Recent work applying the notion of pseudospectrum to gravitational physics showed that the quasinormal mode spectrum of black holes is unstable, with the possible exception of the longest-lived (fundamental) mode. The fundamental mode dominates the expected signal in gravitational wave astronomy, and there is no reason why it should have privileged status. We compute the quasinormal mode spectrum of two model problems where the Schwarzschild potential is perturbed by a small "bump" consisting of either a Pöschl-Teller potential or a Gaussian, and we show that the fundamental mode is destabilized under generic perturbations. We present phase diagrams and study a simple double-barrier toy problem to clarify the conditions under which the spectral instability occurs. Introduction.The advent of gravitational-wave (GW) astronomy [1, 2] and of very long baseline interferometry [3,4] opened exciting new windows to the invisible Universe. Black holes (BHs) play a unique role in the endeavor to test our understanding of general relativity (GR) and in the search for new physics [5][6][7][8][9][10][11].According to the singularity theorems [12,13], classical GR must fail in BH interiors. Quantum mechanics in BH spacetimes also leads to puzzling consequences, such as the information paradox [14][15][16]. It is tempting to conjecture that a theory of quantum gravity will resolve these issues, but the scale and nature of quantum gravity corrections to BH spacetimes is unknown. Uniqueness results in vacuum GR imply that BHs are the simplest macroscopic objects in the Universe [17], and BHs do not "polarize" in binary systems [18][19][20][21][22][23]. The simplicity of BHs (whether isolated or in binaries) implies that they are ideal laboratories to probe the limitations of GR, as long as environmental effects or astrophysical uncertainties can be ignored. In this Letter we ask an important question: is it really possible to ignore environmental effects?One of the tools to test the Kerr geometry is BH spectroscopy [24-26], now a thriving field [27][28][29][30][31][32][33][34]. If a compact binary merger leads to the formation of a rotating BH, as predicted in GR, the spacetime should asymptote to the Kerr metric through a relaxation process during which it can be described as a perturbation of the Kerr metric. The late-time GW signal (the "ringdown") is a superposition of damped exponentials with complex frequencies known as the quasinormal modes (QNMs), which can be computed within perturbation theory as poles of the associated Green's function [35][36][37]. The residues corresponding to these poles in the complex frequency plane dictate the amplitude of the response. To model a ringdown signal using Kerr QNM frequencies in vacuum, we should take into account the surrounding matter (even if it can be considered as a small perturbation). This is the

When gravitational waves (GWs) pass through the nuclear star clusters of galactic lenses, they may be microlensed by the stars. Such microlensing can cause potentially observable beating patterns on the waveform due to waveform superposition and magnify the signal. On the one hand, the beating patterns and magnification could lead to the first detection of a microlensed GW. On the other hand, microlensing introduces a systematic error in strong lensing use-cases, such as localization and cosmography studies. By numerically solving the lensing diffraction integral, we show that diffraction effects are important when we consider GWs in the LIGO frequency band lensed by objects with masses $\lesssim 100 \, \rm M_\odot$. We also show that the galaxy hosting the microlenses changes the lensing configuration qualitatively, so we cannot treat the microlenses as isolated point mass lenses when strong lensing is involved. We find that for stellar lenses with masses $\sim \! 1 \, \rm M_\odot$, diffraction effects significantly suppress the microlensing magnification. Thus, our results suggest that GWs lensed by typical galaxy or galaxy cluster lenses may offer a relatively clean environment to study the lens system, free of contamination by stellar lenses, which can be advantageous for localization and cosmography studies.

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Multiple gauge theories predict the presence of cosmic strings with different mass densities Gµ. We derive an equation governing the perturbations of a rotating black hole pierced by a straight, infinitely long cosmic string along its axis of rotation and calculate the quasinormal-mode frequencies of such a black hole. We then carry out parameter estimation on the first detected gravitational-wave event, GW150914, by hypothesizing that there is a string piercing through the remnant, yielding a constraint of Gµ < 3.8 × 10 −3 at the 90% confidence interval with a comparable Bayes factor with a string-less analysis. In contrast to existing studies which focus on the mutual intersection of cosmic strings, or the cosmic string network, our work focuses on the intersection of a cosmic string with a black hole.

Gravitational lensing describes the bending of the trajectories of light and gravitational waves due to the gravitational potential of a massive object. Strong lensing by galaxies can create multiple images with different overall amplifications, arrival times, and image types. If, furthermore, the gravitational wave encounters a star along its trajectory, microlensing will take place. Our previous research studied the effects of microlenses on strongly-lensed type I images. We extend our research to type II strongly-lensed images to complete the story. Our results are broadly consistent with prior work by other groups. As opposed to being magnified, the type II images are typically demagnified. Moreover, the type II images produce larger mismatches than type I images. Similar to our prior work, we find that the wave optics effects significantly suppress microlensing in the stellar-mass limit. We also discuss the implication of our results for a particularly promising method to use strong lensing to detect microlensing. In the future, it will be crucial to incorporate these microlensed waveforms in gravitational-wave lensing searches.