Strong Gravity

Extreme-mass-ratio inspirals (EMRIs) are the only gravitational-wave sources for the future LISA detector that combine the issue of strong-field complexity with that of long-lived signals. The result is a profoundly difficult inverse problem, with many theoretical and computational challenges presented both by the forward modeling of the predicted EMRI waveform, and by the recovery of an inverse solution for the presence and properties of actual EMRI signals in LISA data. I outline recent progress and ongoing work on both fronts. Specifically, I describe: i) the next generation of accurate, efficient and extensive models for (the response of LISA to) an EMRI signal; and ii) a heuristic study of degeneracy in the space of LISA-observable EMRIs, with a view to informing analysis strategies for EMRI search and characterization.

The astrophysical background of gravitational waves (AGWB) is composed by the incoherent superposition of gravitational wave signals emitted by resolved and unresolved astrophysical sources from the onset of stellar activity until today. In this talk, I  will present a theoretical framework  to characterize the AGWB in terms of energy density and polarization and  I will show predictions for the angular power spectra of the background anisotropy and for its cross-correlations with electromagnetic observables, in the frequency bands accessible by  LIGO/Virgo and  LISA.  I will then discuss the astrophysical content of these observables, give an overview of the state of the art of observations, and highlight future observational challenges.

Gravitational waves from the coalescence of compact binaries provide a unique opportunity to test gravity in strong field regime. In particular, the postmerger phase of the gravitational signal is a proxy for the nature of the remnant.

This is of particular interest in view of some quantum-gravity models which predict the existence of horizonless dark compact objects that overcome the paradoxes associated to black holes. Such dark compact objects can emit a modified ringdown with respect to the black hole case and late-time gravitational wave echoes as characteristic fingerprints.

In this talk, I develop a generic framework to the study of the ringdown of dark compact objects and provide a gravitational-wave template for the echo signal. Finally, I assess the detectability of dark compact objects with current and future gravitational-wave detectors.

The statement that general relativity is deterministic finds its mathematical formulation in the celebrated ‘Strong Cosmic Censorship Conjecture’ due to Roger Penrose. I will present my recent results on this conjecture in the case of negative cosmological constant and in the context of black holes. It turns out that this is intimately tied to Diophantine properties of a suitable ratio of mass and angular momentum of the black hole.

Quasars are the most luminous objects in the universe powered by accretion onto supermassive black holes (SMBHs). They can be observed at the earliest cosmic epochs, providing unique insights into the early phases of black hole, structure, and galaxy formation. Observations of these quasars demonstrate that they host SMBHs at their center, already less than ~1 Gyr after the Big Bang. It has been argued that in order to grow these SMBHs in such short amounts of cosmic time, they need to accrete matter over timescales comparable to the age of the universe, and thus the lifetime of quasars - the integrated time that galaxies shine as active quasars - is expected to be of order ~10^9 yr at a redshift of z~6, even if they accrete continuously at the theoretical maximum limit. 

I will present a new method to obtain constraints on the lifetime of high-redshift quasars, based on measurements of the sizes of ionized regions around quasars, known as proximity zones. The sizes of these proximity zones are sensitive to the lifetime of the quasars, because the intergalactic gas has a finite response time to the quasars’ radiation. Applying this method to quasar spectra at z>6, we discover an unexpected population of very young quasars, indicating lifetimes of only ~10,000 years, several orders of magnitude shorter than expected. I will discuss the consequences of such short lifetimes on the quasars' ionizing power, their black hole mass accretion rates, and highlight tensions with current theoretical models for black hole formation. Furthermore, I will present several modifications to the current SMBH formation paradigm that might explain our findings and show how we aim to disentangle the various scenarios by means of future observations with the upcoming James Webb Space Telescope, in order to shed new light onto the formation and growth of the first SMBHs in the universe.

Most massive stars spend their lives in so close orbit with a companion star that severe mass exchange or even coalescence is inevitable as the stars evolve and swell. A third of massive stars are thus stripped of their fluffy, hydrogen-rich envelopes, leaving the compact helium core exposed. These stripped stars are so hot that most of their radiation is emitted in the ionizing regime. Using evolutionary and spectral models of stripped stars, I will show how they sometimes dominate the ionizing emission from full stellar populations and even significantly contribute to cosmic reionization. With their hard ionizing spectra, stripped stars possibly leave observable traces, for example in the nebular spectrum of distant galaxies. 

Apart from being ionizing sources, stripped stars are also interesting to consider as gravitational wave emitters. Creating a population model, we predict that several stripped stars orbiting compact objects will be detectable by LISA.

The study of compact objects in the strong field regime needs a thorough understanding of the initial value problem in general relativity at the resence of hydrodynamical or magnetohydrodynamical sources. This is a twofold problem that includes general relativistic solutions that represent realistic astrophysical systems at a given moment in time as well as their subsequent evolutions. In this talk I will present the fundamental principles of this endeavor as well as efforts in understanding a great variety of astrophysical systems from binary neutron stars to ergostars and black hole-disks through numerical relativity.

The velocity of a gravitational wave (GW) source provides crucial information about its formation and evolution processes.

Previous studies considered the Doppler effect on the phase of GWs as a potential signature of a time-dependent velocity of the source. However, the Doppler shift only accounts for the time component of the wave vector, and in principle motion also affects the spatial components. In my talk I discuss the latter effect, known as “aberration” for light, for GWs and how it affects the waveform modeling of an accelerating source. I show that the additional aberrational phase shift could be detectable in two astrophysical scenarios, namely, a recoiling binary black hole (BBH) due to GW radiation and a BBH in a triple system.

Further, I discuss how adding the aberrational phase shift in the waveform templates could significantly enhance the detectability of moving sources.

Compact white dwarf (WD) binaries are important sources for space-based gravitational-wave (GW) observatories, and an increasing number of them are being identified by surveys like ELM and ZTF. We study the effects of nonlinear dynamical tides in such binaries. We focus on the global three-mode parametric instability and show that it has a much lower threshold energy than the local wave-breaking condition studied previously. By integrating networks of coupled modes, we calculate the tidal dissipation rate as a function of orbital period. We construct phenomenological models that match these numerical results and use them to evaluate the spin and luminosity evolution of a WD binary. While in linear theory the WD's spin frequency can lock to the orbital frequency, we find that such a lock cannot be maintained when nonlinear effects are taken into account. Instead, as the orbit decays, the spin and orbit go in and out of synchronization.  Each time they go out of synchronization, there is a brief but significant dip in the tidal heating rate. While most WDs in compact binaries should have luminosities that are similar to previous traveling-wave estimates, a few percent should be about ten times dimmer because they reside in heating rate dips. This offers a potential explanation for the low luminosity of the CO WD in J0651. Lastly, we consider the impact of tides on the GW signal and show that LISA and TianGO can constrain the WD's moment of inertia to better than 1% for centi-Hz systems.

To predict the gravitational waves emitted by a black hole binary, one needs to understand the dynamics of the binary in general relativity. No closed form solutions of this problem exist. Instead one must introduce some form of approximation. One such approximation, can be made if one of the components is much heavier than the other, suggesting a perturbative expansion in the mass-ratio. I will review this small mass-ratio (SMR) expansion of the dynamics, and the progress that has been made over the last two decades. In particular, I will discuss some recent results on the convergence of this series, suggesting that at relatively low orders this SMR expansion can be used to model even equal mass binaries.

We comment on the recently introduced Gauss-Bonnet gravity in four dimensions. We argue that it does not make sense to consider this theory to be defined by a set of D->4 solutions of the higher-dimensional Gauss-Bonnet gravity. We show that a well-defined D->4 limit of Gauss-Bonnet Gravity is obtained generalizing a method employed by Mann and Ross to obtain a limit of the Einstein gravity in D=2 dimensions. This is a scalar-tensor theory of the Horndeski type obtained by dimensional reduction methods. By considering simple spacetimes beyond spherical symmetry (Taub-NUT spaces) we show that the naive limit of the higher-dimensional theory to four dimensions is not well defined and contrast the resultant metrics with the actual solutions of the new theory. Theory and solutions in lower dimensions will also be briefly discussed.

We discuss several numerical and analytical studies of the modified gravity theory Einstein dilaton Gauss-Bonnet (EdGB) gravity. This class of modified gravity theories admit scalarized black hole solutions. The theory may then provide significantly different gravitational wave signatures during binary black hole merger as compared to general relativity, so that gravitational wave observations may provide new stringent constraints on EdGB gravity. The theory is also an important member of the Horndeski theories, which have been used to model theories of the early universe, including inflation and ekpyrosis. We describe our investigations of the self-consistency of EdGB gravity for spherically symmetric solutions that differ significantly from those in GR, including scalarized black hole solutions, and find that for solutions where the Gauss-Bonnet corrections to general relativity are not small, the theory no longer admits hyperbolic evolution. We discuss how an effective field theory interpretation of the equations of motion though should always guarantee (locally) hyperbolic evolution for the theory.