The 24th Capra meeting on Radiation Reaction in General Relativity

In the last quarter century, Capra has grown from a mere handful of people to a fully international meeting, which now represents a large diversity of interests. In that time, much progress has been made: many aspects of the first order problem are now in hand, and a multitude of techniques has been formulated for eventually use in full EMRI waveform generation. Yet, much needs to be done, most notably at second order. In Capra meetings of the past, we have often recognized the need to reach out to the younger generation. I think efforts in that direction have clearly been effective. At some Capra meetings, specific problems have often taken focus, both in discussions at the meeting, and in the work that evolves over the coming year. From experience, we know this approach has also clearly paid off. From the discussions that have taken place here, we need to go forward with specific goals for the year ahead, drawing wherever possible on the diversity we now have before us. Great things can be achieved if great problems are tackled. What have we formulated to work on as a community together before we can meet again in 2022?

The open question of whether a black hole can become tidally deformed by an external gravitational field has profound implications for fundamental physics, astrophysics and gravitational-wave astronomy. Love tensors characterize the tidal deformability of compact objects such as astrophysical (Kerr) black holes under an external static tidal field. We prove that all Love tensors vanish identically for a Kerr black hole in the nonspinning limit or for an axisymmetric tidal perturbation. In contrast to this result, we show that Love tensors are generically nonzero for a spinning black hole. Specifically, to linear order in the Kerr black hole spin and the weak perturbing tidal field, we compute in closed form the Love tensors that couple the mass-type and current-type quadrupole moments to the electric-type and magnetic-type quadrupolar tidal fields. For a dimensionless spin ~ 0.1, the nonvanishing quadrupolar Love tensors are ~ 0.002, thus showing that black holes are particularly "rigid" compact objects. We also show that the induced quadrupole moments are closely related to the physical phenomenon of tidal torquing of a spinning body interacting with a tidal gravitational environment.

The prospect of gravitational wave astronomy with EMRIs has motivated increasingly accurate perturbative studies of binary black hole dynamics. Studying the apparent and event horizon of a perturbed Schwarzschild black hole, we find that the two horizons are identical at linear order regardless of the source of perturbation. This implies that the seemingly teleological behaviour of the linearly perturbed event horizon, previously observed in the literature, cannot be truly teleological in origin. The two horizons do generically differ at second order in some ways, but their Hawking masses remain identical. In the context of tidal distortion by a small companion, we also show how the perturbed event horizon in a small-mass-ratio binary is effectively localized in time, and we numerically visualize unexpected behaviour in the black hole’s motion around the binary’s center of mass.

We show that perturbations of massless fields in the Kerr black hole background enjoy a hidden infinite-dimensional ("Love") symmetry in the properly defined near zone approximation. Love symmetry mixes IR and UV modes. Still, this approximate symmetry allows us to derive exact results about static tidal responses (Love numbers) of static and spinning black holes. Generators of the Love symmetry are globally well defined and have a smooth Schwarzschild limit. The Love symmetry contains an SL(2,R)×U(1) subalgebra. Generic regular solutions of the near zone Teukolsky equation form infinite-dimensional SL(2,R) representations. In some special cases these are highest weight representations. This situation corresponds to vanishing Love numbers. In particular, static perturbations of four-dimensional Schwarzschild black holes belong to finite-dimensional representations. Other known facts about static Love numbers also acquire an elegant explanation in terms of the SL(2,R) representation theory.

Black holes are never isolated in realistic astrophysical environments; instead, they are often perturbed by complicated external tidal fields. How does a black hole respond to these tidal perturbations? In this talk, I will discuss both the conservative and dissipative responses of the Kerr black hole to a weak and adiabatic gravitational field. The former describes how the black hole would change its shape due to these tidal interactions, and is quantified by the so-called “Love numbers”. On the other hand, the latter describes how energy and angular momentum are exchanged between the black hole and its tidal environment due to the absorptive nature of the event horizon. In this talk, I will describe how the Love numbers of the Kerr black hole in a static tidal field vanish identically. I will also describe how the Kerr black hole's dissipative response implies that energy and angular momentum can either be lost to or extracted from the black hole, with the latter process commonly known as the black hole superradiance. I will end by discussing how these tidal responses leave distinct imprints on the gravitational waves emitted by binary black holes.

"One of the primary sources for the future space-based gravitational wave detector, the Laser Interferometer Space Antenna, are the inspirals of small compact objects into massive black holes in the centres of galaxies. The gravitational waveforms from such Extreme Mass Ratio Inspiral (EMRI) systems will provide measurements of their parameters with unprecedented precision, but only if the waveforms are accurately modeled. Here we explore the impact of transient orbital resonances which occur when the ratio of radial and polar frequencies is a rational number. We introduce a new Effective Resonance Model, which is an extension of the numerical kludge EMRI waveform approximation to include the effect of resonances, and use it to explore the impact of resonances on EMRI parameter estimation. For one-year inspirals, we find that the few cycle dephasings induced by 3:2 resonances can lead to systematic errors in parameter estimates, that are up to several times the typical measurement precision of the parameters. The bias is greatest for 3:2 resonances that occur closer to the central black hole. By regarding them as unknown model parameters, we further show that observations will be able to constrain the size of the changes in the orbital parameters across the resonance to a relative precision of $10\%$ for a typical one-year EMRI observation with signal-to-noise ratio of 20. Such measurements can be regarded as tests of fundamental physics, by comparing the measured changes to those predicted in general relativity."

In recent work, tidal resonances induced by the tidal field of nearby stars or black holes have been identified as potentially significant in the context of extreme mass-ratio inspirals (EMRIs). These resonances occur when the three orbital frequencies describing the orbit are commensurate. During the resonance, the orbital parameters of the small body experience a ‘jump’ leading to a shift in the phase of the gravitational waveform. We study how common and important such resonances are over the entire orbital parameter space. We find that a large proportion of inspirals encounter a low-order tidal resonance in the observationally important regime.

Everybody talks about EMRIs and IMRIs in connection with LISA and the 2030s. However, inspirals into massive black holes are happening at this very moment. Would we be able to recognize them with our current electromagnetic observations? Even more, are we maybe observing these inspirals at the very moment without realising it? We simulated accretion-disks perturbed by light perturbers and deduced that the orbital periods show in the disk variability. I present a list of candidate sources that, based on their variability periods and period derivatives, may contain ongoing inspirals into massive black holes.

"We consider the motion of nonspinning, compact objects orbiting around a Kerr black hole with tidal couplings. The tide-indcued quadrupole moment modifies both the orbital energy and out-going fluxes, so that over the inspiral timescale there is an accumulative shift in the orbital and gravitational wave phase. Previous studies on compact object tidal effects have been carried out in the Post-Newtonian (PN) and Effetive-One-Body (EOB) formalisms. In this work, within the black hole perturbation framework, we propose to characterize the tidal influence in the expansion of mass ratios, while higher-order PN corrections are naturally included. For the equatorial and circular orbit, we derive the leading order, frequency depedent tidal phase shift which agrees with the Post-Newtonian result at low frequencies but deviates at high frequencies. We also find that such phase shift has weak dependence (≤ 10%) on the spin of the primary black hole. Combining this black hole perturbation waveform with the Post-Newtonian waveform, we propose a frequency-domain, hybrid waveform that shows comparable accuracy as the EOB waveform in characterizing the tidal effects, as calibrated by numerical relativity simulations. Further improvement is expected as the next-leading order in mass ratio and the 2PN tidal corections are included. This hybrid approach is also applicable for generating binary black hole waveforms."

Extreme Mass Ratio Inspirals (EMRIs) can be classified as dry EMRIs and wet EMRIs based on their formation mechanisms. Dry (or the" loss-cone") EMRIs, previsouly considered as the main EMRI sources for the Laser Interferometer Space Antenna, are primarily produced by multi-body scattering in the nuclear star cluster and gravitational capture. In this Letter, we highlight an alternative EMRI formation channel:(wet) EMRI formation assisted by the accretion flow around accreting galactic-center massive black holes (MBHs). In this channel, the accretion disk captures stellar-mass black holes that are intially moving on inclined orbits, and subsequently drives them to migrate towards the MBH-this process boosts the formation rate of EMRIs in such galaxies by orders of magnitude. Taking into account the fraction of active galactic nuclei where the MBHs are expected to be rapidly accreting, we forecast that wet EMRIs will contribute an important fraction of all EMRIs observed by spaceborne gravitational wave detectors and likely dominate for MBH hosts heavier than a few $10^5 M_\odot$.