General Relativity & Quantum Cosmology

Gravitational wave observations are beginning to reveal the nature of the dark side of our universe. The Advanced LIGO and Virgo detectors have observed dozens of binary black hole mergers during the recent third observing run and, with planned sensitivity improvements, expect to observe significantly more binary black hole mergers in future observing runs. The combination of the increased number of detections  and the sheer volume of data associated with each detection provides a significant data analysis challenge. In recent years, various machine learning approaches such as convolutional neural networks have been explored as a basis for rapid analyses for gravitational wave data. This seminar will give a brief introduction to current transient gravitational wave data analysis methodology and highlight novel applications of machine learning for rapid detection of binary black holes and rapid inference of their astrophysical properties. The use of generative machine learning algorithms for transient gravitational wave signal generation will also be discussed.

A fundamental problem of quantum gravity is to understand the quantum evolution of black holes.  While aspects of their evolution are understood asymptotically, a more detailed description of their evolving wavefunction can be provided.  This gives a possible foundation for studying effects that unitarize this evolution, which in turn may provide important clues regarding the quantum nature of gravity.

The issue of whether quantum effects can affect gravity at cosmological distances still lacks a fundamental understanding, but there are indications of a non-trivial gravitational infrared dynamics. This possibility is appealing for building alternatives to the standard cosmological model and explaining the accelerated expansion of the Universe. In this talk I will discuss some large scale modifications of general relativity due to nonlocal terms, which are assumed to arise at the level of quantum effective action. Nonlocality is a general feature of quantum effective actions for theories with massless degrees of freedom and dynamical mass generation is a typical non-perturbative IR effect. Among several models, cosmological requirements select a single structure of the nonlocal term describing a mass for the conformal mode of the metric. The model fits very well cosmological data and has strong signatures in the tensor sector that could be tested in the future by gravitational-wave detections.

Since their first discovery in 2015, gravitational-wave observations yielded several "surprises." The LIGO and Virgo observatories detected more and heavier black holes than anticipated; the first object in the lower mass gap was found; and LIGO announced the discovery of a particularly heavy black hole that could have not come from stellar core collapse. The surprises point to the possibility that some of LIGO/Virgo's black hole mergers occurred in the dense accretion disks of active galactic nuclei (AGNs). AGNs act like black hole assembly lines, resulting in multiple consecutive mergers that create heavier and faster-spinning black holes. I will discuss what we currently know about AGN-assisted mergers and which of LIGO/Virgo's events are suspects. I will finally discuss the prospects of multi-messenger observations from AGN assisted mergers.

Zoom Link: https://pitp.zoom.us/j/93121526365?pwd=c1VCTjBEZnlXYk5HVTFObVBadHlxQT09

The model of Causal Dynamical Triangulations (CDT) is a background-independent and diffeomorphism-invariant approach to quantum gravity,

which provides a lattice regularization of the formal gravitational path integral. The framework does not involve any coordinate system and employs only geometric invariants. For a Universe with toroidal spatial topology, we can introduce coordinates using classical scalar fields with periodic boundary conditions with a jump. The field configurations reveal pictures of cosmic voids and filaments surprisingly similar to the ones observed in the present-day Universe. I will discuss the impact of dynamical matter fields on the geometry of a typical quantum universe in the four-dimensional CDT model and explain several observed phenomena. In particular, a phase transition is triggered by the change of the scalar field jump amplitude. This discovery may have important consequences for quantum universes with non-trivial topology since the phase transition can change the topology to a simply connected one.

Gravitational waves provide a unique observational handle on the properties of strong, dynamical gravity. Ringdowns, in particular, cleanly encode information about the structure of black holes, allowing us to test fundamental principles like the no-hair theorem and the area law. In this talk, I will review the status of this effort, including recent observational results and remaining challenges.

In this talk I will discuss how the generators of radial conformal symmetries in Minkowski space-time are related to the generators of time evolution in conformal quantum mechanics. Within this correspondence I will show that in conformal quantum mechanics the state corresponding to the inertial vacuum for a conformally invariant field in Minkowski space-time has the structure of a thermofield double. The latter is built from a bipartite "vacuum state" corresponding to the ground state of the generators of hyperbolic time evolution, which cover only a portion of the time domain. When such generators are the ones of conformal Killing vectors mapping a causal diamond in itself and of dilations, the temperature of the thermofield double reproduces, respectively, the diamond temperature and the Milne temperature perceived by observers whose constant proper time hyper-surfaces define a hyperbolic slicing of the future cone. I will point out how this result indicates that, for conformal invariant fields, the fundamental ingredient for vacuum thermal effects in flat-space time is the non-eternal nature of the lifetime of observers rather than their acceleration.

Over forty detections of binary-black-hole mergers have been made during the first three observing runs of the LIGO and Virgo detectors. With this larger number of measurements of increasing accuracy, many of the remarkable predictions of general relativity for strongly curved, dynamical spacetimes will be able to be studied observationally. In this talk, I will discuss one class of strong-gravity phenomena, called gravitational-wave memory effects, which are predictions of general relativity that are most prominent in systems with high gravitational-wave luminosities, like binary black holes. Memory effects are characterized by changes in the gravitational-wave strain and its time integrals that persist after a transient signal passes by a detector. I will summarize the computation of these effects and the prospects for current and planned future gravitational-wave detectors to detect memory effects from black-hole mergers; in particular, there could be evidence for the memory effect in just a few years of advanced LIGO, Virgo, and KAGRA data at their design sensitivities. I will also review what observing gravitational-wave memory effects can teach us about the symmetries and conserved quantities around isolated systems like binary-black-hole mergers. Time permitting, I will present results on memory effects in scalar-tensor theories of gravity and on subleading memory effects.

We are experimentally investigating possible departures from the standard quantum mechanics’  predictions at the Gran Sasso underground laboratory in Italy. In particular, with radiation detectors we are searching signals predicted by the collapse models (spontaneous emission of radiation) which were proposed to solve the “measurement problem” in quantum physics and signals coming from a possible violation of the Pauli Exclusion Principle. 

I shall discuss our recent results published in Nature Physics under the title “Underground test of gravity-related wave function collapse”, where we ruled out the natural parameter-free version of the gravity-related collapse model.  I shall then present more generic results on testing CSL (Continuous Spontaneous Localization) collapse models and discuss future perspectives.

Finally, I shall briefly present the VIP experiment with which we look for possible violations of the Pauli Exclusion Principle by searching for “impossible” atomic transitions and comment the impact of this research in relation to Quantum Gravity models.

 

Core-Collapse Supernovae are a fantastic laboratory to study fundamental physics.  The messengers from the core, neutrino and gravitational waves, carry a wealth of information about the dynamics, thermodynamics, underlying physics, and structure of massive stars at the end of their lives.  In this talk, I will discuss some of the ways we can use supernovae to probe this physics.  From quark-hadron phase transitions emitting unique gravitational wave and neutrino signals, to neutrinos telling us precise information about the distance to and progenitor mass (!) of galactic supernovae, even before the optical signal is emitted.  I will also talk about current efforts to model the population of core-collapse supernovae using parameterized 1D models.

 

Asymptotically safe gravity is one of the most conservative approaches to quantum gravity. It relies on the framework of quantum field theory and the Wilsonian renormalization group. Recently, questions and open issues have been discussed both within and outside its community. This week, instead of a seminar, we will have a debate between John Donoghue ("A Critique of the Asymptotic Safety Program", arXiv:1911.02967) and Roberto Percacci ("Critical reflections on asymptotically safe gravity", arXiv:2004.06810), who will critically discuss the status of the field, and highlight its strengths and challenges. 

The X-ray source Cygnus X-1 was discovered by sounding rockets in 1964 and is perhaps the oldest and best known "black hole candidate".   Its celestial coordinates were poorly known until 1971, when simultaneous radio flares and X-ray flares were observed which enabled the X-ray source to be associated with a 9th magnitude O-star known as HDE 226868.  This star was subsequently shown to be a single-lined spectroscopic binary with an orbital period of about 5.6 days.  However, owing to difficulties with measuring the system geometry and the distance to the source, the mass of the dark companion remained elusive.  In 1974, Stephen Hawking and Kip Thorne famously made a wager on the nature of the dark companion in Cygnus X-1, with Hawking wagering that the dark companion was not a black hole.  Eventually the consensus was that Cygnus X-1 does indeed contain a black hole, and Hawking conceded the bet in 1990.    In the following years many estimates for the black hole mass appeared in the literature, and most of them had large uncertainties, mainly owing to the difficulty of measuring the distance to the source.  In this talk I will describe how we were able to use VLBI radio observations to measure a precise distance to the source, and how, in turn, this precise distance allowed us to obtain precise masses for the component stars in Cygnus X-1.  Using the refined distance and geometry of the binary, it is possible to measure the relativistic spin parameter a* of the black hole using the X-ray continuum fitting technique.  We find that the black hole spin is nearly maximal with  a* > 0.9985 (3σ).