohertz Gravitational Waves

TL;DR

The paper discusses the detection of nanohertz gravitational waves via pulsar timing arrays, exploring their astrophysical and cosmological origins, and highlights how these signals can inform us about the early Universe and fundamental physics.

Contribution

It provides a comprehensive analysis of potential gravitational wave sources in the nanoHz range, including astrophysical black-hole binaries and early Universe phenomena, with implications for physics beyond the standard model.

Findings

Detection of a gravitational wave signal in PTA data.

Astrophysical and cosmological sources can explain the observed signals.

PTA data constrains models of the early Universe and fundamental physics.

Abstract

Evidence of a gravitational wave (GW) signal has emerged in pulsar timing array (PTA) data, opening a new window into the nanoHz GW Universe. We explore the physics of GW signals potentially explaining the data, with a primary focus on GW backgrounds (GWBs), considering both astrophysical and cosmological origins. We describe how: (i) An astrophysical nanoHz GWB emerges as the superposition of individual signals from inspiralling massive black-hole binaries (MBHBs); (ii) Environment coupling, eccentricity, and sparse sampling, affect the MBHB signal spectrum and statistical properties, causing great uncertainty in theoretical predictions, but simultaneously offering a handle to discriminate a potential astrophysical origin; (iii) PTA data offers unprecedented opportunities to constrain high-energy physics beyond the standard model, by probing early Universe GWBs, originated during or after inflation; (iv) Different early Universe GWBs, typically created by non-linear and out-of-equilibrium dynamics, can explain the PTA data, as e.g. from inflation scenarios, first order phase transitions, or topological defects; (v) The PTA detection of GWs opens a new window to explore the Universe, with profound implications for astrophysics and particle physics, probing e.g. the equation of state of the early Universe, the origin of the cosmological perturbations, the nature of the dark matter, or whether exotic objects like primordial black holes or cosmic strings exist.