Environmental Effects for Gravitational-wave Astrophysics

TL;DR

The paper investigates whether astrophysical environments around gravitational-wave sources can pollute GW signals enough to hinder detections or bias tests of General Relativity (GR). It analyzes matter effects during inspiral and ringdown for both comparable-mass binaries and extreme mass-ratio inspirals (EMRIs) in the context of a space-based detector like eLISA, using conservative environmental models (accretion disks, dark matter, electromagnetic fields, cosmology) to bound dephasings and QNM shifts. The main findings show that environmental effects are typically negligible for detection and parameter estimation, with notable exceptions: EMRIs in geometrically-thin, radiatively efficient disks and, to a lesser extent, DM spikes in satellite galaxies could produce measurable deviations; however, ringdown is generally governed by the isolated BH QNMs, allowing precise mass measurements at the level of about $\sim 0.1\%$. The study also provides a theory-agnostic framework to place intrinsic limits on modified gravity tests, showing that environmental effects can set conservative bounds that must be considered when testing GR with gravitational waves. Overall, the results support the viability of precision GW astronomy with space-based detectors while highlighting a few rare environments that could yield extra astrophysical information and require careful modeling to avoid masking new physics.

Abstract

The upcoming detection of gravitational waves by terrestrial interferometers will usher in the era of gravitational-wave astronomy. This will be particularly true when space-based detectors will come of age and measure the mass and spin of massive black holes with exquisite precision and up to very high redshifts, thus allowing for better understanding of the symbiotic evolution of black holes with galaxies, and for high-precision tests of General Relativity in strong-field, highly dynamical regimes. Such ambitious goals require that astrophysical environmental pollution of gravitational-wave signals be constrained to negligible levels, so that neither detection nor estimation of the source parameters are significantly affected. Here, we consider the main sources for space-based detectors -- the inspiral, merger and ringdown of massive black-hole binaries and extreme mass-ratio inspirals -- and account for various effects on their gravitational waveforms, including electromagnetic fields, cosmological evolution, accretion disks, dark matter, "firewalls" and possible deviations from General Relativity. We discover that the black-hole quasinormal modes are sharply different in the presence of matter, but the ringdown signal observed by interferometers is typically unaffected. The effect of accretion disks and dark matter depends critically on their geometry and density profile, but is negligible for most sources, except for few special extreme mass-ratio inspirals. Electromagnetic fields and cosmological effects are always negligible. We finally explore the implications of our findings for proposed tests of General Relativity with gravitational waves, and conclude that environmental effects will not prevent the development of precision gravitational-wave astronomy.

Figures (1)

• Figure 1: (a) Deviations of the real and imaginary parts of the fundamental $l=2$ QNM frequencies for a BH with mass $M$ surrounded by a thin shell of mass $\delta M=0.01 M$ located at $r_0$. In the large-$r_0$ limit, $\delta_R$ and $\delta_I$ grow linearly. (b) Gravitational-axial waveform for the scattering of a gaussian profile off a thin shell - BH configuration with $r_0=60 GM/c^2$ and $\delta M=M$. The gaussian is initially placed close to the ISCO, with width $\sigma=4GM/c^2$. The effect of matter appears in the time domain as a second pulse at late times, but the isolated BH QNMs dominate most of the response.