When Vacuum Breaks: A Self-Consistency Test for Astrophysical Environments in Extreme Mass Ratio Inspirals

Gravitational-wave signals are typically interpreted under the vacuum hypothesis, i.e., assuming negligible influence from the astrophysical environment. This assumption is expected to break down for low-frequency sources such as extreme mass ratio inspirals (EMRIs), which are prime targets for the Laser Interferometer Space Antenna (LISA) and are expected to form, at least in part, in dense environments such as active galactic nuclei or dark-matter spikes or cores. Modeling environmental effects parametrically is challenging due to the large uncertainties in their underlying physics. We propose a nonparametric test for environmental effects in EMRIs, based on assessing the self-consistency of vacuum parameter posteriors inferred from different portions of the signal. Our results demonstrate that this test can reveal statistically significant inconsistencies from vacuum signals—arising from, e.g., incomplete modeling, environmental effects, or deviations from general relativity—without introducing additional parameters or assumptions about the underlying physics.