High-Frequency Gravitational Waves from the Galactic Pulsar Population

TL;DR

This study models high-frequency gravitational waves from the Galactic pulsar population, revealing a Standard-Model foreground in the MHz band that could obscure primordial signals, despite current detection limitations.

Contribution

It introduces a detailed population-level model of gravitational waves from pulsar polar caps, incorporating realistic physics and a full Fourier-space framework.

Findings

Foreground in MHz band overlaps with early Universe gravitational-wave signals.

Predicted strain is below current detection sensitivity.

Model normalization is sensitive to assumptions.

Abstract

The high-frequency gravitational-wave band is often discussed primarily in the context of new physics, but realistic Standard-Model foregrounds remain incompletely characterized. We investigate pulsar polar caps as a physically motivated astrophysical source of high-frequency gravitational waves, generated by repeated discharge cycles in compact near-surface plasma gaps. Our baseline result is population-level: we construct the signal from the Galactic normal-pulsar population rather than from a single especially favorable object. To do so, we calibrate the source dynamics with particle-in-cell simulations performed at real physical scales, with physical pulsar parameters mapped directly onto numerical scales, and then lift the resolved longitudinal discharge to a cap-scale emission model. The gravitational-wave signal is computed in a full Fourier-space framework, retaining finite-source, geometric, and polarization effects explicitly. Within this treatment, the dominant contribution is not the purely electric channel emphasized in some earlier simplified approaches, but a source channel involving the large background magnetic field and discharge-induced transverse fluctuations of magnetic field. Integrating this description over a normal-pulsar population, we find an astrophysical foreground in the MHz-scale high-frequency band that can overlap with and partially obscure the thermal gravitational-wave signal sourced by the plasma of the early Universe. At the same time, the normalization remains sensitive to the modeled assumptions. Although the predicted strain remains far below current experimental sensitivity, pulsar polar caps provide a concrete Standard-Model foreground benchmark in a band often treated as nearly background-free. Alternative source configurations further broaden the plausible signal range around this baseline.