Search Talks

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.

Understanding gravity in the framework of quantum mechanics is one of the great challenges in modern physics. Along this line, a prime question is to find whether gravity is a quantum entity subject to the rules of quantum mechanics. It is fair to say that there are no feasible ideas yet to test the quantum coherent behaviour of gravity directly in a laboratory experiment. Here, I will introduce an idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator. I will show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. I will provide a prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, through simple correlation measurements between two spins: one embedded in each test mass. Fundamentally, the above entanglement is shown to certify the presence of non-zero off-diagonal terms in the coherent state basis of the gravitational field modes.

I discuss general argument to show that if a physical system can mediate locally the generation of entanglement between two quantum systems, then it itself must be non-classical. Remarkably, the argument does not assume any classical or quantum formalism to describe the mediating physical system: the result follows from general information-theoretic principles. This argument provides a robust and general theoretical basis for recently proposed tests of non-classicality in gravity, based on witnessing gravitationally-induced entanglement in quantum probes.

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.

Renormalisation in curved spacetimes is an involved subject. In contrast to renormalisation in a flat spacetime, the standard momentum representation is not directly available. Nevertheless, the momentum dependence of correlation functions is crucial to deciding whether a theory is unitary and causal. I will discuss how to define a notion of momentum dependence in gravity on a fundamental level. With this at hand, one can discuss an important quantum field theory observable: scattering cross sections. Taking the example of gravity-mediated scalar scattering, I will discuss conditions that a quantum field theory of gravity has to fulfil to have a well-behaved scattering amplitude. These can be satisfied without the introduction of massive higher spin modes as is done in string theory. Finally, I will review the status of first principle calculations of the non-perturbative momentum dependence of quantum gravity correlation functions.

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 Raychaudhuri equation predicts the convergence of geodesics and gives rise to the singularity theorems. The quantum Raychaudhuri equation (QRE), on the other hand, shows that quantal trajectories, the quantum equivalent of the geodesics, do not converge and are not associated with any singularity theorems. Furthermore, the QRE gives rise to the quantum corrected Friedmann equation. The quantum correction is dependent on the wavefunction of the perfect fluid whose pressure and density enter the Friedmann equation. We show that for a suitable choice of the wavefunction this term can be interpreted as a small positive cosmological constant.

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.

We will argue that even with semiclassical gravity, it can be shown that a copy of  all the information on a Cauchy slice resides near the boundary of the slice. We will first demonstrate this in asymptotically global AdS, and then in four-dimensional asymptotically flat space. We will then describe a physical protocol that can be used to verify this property at low-energies and within perturbation theory. This property of gravity implies that information about the black-hole interior is always present "outside" the black hole, which leads to a fresh perspective on the information paradox.

References:
1) https://arxiv.org/abs/2002.02448
2) https://arxiv.org/abs/2008.01740

The computation of transition amplitudes in Loop Quantum Gravity is still a hard task, especially without resorting to large-spins approximations. In Marseille we are actively developing a C library (sl2cfoam) to compute Lorentzian EPRL amplitudes with many vertices. We have already applied this tool to obtain interesting results in spinfoam cosmology and on the so-called flatness problem of spinfoam models. I will highlight the main aspects of our software library, describe the current applications and also introduce undergoing developments (Montecarlo over spin space, tensor contractions, GPU offloading) that will greatly enhance the library's speed and capabilities.

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.