The scientific potential of space-based gravitational wave detectors

TL;DR

Space-based gravitational wave detectors operating in the millihertz band have immense scientific potential for understanding black hole mergers, galaxy formation, stellar populations, and testing general relativity in strong fields.

Contribution

This paper reviews the potential sources and scientific applications of millihertz gravitational wave observations, highlighting their importance for astrophysics, cosmology, and fundamental physics.

Findings

Detection of massive black hole mergers will trace galaxy assembly.

Observation of stellar-mass objects near black holes offers tests of general relativity.

Millihertz gravitational waves from galactic binaries will inform stellar population studies.

Abstract

The millihertz gravitational wave band can only be accessed with a space-based interferometer, but it is one of the richest in potential sources. Observations in this band have amazing scientific potential. The mergers between massive black holes with mass in the range 10 thousand to 10 million solar masses, which are expected to occur following the mergers of their host galaxies, produce strong millihertz gravitational radiation. Observations of these systems will trace the hierarchical assembly of structure in the Universe in a mass range that is very difficult to probe electromagnetically. Stellar mass compact objects falling into such black holes in the centres of galaxies generate detectable gravitational radiation for several years prior to the final plunge and merger with the central black hole. Measurements of these systems offer an unprecedented opportunity to probe the predictions of general relativity in the strong-field and dynamical regime. Millihertz gravitational waves are also generated by millions of ultra-compact binaries in the Milky Way, providing a new way to probe galactic stellar populations. ESA has recognised this great scientific potential by selecting The Gravitational Universe as its theme for the L3 large satellite mission, scheduled for launch in ~2034. In this article we will review the likely sources for millihertz gravitational wave detectors and describe the wide applications that observations of these sources could have for astrophysics, cosmology and fundamental physics.