General Relativity & Quantum Cosmology

The standard cosmological model informed by cosmic microwave background (CMB) fluctuations makes a precise prediction for the growth of matter density fluctuations over cosmic time on linear scales. A variety of cosmological observables offer independent and complementary ways of testing this prediction, but results have been mixed, with many constraints on the amplitude of structure being 2-3σ lower than the expectation from Planck primary CMB anisotropies. Could this be due to new physics? Or could this be due to complexities like feedback from active galaxies? I will show how gravitational lensing of the CMB has and will continue to provide insights into this problem, highlighting recent results from the Atacama Cosmology Telescope and the Simons Observatory.

Over the past 25 years, particle physics has grown and thrived by making connections with other fields of physics and the physical sciences in general. As we look forward to the next 25 years, the flexible toolbox of high-energy physics can be put to use on a number of problems outside the traditional application of collider physics, and we should keep an open mind and go where the data leads us. I will give several examples of fruitful interdisciplinary collaborations in particle physics from my own research career, including condensed matter physics, quantum chemistry, quantum sensing, and machine learning.

Fast Radio Bursts are a recently discovered phenomenon consisting of brief (typically few millisecond) bursts of radio waves coming from far outside our Milky Way galaxy, indeed from cosmological distances. Their origin is unknown. I will review what is known about these mysterious sources, and how they can act as novel probes of the matter distribution in the Universe. I will focus on results from the CHIME Fast Radio Burst Project, which uses a new Canadian digital radio telescope that is revolutionizing our view of the fast transient sky. I will also introduce the CHIME/FRB Outriggers, which will enable precise sky localizations for >1000 CHIME FRBs, hence permit host galaxy ID and redshift determinations.

To date only one astronomical event has been observed in both gravitational waves and electromagnetic waves -- the merger of two neutron stars known as GW170817. This event was detected in gamma rays simultaneously with gravitational waves, but was poorly localized initially. No other counterparts were detected until localization was improved leading to an 11 hour dearth of data in other EM wavelengths. GW170817 also demonstrated that realistic neutron star mergers may have off-axis GRB observations that could be sub-threshold in modern instruments. Here we describe an ongoing coordinated effort to detect binary neutron stars before they merge using gravitational waves and to slew NASA's Swift observatory to catch prompt potentially sub-threshold GRB and x-ray emission. If successful, this ambitious project would pin down the event location allowing for prompt follow-up observations across all other wavelengths. Multimessenger observations of binary neutron star mergers (gravitational waves and electromagnetic waves) have deep implications for nuclear physics, strong gravity and cosmology.

Compact, spinning, and horizonless spacetimes can develop an ergoregion, where massless negative-energy states are quasi-trapped and drive the ergoregion instability. I will briefly review the linear mechanism and then describe recent progress in understanding the nonlinear evolution. Nonlinear mode coupling can amplify high-frequency modes through a turbulent direct cascade inside the ergoregion. Gravitational backreaction leads to an enhancement of the unstable process, and ultimately, black hole formation. I will illustrate the relevant dynamics and discuss implications for strongly gravitating horizonless systems.

The Perimeter Institute has provided me with 16 years of interactions, engagement and stimulating discussion. I'll describe some of my happy times at Perimeter and focus on one research area of interest: ultralight axions as a cosmological component. I'll describe constraints from the cosmic microwave background and large scale clustering of matter, through to novel constraints from voids and future prospects from stellar streams.

In this talk I will discuss one of the frontiers of both theory and data analysis in gravitational wave astronomy - understanding the ringing of black holes and probing them from real data. I will review past efforts started from Chandrasekhar, Detweiler, et al in analyzing modes of black holes and explain what we currently understand in both linear and nonlinear wave properties, as well as the corresponding detection aspect. At last I will show a few pressing problems and where we will be heading.

I will discuss BBN and a new python-based tool (PRyMordial) which allows one to easily simulate it both in the context of a standard cosmological model as well as in various scenarios of physics beyond the Standard Model. I’ll discuss how BBN provides a unique probe of physics relevant for the ~ MeV scale, and how it constrains or hints at modifications to the standard picture.