Snowmass2021 Cosmic Frontier White Paper: Fundamental Physics and Beyond the Standard Model

TL;DR

Gravitational wave detectors offer powerful means to test fundamental physics, explore strong-field gravity, and potentially uncover new physics beyond the Standard Model through observations of black holes and neutron stars.

Contribution

This white paper highlights the potential of upcoming gravitational wave observations to probe fundamental physics and test general relativity in extreme regimes.

Findings

Detection of black hole binaries will test general relativity.

Extreme mass-ratio inspirals can reveal near-horizon modifications.

Gravitational waves can detect ultralight bosons and constrain dark matter properties.

Abstract

Gravitational wave detectors are formidable tools to explore strong-field gravity, especially black holes and neutron stars. These compact objects are extraordinarily efficient at producing electromagnetic and gravitational radiation. As such, they are ideal laboratories for fundamental physics and have an immense discovery potential. The detection of black hole binaries by third-generation Earth-based detectors, space-based detectors and pulsar timing arrays will provide exquisite tests of general relativity. Loud "golden" events and extreme mass-ratio inspirals can strengthen the observational evidence for horizons by mapping the exterior spacetime geometry, inform us on possible near-horizon modifications, and perhaps reveal a breakdown of Einstein's gravity. Measurements of the black-hole spin distribution and continuous gravitational-wave searches can turn black holes into efficient detectors of ultralight bosons across ten or more orders of magnitude in mass. A precise monitoring of the phase of inspiralling binaries can constrain the existence of additional propagating fields and characterize the environment in which the binaries live, bounding the local dark matter density and properties. Gravitational waves from compact binaries will probe general relativity and fundamental physics in previously inaccessible regimes, and allow us to address fundamental issues in our current understanding of the cosmos.