Colloquium

General relativity taught us that space time is dynamical and quantum theory posits that dynamical objects are quantum. Hence the Newtonian notion of space time as a passive stage where physics takes place needs to be replaced by a notion of quantum space time which is interacting with all other quantum matter fields.

I will present recent developments that aim at constructing quantum space time. In particular I will explain how  topological quantum field theories give rise to new quantum geometry realizations and how these serve as starting points  for the construction of a  dynamics  of quantum gravity, which is consistent over all scales.  Such a dynamics will then determine the properties of quantum space time.  

Topological phases of matter serve as one of the most striking examples of the richness and novelty of quantum systems with many degrees of freedom.  In contrast to conventional matter, they are characterized by both non-local properties and non-classical notions such as entanglement.  I will discuss two broad categories of topological phases, distinguished by whether or not they possess fractionalized “anyon” excitations that are neither bosons nor fermions.  I will demonstrate that entanglement not only provides an understanding of such phases but also enables the transmutation between these two categories of topological phases. 

We normally think of large accelerators and large-scale cosmic events when we consider the frontiers of elementary particle physics, pushing to understand the universe at higher and higher energy scales. However, several tabletop low-energy experiments are posed to discover a wide range of new physics beyond the Standard model, where feeble interactions require precision measurements rather than high energies.  In our experiments, high-Q resonant sensors enable ultra-sensitive force and field detection. In this talk I will describe two applications of these sensors in searches for new physics, based on techniques in atomic-molecular-and optical (AMO) physics. First, I will discuss an experiment which uses laser-cooled optically trapped silica nanospheres to search for corrections to Newtonian gravity at micron distances with zeptonewton sensitivity.  Finally, I will discuss the Axion Resonant InterAction Detection Experiment (ARIADNE), a new precision magnetometry experiment using laser-polarized 3-He gas to search for a notable dark-matter candidate: the QCD axion.  

I present three possible non-standard additions to cosmology.  First I show that a very long early period of inflation could exist in which parameters evolve, or 'relax', to seemingly fine-tuned values.  Next, I show that even if cosmic inflation existed, a period after inflation with anisotropic stress can dramatically affect super-horizon modes and thus the imprint on the cosmic microwave background.  Finally, I show that cosmological singularities can be avoided by a bounce without using exotic matter that violates the Null Energy Condition, but by the addition of vorticity in compact extra dimensions.

In 1983, Nielsen and Ninomiya predicted that the Adler-Bell-Jakiw (or chiral) anomaly should be observable in a crystal that has protected Dirac states in the bulk (3+1 D). Following recent progress in the field of Topological Quantum Matter, the anomaly has now been observed, most clearly in the two semimetals Na3Bi and GdPtBi. I will discuss the general problem of realizing Weyl Fermions in semimetals, and explain what the chiral anomaly is in condensed matter. I will remark on its historical context, starting with pion decay. Several tests that support our conclusion will be described.

*Supported by the Moore Foundation, ARO and NSF.

Population expansions are ubiquitous in nature. They control the speed of many important dynamical processes, including multicellular development, biological evolution and epidemic outbreaks. Yet, the theoretical description of spreading behaviors has been limited largely to mean-field models that ignore the randomness inherent to living systems. In this talk, I present theoretical arguments and experimental results that elucidate how noise influences spreading processes on many scales, ranging from cellular scales, where jamming cells impede their own expansion, to global scales, where epidemic spread relies on rare long-range jumps.  Our results underscore that carrying excellent genes does not guarantee success in evolution - the pure luck of being in the right place at the right time can be equally, or more, important.

It has long been believed that Stradivari and his contemporaries in 18th Century Italy built violins with playing qualities unmatched by later makers. However, a team of researchers led by Claudia Fritz and Joseph Curtin have shown that under double-blind conditions neither professional violinists nor experienced listeners can tell Old Italian violins from new ones at better than chance levels. Moreover, players and listeners tend to prefer the new. Violin-maker, researcher, and MacArthur Fellow Joseph Curtin will discuss recent developments in violin science, and his own interest in measuring violin sound in order to establish objective parameters for violin quality.

With two confident binary black hole mergers already detected in their first observing run, the advanced LIGO detectors are expected to detect hundreds more in coming years.  We are poised to learn more about compact binary (e.g., BNS, NSBH, BBH) formation than ever before, but doing so will demand both detailed characterization of individual events, and inference of the properties of the underlying population as a whole.  I will present the methods behind the detection and characterization of LIGO's binary black hole mergers, what they have taught us about compact binaries thus far, and what they will potentially teach us about compact binary formation in the future.

In 2015 the LIGO detectors observed gravitational waves from two distinct stellar-mass binary black hole mergers. This long awaited feat now opens avenues to explore astrophysical questions which cannot, or are difficult to, be answered purely by electromagnetic means. Massive stars which end their lives in a pair-instability supernova are not thought to leave a remnant behind, meaning there should exist a gap in the black hole mass spectrum. In this talk I will discuss whether LIGO observations can tell us something about this apparent mass gap. 

LIGO is currently operating in its second science run, and like its first, alerts are sent to electromagnetic partners whenever a candidate event is identified. I will also discuss the efforts being made to capture the signature of an electromagnetic counterpart, some of the facilities involved and the hurdles which need to be overcome to make a confident association between a gravitational-wave signal and an electromagnetic transient.

Galaxy mergers are a standard aspect of galaxy formation and evolution, and most (likely all) large galaxies contain supermassive black holes. As part of the merging process, the supermassive black holes should in-spiral together and eventually merge, generating both continuous gravitational waves and a background of gravitational radiation in the nanohertz to microhertz regime.  An array of precisely timed pulsars spread across the sky can form a galactic-scale gravitational wave detector in the nanohertz band. I describe the current efforts to develop and extend the pulsar timing array concept, together with recent limits which have emerged from international efforts to constrain astrophysical phenomena at the heart of supermassive black hole mergers.

The spectral action functional of noncommutative geometry provides a model of Euclidean (modified) gravity, possibly coupled to matter. The terms in the large energy asymptotic expansion of the spectral action can be computed via pseudodifferential calculus. In the case of highly symmetric spacetimes, like Robertson-Walker metrics and Bianchi IX gravitational instantons, there is a richer arithmetic structure in the spectral action, and the terms in the asymptotic expansion are expressiblein terms of periods of motives and of modular forms. This reveals a new occurrence of interesting periods and motives in high-energy physics.