Colloquium

Two years ago the Laser Interferometer Gravitational-wave Observatory.

(LIGO) announced the first direct detection of gravitational waves; minute distortions in space-time caused by cataclysmic events far away in the universe.  Very recently, the merger of a binary neutron star system was detected by both of the Advanced LIGO detectors and the Advanced Virgo detector in Italy, triggering a massive follow-up campaign by ground and space-based telescopes.  A counterpart to the gravitational-wave source was located, and transient emission was detected from gamma rays to radio.  I will talk about the sources of the signals we detected, the physics behind the detectors, and prospects for the future of this emerging field.

CHIME is a new interferometric telescope at radio frequencies 400-800 MHz.The mapping speed (or total statistical power) of CHIME is among the largestof any radio telescope in the world, and the technology powering CHIME could be used to build telescopes which are orders of magnitude more powerful.This breakthrough sensitivity has the power to revolutionize radio astronomy, but meeting the computational challenges will require breakthroughs on the algorithmic side.  I'll give a status update on CHIME, with an emphasis on new algorithms developed at Perimeter to search for fast radio bursts (FRB's) and pulsars.

Diamond anvil cells and shock techniques now permit the study of matter under multimegabar (i.e. several hundred GPa) pressures. The properties of matter in this pressure regime differ drastically from those known at 1 atm. Just how different physics and chemistry are at high pressure and the role that a chemical intuition for bonding and structure can have in understanding matter at high pressures will be explored in this lecture. I will discuss in detail an overlapping hierarchy of responses to increased density, consisting of (a) squeezing out van der Waals space (for molecular crystals); (b) increasing coordination; (c) decreasing the bond length of covalent bonds and the size of anions; and (d) an extreme regime of electrons moving off atoms and new modes of correlation. Examples of the startling chemistry and physics that emerge under such extreme conditions will alternate in this account with qualitative chemical ideas about the bonding involved.

Quantum field theories play an important role in many condensed matter systems for their description at low energies and long length scales. In 1+1 dimensional critical systems the energy spectrum and the spectrum of scaling dimensions are intimately related in the presence of conformal symmetry. In higher space-time dimensions this relation is more subtle and not well explored numerically. In this talk we motivate and review our recent effort to characterize 2+1 dimensional quantum field theories using computational techniques 2+targetting the energy spectrum on a spatial torus. We discuss several examples ranging from the O(N) Wilson Fisher theories and Gross-Neveu-Yukawa theories to deconfinement- confinement transitions in the context of topological ordered systems. We advocate a phenomenological picture that provides insight into the operator content of the critical field theories.

Red supergiants (RSGs) are the helium-fusing evolved descendants of moderately massive (10-25Mo) stars, the result of a near-horizontal evolution across the top of the Hertzsprung-Russell diagram following their time on the main sequence. As the coldest and largest (in physical size) members of the massive star population, these stars represent a significant evolutionary extreme and serve as ideal "magnifying glasses" for scrutinizing our current understanding of massive stars and their role in the universe. RSGs are significant dust producers in young stellar populations, the observationally-confirmed progenitors of core-collapse supernovae, and a crucial step in the formation and population statistics of massive interacting binaries (including those that will ultimately produce compact object binaries and gravitational waves). Observations of RSGs can also be used to test stellar evolution and population models and as metallicity indicators in nearby galaxies. This talk will provide an overview of our field's knowledge of RSGs, identify some of the most pressing current questions about these stars, and consider the importance of RSGs in the coming decade as the next generation of observatories - including JWST, WFIRST, and the ELTs - comes online.

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? 

Two seemingly different quantum field theories may secretly describe the same underlying physics — a phenomenon known as “duality”. Duality has been proved powerful in condensed matter physics, since many difficult questions can be drastically simplified in certain “dual” pictures. This is especially valuable for strongly interacting many-body problems, for which traditional tools (such as perturbation theory) are often not applicable. 

Recent developments on dualities have also revealed deep connections between several previously unrelated topics in modern condensed matter physics, including topological insulators, fractional quantum Hall effects and quantum phase transitions. Connections were also made between dualities in condensed matter physics and in high energy physics. I will give a brief review of some of these developments.

The study of black holes has revealed a deep connection between quantum information and spacetime geometry. Its origin must lie in a quantum theory of gravity, so it offers a valuable hint in our search for a unified theory. Precise formulations of this relation recently led to new insights in Quantum Field Theory, some of which have been rigorously proven. An important example is our discovery of the first universal lower bound on the local energy density. The energy near a point can be negative, but it is bounded below by a quantity related to the information flowing past the point.

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.

Hidden-variables theories account for quantum mechanics in terms of a particular 'equilibrium' distribution of underlying parameters corresponding to the Born rule. In the most well-studied example, the pilot-wave theory of de Broglie and Bohm, it is well established that the Born rule may be understood to arise from a process of dynamical relaxation. This 'quantum relaxation' may have taken place in the very early universe and could have left imprints on the cosmic microwave background (CMB). Such imprints amount to signatures of the decay of early violations of the Born rule. In this colloquium we summarise recent progress in making detailed predictions and in comparing them with the reported large-scale anomalies in the CMB data.

One of the most enduring mysteries in particle physics is the nature of the non-baryonic dark matter that makes up 85% of the matter in the universe. For several decades, most searches for this mysterious substance have focused on Weakly Interacting Massive Particles (WIMPs). Recently, there has been a surge in theoretical interest in ultra-light-field dark matter candidates, including QCD axions (spin 0 bosons) and hidden photons (spin 1 bosons), which can be probed through their coupling to electromagnetism or nuclear spin. I will discuss general principles of efficiently searching for the direct detection of the electromagnetic coupling of these candidates, how the sensitivity of these experiments can be improved by the exploitation of quantum correlations in the electromagnetic signals that they produce, and describe the Dark Matter Radio, and experiment searching for axions and hidden photons in the frequency range of 1 kHz through 100 MHz.