Talks

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.

In this talk, we explore new techniques to probe the analytic structure of scattering amplitudes in perturbative Quantum Field Theory. The goal of this approach is to leverage symmetries, limits, and analyticity in order to circumvent the explicit evaluation of multi-loop Feynman integrals. First, we generalize the cutting rules by relating sequential discontinuities (discontinuities of discontinuities) to multiple cuts through the corresponding Feynman diagram. Then, we determine the logarithmic branch-cut structure of massive scattering amplitudes by expanding around their branch points. As a corollary, we prove a conjectured bound on the transcendental weight of polylogarithmic Feynman integrals. These results pave a new path towards bootstrapping scattering amplitudes in perturbation theory.

Finding cosmological models that are consistent with a wide range of observations, including those that indicate a high Hubble constant (the current rate of expansion of space), has proved to be very challenging. In this talk I explore why. Our exploration leads us to fundamental sources of length scales in cosmological models: gravitational collapse time scales and photon electron-scattering mean free paths. We find that scaling down these quantities leaves cosmic microwave background (CMB) maps, and many other cosmological observables, nearly invariant, while boosting up the expansion rate. I then introduce a model that takes advantage of this symmetry to boost up model predictions of the Hubble constant given CMB and other cosmological data. Gravitational collapse time scales are scaled down from standard cosmological values by the introduction of a `mirror world' dark sector, and photon mean free paths are scaled down by reducing the amount of helium, thereby freeing up more electrons. I speculate about alternative ways to reduce photon mean free paths, as consistency with the Riess et al. (2021) measurement of the Hubble constant requires there to be less helium in the universe than is observed. 

Redshift space distortions and weak gravitational lensing have been used as a powerful combination of growth data to test gravity. In particular, combining these two probes where spectroscopic and photometric surveys overlap, can yield much stronger dark energy and growth constraints than a combination of independent measurements of the two. However, this approach is limited due to a fairly small overlapping area between spectroscopic and photometric surveys. In this talk, I will introduce a new method to optimally combine spectroscopic and photometric surveys, using the DMASS galaxy sample as gravitational lenses. The new approach can extract the full statistical power of photometric surveys beyond the overlapping area. I will illustrate how this approach with DMASS improves cosmological constraints in the frame of modified gravity and will show its application to future surveys having a limited overlap such as DESI and LSST.

There are many different interpretations of quantum mechanics. Among them, QBism and Rovelli's Relational Quantum Mechanics (RQM) are special because they both propose that reality itself is produced relative to "observers". For QBism, observers are defined as rational decision-making "agents", while in RQM any physical system can be an observer. But both interpretations agree that reality is shaped by what happens when observers encounter the world external to themselves. In this talk I will try to understand what these interpretations imply for the ongoing problem of defining an ontological model of quantum mechanics.

I will describe a concrete, computable example of holographic geometry emerging purely from the combinatorics of large matrices. Correlation functions of determinant operators in a matrix-valued free chiral algebra will be matched to holomorphic curves in SL(2,C).

Zoom Link: https://pitp.zoom.us/j/99341570607?pwd=TVVmSkF3d1haa0hBOWlPeDhGNi9aQT09

In the asymptotic safety approach to quantum gravity it is conjectured that a nonperturbative ultraviolet completion exists for Einstein's metric gravity and its perturbative effective description in four dimensions. Such completion would circumvent the notorious problems of renormalizability and predictivity at high energies of the quantum theory. I want do discuss the conjecture armed with pragmatism and avoiding preconceived ideologies. I also want to present some perturbative results in two dimensions, that can be used to argue the validity of the conjecture above two dimensions, and to discuss the implications for the current status of the approach.