Scientific Series

We introduce the basic elements of tensorial group field theories (TGFTs) for quantum gravity, emphasizing how they encode quantum geometry and their relation with canonical loop quantum gravity and spin foam models. Next, we discuss briefly the issue of continuum limit and how it could be understood in this framework.

In the bulk of the talk, we overview the work on the extraction of an effective cosmological dynamics from TGFTs, inspired by the idea of the universe as a quantum gravity condensate. In this context, we emphasize: the need for appropriately coarse-grained states capturing collective dynamics and the role of relational observables and their construction. We discuss what the theory says (so far) about the fate of the big bang singularity at the beginning of our universe and how it suggests a quantum gravity origin for (phantom) dark energy.

In the last couple of years it has been demonstrated that black hole spacetimes contain many more marginally outer trapped surfaces (MOTS) than had been previously recognized. For example, there is an infinite family of axially symmetric MOTS even in the Schwarzschild solution, of which the apparent horizon is only the first element. In a recent series of papers (arXiv: 2104.10265, 2104.11343, 2104.11343) we demonstrated that these exotic new MOTS play a key role in black hole mergers and, in fact, are the missing pieces needed to complete the apparent horizon “pair of pants” diagram. This merger turns out to be far richer than that of event horizons and, staying with clothing analogies, is better represented by a rococo ball gown than a pair of pants. 

In this talk, I will overview the techniques that we used to find these surfaces, explain the role played by the exotic MOTS in resolving the merger of apparent horizons during a (head-on, non-rotating) black hole merger and also show how the stability operator brings order to what initially appears to be a chaotic melee of (often self-intersecting!) MOTS as they create, annihilate and weave through time. The stability operator was initially introduced by Andersson, Mars and Simon to characterize when a MOTS will smoothly evolve in time, but I will show that it can also be fruitfully understood as the Jacobi operator for MOTS.

We give a complete characterization of the (non)classicality of all stabilizer subtheories. First, we prove that there is a unique nonnegative and diagram-preserving quasiprobability representation of the stabilizer subtheory in all odd dimensions, namely Gross’s discrete Wigner function. This representation is equivalent to Spekkens’ epistemically restricted toy theory, which is consequently singled out as the unique noncontextual ontological model for the stabilizer subtheory. Strikingly, the principle of noncontextuality is powerful enough (at least in this setting) to single out one particular classical realist interpretation. Our result explains the practical utility of Gross’s representation, e.g. why (in the setting of the stabilizer subtheory) negativity in this particular representation implies generalized contextuality, and hence sheds light on why negativity of this particular representation is a necessary resource for universal quantum computation in the state injection model. This last fact, together with our result, implies that generalized contextuality is also a necessary resource for universal quantum computation in this model. In all even dimensions, we prove that there does not exist any nonnegative and diagram-preserving quasiprobability representation of the stabilizer subtheory, and, hence, that the stabilizer subtheory is contextual in all even dimensions.

Ontic structural realism is a form of scientific realism based on quantum mechanics in two ways:
(i) particles are not taken to be individual entities because they are not distinguishable; and, (ii) entanglement is taken to be relational structure that does not reduce to the state of parts and their causal interactions.

Furthermore, the idea of modal structure in OSR is exemplified by the way quantum mechanics sits between Bell-inequality violation and no-superluminal signalling. However, what should the advocate of OSR say about the measurement problem and decoherence? Wallace and Saunders combines OSR with Everettianism and argue that they need each other. Are Ladyman and Ross justified in their reluctance to agree with him?

We discuss how we can use numerical-relativity simulations to derive gravitational-wave and electromagnetic models describing the binary  neutron star coalescence. We show how these models can be used within a multi-messenger framework to derive new constraints on the neutron-star equation of state and the Hubble constant. For this purpose, we analyze the gravitational wave signal GW170817 and its electromagnetic counterparts AT2017gfo and GRB170817A, together with X-ray observations by NICER, radio observations of massive pulsars, and nuclear theory computations. Similarly, we also discuss that a non-detection of a kilonova for the second binary neutron-star merger detection GW190425 placed constraints on the properties of the system.

Zoom Link: https://pitp.zoom.us/j/99911822549?pwd=NXNNMWJVOUhMTGJSSGYwWUN2NEcxQT09

An extension of general relativity (GR) obtained by adding local quadratic terms to the action will be considered. Such theory can be a viable UV completion of GR. The additional terms soften gravity above a certain energy scale and render gravity renormalizable. The presence of 4 derivatives implies via the Ostrogradsky theorem that the classical Hamiltonian is unbounded from below. Nevertheless, I will argue that the relevant solutions are not unstable, but metastable: when the energies are much below a threshold (that is high enough to describe the whole cosmology) runaways are avoided. Remarkably, the chaotic inflation theory of initial conditions ensures that such bound is satisfied and testable implications for the early universe will be discussed. I will also argue that the basic unitarity condition is satisfied when the theory is correctly formulated at the quantum level. Moreover, thanks to the UV softening of gravity, sufficiently light objects must be horizonless and I will discuss explicit analytic examples of horizonless ultracompact objects, which have interesting physical implications.

I discuss the question how string theory achieves a sum over bulk geometries with fixed asymptotic boundary conditions. I analyze this problem with the help of the tensionless string on AdS3xS3xT4 (with one unit of NS-NS flux) that was recently understood to be dual to the symmetric orbifold of T4. I argue that large stringy corrections around a fixed background can be interpreted as different semiclassical geometries, thus making a sum over semi-classical geometries superfluous.

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.

With the race for quantum computers in full swing, researchers became interested in the question of what happens if we replace a supervised machine learning model with a quantum circuit. While such "supervised quantum models" are sometimes called "quantum neural networks", their mathematical structure reveals that they are in fact kernel methods with kernels that measure the distance between data embedded into quantum states. This talk gives an informal overview of the link, and discusses the far-reaching consequences for quantum machine learning.

Detecting ultralight axion dark matter has recently become one of the benchmark goals of future direct detection experiments. I will discuss a new idea to detect such particles whose mass is well below the micro-eV scale, corresponding to Compton wavelengths much greater than the typical size of tabletop experiments. The approach involves detecting axion-induced transitions between two quasi-degenerate resonant modes of a superconducting accelerator cavity. Compared to more traditional setups, the sensitivity is parametrically enhanced for ultralight axions, allowing for the exploration of vast new areas of parameter space relevant to the QCD axion and astrophysically long-ranged fuzzy dark matter.

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.