General Relativity & Quantum Cosmology

We are fortunate to live in an era of great discoveries in particle physics and cosmology, and most of the theoretical understanding that made this possibile is based on effective field theories. In this talk, I will show how these powerful techniques can be applied across the spectrum of theoretical physics, and allow us to draw unexpected connections among very different systems. To illustrate this, I will discuss two interesting but very different phenomena, and show how they can both be described using a point-like particle effective theory. The first phenomenon I will consider is superradiance---a dissipative phenomenon by which a spinning object can amplify the intensity of scattered radiation, or be unstable by creation of particles in a bound state. This phenomenon is particularly interesting when applied to spinning black holes, and in this context it has recently received significant attention as a possible probe of ultra-light particle beyond the Standard Model. The second phenomenon I will discuss is the existence of roton excitations in superfluid He4. By treating rotons as point-like particles--albeit of a very special kind--I will write down an effective theory that describes the motion of rotons in the medium at equilibrium as well as their couplings to sound waves and generic fluid flows. This approach allows us to correct classic results by Landau and Khalatnikov on roton-phonon scattering, and to pose and answer the intriguing question: do rotons sink or float? 

We argue that in quantum gravity there is no stable equilibrium state corresponding to the Born rule. Our main argument rests on the continued controversy over the physical meaning of the Wheeler-DeWitt equation. We suggest that attempts to interpret it are hampered by the conventional assumption that probabilities should be governed by a fixed Born rule. It is possible to abandon this assumption in a de Broglie-Bohm interpretation. We argue that a stable Born rule emerges only in the limiting Schrödinger regime in which the system is described by an effective (time-dependent) Schrödinger equation with an appropriate conserved current. We consider a regime in which quantum-gravitational corrections to the  Schrödinger equation induce a non-conservation of the standard current. Such a regime can be consistently described in de Broglie-Bohm theory. As a result, quantum-gravitational effects can generate non-equilibrium deviations from the Born rule even starting from an initial equilibrium state. We provide estimates for such effects in an inflationary de Sitter background.

The recent detection of the binary neutron star merger GW170817 by LIGO and Virgo was followed by a firework of electromagnetic counterparts across the entire electromagnetic spectrum. In particular, the ultraviolet, optical, and near-infrared emission is consistent with a kilonova that provided strong evidence for the formation of heavy elements in the merger ejecta by the rapid neutron capture process (r-process). In this talk, I will discuss the state of the art in modeling neutron star mergers from first principles, which represents a multi-physics challenge involving all four fundamental forces and petascale computing. I will present recent results from general-relativistic magnetohydrodynamic simulations and discuss possible scenarios and mass ejection mechanisms that can give rise to the observed kilonova features. In particular, I will argue that massive winds from neutrino-cooled post-merger accretion disks most likely synthesized the heavy r-process elements in GW170817. I will show how this finding (at least partially) concludes the quest for the cosmic origin of the heavy elements, which has been an enduring mystery for more than 70 years.

In general relativity, the effective-one-body (EOB) approach, which consists in reducing the two-body dynamics to the motion of a test particle in an effective static, spherically symmetric metric, has proven to be a very powerful framework to describe analytically the coalescence of compact binary systems.

In this seminar, we address its extension to modified gravities, considering first the example of massless scalar-tensor theories (ST). We reduce the ST two-body dynamics, which is known at second post-Keplerian order, to a simple parametrized deformation of the general relativistic EOB Hamiltonian, and estimate the ST corrections to the strong-field regime; in particular, the ISCO location and orbital frequency.

We then discuss the class of Einstein-Maxwell-dilaton (EMD) theories, which provide simple examples of "hairy" black holes. We compute the EMD post-Keplerian two-body Lagrangian, and show that it can, as well, be incorporated within the EOB framework. Finally, we highlight that, depending on their scalar environment, EMD black holes can transition to a regime where they strongly couple to the scalar and vector fields, inducing large deviations from the general relativistic two-body dynamics.

Gravitational waves from the mergers of five binary black holes and one binary neutron star were detected in the past two years by the advanced LIGO and Virgo detectors. These detections allowed our Universe to be observed in gravitational waves for the first time, and they have tested the predictions of general relativity for dynamical and strongly gravitating systems. I will discuss these results and also highlight a few additional examples of ways in which gravitational waves can shed light on open questions in theoretical physics and astrophysics. One involves the gravitational-wave memory effect, which is a constant change in the gravitational-wave strain produced by the energy that gravitational waves and matter carry away from an isolated system. I will describe the challenges in detecting the memory with LIGO and Virgo, and how the memory is related to the symmetries and conserved quantities of isolated gravitating systems. A second involves using precision astrometry to detect a stochastic background of gravitational waves from astrophysical, and potentially even cosmological, sources. With the upcoming data release from the Gaia mission, it will likely be possible to place improved constraints on the stochastic background at frequencies higher than those coming from the cosmic microwave background, but below those of pulsar timing searches. I will show how these constraints can be determined. Finally, I will discuss gravitational waves from a compact object orbiting an intermediate mass black hole surrounded by a dark matter spike. I will describe how details about the dark matter distribution can be imprinted in the emitted gravitational waves.

Singularities, boundary points of spacetime beyond which no extension is possible, continue to intrigue both mathematicians and physicists since they are places where our current understanding of physical law breaks down. The question of whether they exist in physical situations is still an open one. Fifty years ago, Hawking and Penrose developed the first general model independent singularity theorems. These theorems showed that singularities have to exist in any spacetime that satisfies certain properties. Some of these properties are mild assumptions but others, called energy conditions, depend on matter content and are more problematic. For both classical and quantum fields, violations of these conditions can be observed in some of the simplest of cases. Therefore there is a need to develop theorems with weaker restrictions, namely energy conditions averaged over an entire geodesic and quantum inequalities, weighted local averages of energy densities. In this work we investigate the strong energy condition in the presence of both classical and quantum non-minimally coupled scalar fields and derive bounds in each case. In the quantum case these bounds take the form of a set of state-dependent quantum energy inequalities valid for the class of Hadamard states. Finally, we show how the quantum inequalities derived can be used as an assumption to a modified Hawking singularity theorem.

Although entanglement harvesting was first posited over 25 years ago, it is only in recent years that this phenomenon has been the subject of active study. The basic idea of entanglement harvesting is to transfer correlations from the vacuum of some quantum field to a pair of detectors. The result provides a new probe of the structure of spacetime via quantum correlations. I shall describe recent work on some of the first results in harvesting entanglement in curved spacetime, in particular anti de Sitter spacetime and black holes.

The observations of gravitational waves from coalescing compact binary systems allow us to test gravity in its strong field regime. In order to better constrain alternative theories of gravity, one has to build template waveforms for these theories. In this talk, I will present a post-Newtonian Lagrangian approach adapted to the specificities of scalar-tensor theories. I will derive the equations of motion of a compact binary system at 3PN order in harmonic coordinates. This result is primordial in order to compute the scalar and gravitational waveforms at 2PN order. I will conclude by a short discussion on tidal effects.

In both Causal Set Quantum Gravity as well as in the String Landscape, we face the challenging tasks of sifting through large state spaces and searching for the set of solutions which best model our physical universe. I demonstrate in this talk how efficient parallel algorithms can give us access to areas of physics previously unstudied due to computational barriers. I first present new methods to accelerate the evolution of causal set Markov chains, which enables us to look for the spontaneous emergence of manifoldlike structure. Then, I use a similar graph theoretic approach to show how a dynamic vacuum selection mechanism naturally emerges in F-Theory and Type IIB String Theory in the context of eternal inflation. We will discuss in detail relevant numerical aspects, including practical limitations and algorithms from graph theory, and how machine learning can be used in future work.

TBA In the framework set by the AdS/MERA conjecture, we investigate a generalisation of the Tensor Network description of bulk geometry in the language of Group Field Theories, a promising convergence of insights and results from Matrix Models, Loop Quantum Gravity and simplicial approaches. We establish a first dictionary between Group Field Theory and Tensor Network states. With such a dictionary at hand, we target the calculation of the Ryu-Takayanagi formula recently derived for Random Tensor Networks in the quantum gravity formalism.

In this talk I will discuss issues and possibilities to outline Quantum Gravity Phenomenology using cosmological and astrophysical data. After a brief review on some formal aspect of the problem I will focus on the analysis of in-vacuo dispersion features for GRB (gamma-ray-burst) neutrinos of energy in the range of 100 TeV, and for GRB photons with energy in the range of 10 GeV. I will introduce a strategy of data analysis which has the advantage of being applicable to several alternative possibilities for the laws of propagation of neutrinos and other particles in a quantum spacetime. Far from being conclusive, the results to be shown should nonetheless motivate a vigorous program of investigation following the methodology of multi-messenger astronomy.

In this talk, I will show that light cones in MInkowski spacetime are a beautiful analoue of black hole horizons in curved spacetime. To do so, I will prove the analogue of the four laws of black hole thermodynamics in this setting. This is what we called light cone thermodynamics. More precisely, I will consider null surfaces defined by the out-going and in-falling wave fronts emanating from and arriving at a sphere in Minkowski spacetime. Such null surfaces, made of pieces of light cones, are bifurcate conformal Killing horizons for suitable conformally stationary observers. They can be extremal and non-extremal depending on the radius of the shining sphere. Such conformal Killing horizons have a constant light cone (conformal) temperature, given by the standard expression in terms of the surface gravity. Considering exchanges of conformally invariant energy across the conformal horizon, one can prove, in perturbation theory, a first law where entropy changes are given by 1/4 of the changes of the area of the bifurcate surface in Planck units. In the extremal case they become light cones associated with a single event; these have vanishing temperature as well as vanishing entropy. I will conclude with possible generalisations and applications of such results.