Can environmental effects spoil precision gravitational-wave astrophysics?

TL;DR

The paper rigorously quantifies how astrophysical environments—accretion disks, magnetic fields, cosmological expansion, dark matter halos, and beyond-GR physics—alter gravitational-wave signals during BH ringdown and compact-object inspirals. Employing a broad, largely perturbative framework and multiple toy and realistic models, the authors show that environmental effects can induce new QNM branches and resonances in ringdowns while generally preserving the dominant vacuum-like response within detector bands; they also quantify dephasing and orbital changes in inspirals, finding that for most eLISA-like scenarios thick-disk environments render environmental corrections subdominant to self-force effects, though thin-disk regimes can produce sizable dephasings. A key outcome is the recommendation to use at least two-mode ringdown templates and to adopt a parametrized ringdown approach to disentangle environmental and beyond-GR signals. Collectively, the work provides practical order-of-magnitude guidance for interpreting future GW observations, constraining modified gravity theories, and potentially using GW data to probe the environments around compact objects. The study highlights intrinsic limits on strong-field gravity tests set by unknown matter, while outlining concrete methodological paths to mitigate these degeneracies in precision GW physics.

Abstract

[abridged abstract] No, within a broad class of scenarios. With the advent of gravitational-wave (GW) astronomy, environmental effects on the GW signal will eventually have to be quantified. Here we present a wide survey of the corrections due to these effects in two situations of great interest for GW astronomy: the black hole (BH) ringdown emission and the inspiral of two compact objects. We take into account various effects such as: electric charges, magnetic fields, cosmological evolution, possible deviations from General Relativity, firewalls, and various forms of matter such as accretion disks and dark matter halos. Our analysis predicts the existence of resonances dictated by the external mass distribution, which dominate the very late-time behavior of merger/ringdown waveforms. The mode structure can drastically differ from the vacuum case, yet the BH response to external perturbations is unchanged at the time scales relevant for detectors. This is because although the vacuum Schwarzschild resonances are no longer quasinormal modes of the system, they still dominate the response at intermediate times. Our results strongly suggest that both parametrized and ringdown searches should use at least two-mode templates. Our analysis of compact binaries shows that environmental effects are typically negligible for most eLISA sources, with the exception of very few special extreme mass ratio inspirals. We show in particular that accretion and hydrodynamic drag generically dominate over self-force effects for geometrically thin disks, whereas they can be safely neglected for geometrically thick disk environments, which are the most relevant for eLISA. Finally, we discuss how our ignorance of the matter surrounding compact objects implies intrinsic limits on the ability to constrain strong-field deviations from General Relativity.

Figures (18)

• Figure 1: Toy model of two rectangular barriers of height $V_0$ and $V_1\ll V_0$. For $V_1=0$ one recovers the original toy model considered by Chandrasekhar and Detweiler Chandrasekhar:1975zza.
• Figure 2: QNMs of the double-barrier system. Top left: Some modes of the toy model of two rectangular barriers of height $V_0$ and $V_1\ll V_0$ as functions of the second barrier position, $b$. For definiteness, we focus on $V_0=16/a^2$, $V_1=10^{-3}/a^2$ and $c-b=0.1a$. Top right: the same but for the fundamental mode only as a function of $b$ with $V_1=10^{-3}/(ab)$ and for different values of $c-b\equiv L$. Qualitatively similar results hold for different choices of the parameters. Bottom left: Eigenfunctions of the double barrier potential. We show the first three QNMs, the fundamental mode in the vacuum case ($\omega_v a=0.466-0.710i$) and two intermediate modes whose real part is close to that of $\omega_v$. Bottom right: QNM spectrum (cf. also Table \ref{['tab:spectrum']}). In both panels we set $V_0=16/a^2$, $V_1=10^{-3}/a^2$, $b=10a$, $c=11a$, but different choices of the parameters would give similar results.
• Figure 3: Left: Waveforms for a gaussian packet in the double barrier potential. Top panel: the initial packet is located on the left of the first barrier, $x_0=-2a$ and $\sigma=a$. Bottom panel: the initial packet is located between the two barriers, $x_0=5a$ and $\sigma=a$. For both panels: $V_0=16/a^2$ and $c=b+a$. Other choices give qualitatively similar results.
• Figure 4: Left panels: percentage deviations of the real and imaginary parts of the QNMs of a Schwarzschild BH surrounded by a thin-shell with respect to the case of an isolated BH (with the same horizon mass) as a function of the shell mass $\delta M$ and for different values of the shell radius, $r_0$ and $l=2$. Note that isospectrality is mildly broken and that polar modes refer to $v_s=0$ (the dependence on $v_s$ is shown in Fig. \ref{['fig:modes_EOS']}). Right: The same as a function of $r_0$ for different values of $\delta M$. Top panels refer to to the case $r_0\lesssim10 M$, whereas bottom panels refers to $r_0\gtrsim 10M$.
• Figure 5: Left panels: linear coefficients of $\delta_R$ and $\delta_I$ in the small $\delta M/M$ limit as functions of the shell radius $r_0$ for $l=2$ and for the real and imaginary parts. In the polar case we have considered $v_s=0$. Right panels: real and imaginary parts of the QN fundamental frequency for $l=2$ polar modes as a function of $v_s$ for $\delta M=10^{-2}M$ and different values of $r_0$.