Talks

Recently a new solution to the hierarchy problem was proposed which makes use of the cosmological evolution of a light scalar field, a scanner, instead of symmetry or anthropic arguments to select a small Higgs mass. In the original proposal this scanner field could be the QCD axion and thus such class of solution became known as ``relaxion’’. Two central features required of the relaxion are a reason for why a small Higgs mass is special from the scanner’s viewpoint and a mechanism fo the scanner to dissipate its energy in order to stop in a value associated with small Higgs mass. In this talk we propose a novel mechanism that achieves both goals and opens new possibilities for the relaxion. We show that particle production can create an effective friction force for the relaxion and show how it can be used to ``select’’ the TeV scale as a special energy scale from reasonable initial conditions. This allows for the scanning to happen much faster than in previous relaxion models, which usually require very large amounts of inflation, and also allows for the scanning to take place after inflation has ended.

Pulsars are some of physics and astrophysics’ most exotic objects, and they have already earned two Nobel Prizes. We currently know of about 2500 of them in our Galaxy, but a small subset, the millisecond pulsars (MSPs), are truly remarkable. These systems are notoriously hard to detect, yet their numbers have more than doubled in the past 5 years via surveys using the world’s most sensitive
telescopes, new instrumentation, and huge amounts of computing. Specialized “timing” observations of these systems, accounting for each and every one of the billions of rotations of the stars, are providing fantastic results in basic physics. In this talk I’ll focus on the efforts to directly detect gravitational waves from super-massive black hole binaries, make strong-field tests of general relativity, and determine the nature of the densest form of matter known in the universe.

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.