Talks

The problem of time is often discussed as an obstacle in the canonical quantisation of gravitational systems: general covariance means there is no preferred time parameter with respect to which evolution could be defined. We can instead characterise dynamics in relational terms by defining one degree of freedom to play the role of an internal clock for the other variables; this leads to a multiple choice problem of which variable should play the role of clock. I will review recent results obtained in a quantum cosmological model with three dynamical degrees of freedom: a volume or scale factor variable for the geometry, a massless scalar matter field, and a perfect fluid. Each of these variables can be used as a clock for the other two. We obtain three different theories which, if we require them to have unitary time evolution with respect to the given clock, make very different statements about the fate of the Universe. Only one resolves the classical singularity, and only one leads to a quantum recollapse of the Universe at large volume. Nonclassical behaviour arises whenever a classical solution terminates in finite time so that reflecting boundary conditions are needed to make the theory unitary. We discuss general implications for a canonical quantisation of gravitational systems.

Zoom Link: https://pitp.zoom.us/j/95556457739?pwd=S0dZUVYwTTBOaFNpNGcrc2ladHZ5QT09

In this talk, I will present how gravitational-wave (GW) measurements can be utilized to search for ultralight bosons and primordial black holes (PBHs), which can be the keys to some of the unsolved questions about our Universe. In the first half, I will discuss the current constraints on excluding the mass of ultralight bosons around ~10^-13 eV, based on the mass-spin measurements of BBHs from the second GW catalog (GWTC-2). These constraints are driven by the phenomenology called “superradiance”, which extracts the angular momentum of a rapidly spinning black hole to form a slowly spinning black hole-boson cloud system. In the second half, I will illustrate how the distance measurements of very high-redshift BBHs made by the next-generation GW detectors may provide cleaner hints or constraints for the existence of PBHs in the future.

Zoom Link: https://pitp.zoom.us/j/95012157554?pwd=UWdBZ1FOTSt2dVlBaS81MjRLbVBvUT09

Classical machine learning (ML) provides a potentially powerful approach to solving challenging quantum many-body problems in physics and chemistry. However, the advantages of ML over more traditional methods have not been firmly established. In this work, we prove that classical ML algorithms can efficiently predict ground state properties of gapped Hamiltonians in finite spatial dimensions, after learning from data obtained by measuring other Hamiltonians in the same quantum phase of matter. In contrast, under widely accepted complexity theory assumptions, classical algorithms that do not learn from data cannot achieve the same guarantee. We also prove that classical ML algorithms can efficiently classify a wide range of quantum phases of matter. Our arguments are based on the concept of a classical shadow, a succinct classical description of a many-body quantum state that can be constructed in feasible quantum experiments and be used to predict many properties of the state. Extensive numerical experiments corroborate our theoretical results in a variety of scenarios, including Rydberg atom systems, 2D random Heisenberg models, symmetry-protected topological phases, and topologically ordered phases.

We will begin by explaining the black hole information paradox, starting from first principles. We will then explain how computations in string theory yield a resolution of this paradox. When we make a bound state of strings and branes, then this bound state is found to swell up into a horizon sized `fuzzball'; this fuzzball radiates like a normal body without any information loss. The existence of these fuzzballs implies a new picture for the quantum gravitational vacuum, where the virtual fluctuations resemble the scale free fluctuations at a second order phase transition, rather than being confined to within the planck scale.  We will see how this `vecro' picture of the vacuum might give a resolution to several puzzles we face in cosmology, like the origin of energy needed for inflation and the existence of a cosmological constant.

Thanks to integrability, we have access to a significant portion of the conformal data of planar N=4 SYM: in particular, the scaling dimensions of all single-trace operators at finite 't Hooft coupling. Can we solve the theory by using these data as an input for the conformal bootstrap? 

We recently started to explore this question in a simplified setup: the one-dimensional CFT living on a supersymmetric Wilson line embedded in the 4D theory. 

After reviewing how integrability describes the spectrum of this 1D CFT, I will discuss how, using the numerical conformal bootstrap with a minimal input from the spectral data, it is possible to compute with good precision a non-supersymmetric OPE coefficient at finite 't Hooft coupling. I will conclude discussing the open questions and future perspectives. Based on 2107.08510 with N.Gromov, J.Julius and M.Preti.

Sterile neutrinos have been proposed to tackle a number of outstanding questions in physics, including the phenomena of dark matter, neutrino masses and the baryon asymmetry of the Universe. I will review current limits on GeV-, MeV- and keV-scale sterile neutrinos from cosmology and astrophysics. In particular, I will focus on how primordial abundance determinations, Cosmic Microwave Background observations and stellar kinematic inferences from dwarf spheroidal galaxies allow us to set robust constraints across this mass scale. I will then end with a short discussion on how sterile neutrinos could relax cosmological bounds on neutrino masses and what implications this would have for experiments like KATRIN.
 

The large scale distribution of matter in the Universe contains the answers to many mysteries, such as the nature of dark matter, the reionization of the Universe, and the growth of galaxies. Cosmological simulations are the only way to understand these questions. I will talk about how modern current simulation models, work, discuss some new models and improvements in our latest simulation runs, especially our implementations of reionization and cosmology. I will then talk about some new work to dramatically expand the region of applicability of these simulations using machine learning. This can both to expand their dynamic range and combine different simulations to infer the physical parameters of the Universe.

Traditionally, quantum many-body theory has focussed on ground states and equilibrium properties of spatially extended systems, such as electrons and spins in crystalline solids. In recent years “noisy intermediate scale quantum computers” (NISQ) have emerged, providing new opportunities for controllable non-equilibrium many-body systems. In such dynamical quantum systems the inexorable growth of non-local quantum entanglement is expected, but monitoring (by making projective measurements) can compete against entanglement growth. In this talk I will overview theoretical work exploring the behavior of “monitored” quantum circuits, which can exhibit a novel quantum dynamical phase transition between a weak measurement phase and a quantum Zeno phase, the former which we characterize in detail. Accessing such physics in the lab is challenged by the need for post-selection, which might be circumnavigated by decoding using active error correction, conditioned on the measurement outcomes, as will be described in systems with Z2 symmetry.

Zoom Link: https://pitp.zoom.us/meeting/register/tJcqc-ihqzMvHdW-YBm7mYd_XP9Amhypv5vO

I will discuss the dynamical friction experienced by black holes moving through scalar or “wave-like” dark matter. This has been studied analytically and numerically in the non relativistic case, which is applicable to its effect in “fuzzy dark matter” halos where the scalar wavelength is on galactic scales. On smaller scales, dynamical friction from dark matter clouds formed from accretion or superradiance may cause a dephasing in the gravitational wave signal in LISA EMRIs. I will discuss our recent numerical work (https://arxiv.org/abs/2106.08280) to extend the description of dynamical friction to relativistic scalars, in order to treat this case.

Zoom Link: https://pitp.zoom.us/j/98815587081?pwd=a3FvcUtTaWFUejBpR1Q3QUhRRzF4dz09

The information loss paradox is usually stated as an incompatibility between general relativity and quantum mechanics. However, the assumptions leading to the problem are often overlooked and, in fact, a careful inspection of the main hypothesises suggests a radical reformulation of the problem. Indeed, I will present a thought experiment that shows the existence of an incompatibility between (i) the validity of the laws of general relativity to describe infalling matter far from the Planckian regime, and (ii) the so-called central dogma which states that as seen from an outside observer a black hole behaves like a quantum system whose number of degrees of freedom is proportional to the horizon area. We critically revise the standard arguments in support of the central dogma, and argue that they cannot hold true unless some new physics is invoked even before reaching Planck scales. This leads to the counterintuitive result that the information loss problem, in its current formulation, is not necessarily related to any loss of information or lack of unitarity. Semiclassical general relativity and quantum mechanics can be perfectly compatible before reaching the final stage of the black hole evaporation where, instead, a consistent theory of quantum gravity is needed to make any prediction.

Zoom Link: https://pitp.zoom.us/j/94637978136?pwd=VmlWa1BJU3I1UGVRNjliMEpUTlo2dz09

The double copy relates scattering amplitudes in gauge theory and gravity. But interaction potentials (and spacetime metrics) can be extracted from amplitudes, and so the double copy leads to a relationship between classical solutions of gauge theory and gravity. In this talk I will describe this relationship, provide a perspective on the Schwarzschild metric as a “square” of the Coulomb charge, and take a look at the “square root” of the Kerr metric.