Cosmology

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.

A number of theories predict heavy dark matter, including WIMPs and high mass composite states formed in the early universe. Discovering the heaviest dark matter candidates, with a unit mass in excess of a microgram, requires methods beyond traditional underground experiments. For high mass dark matter searches in general (including WIMPS) new search methods will be necessary in the coming decades. I will discuss the most trenchant past searches for high mass dark matter along with future prospects.

Dark matter (DM) remains one of the most enduring mysteries in fundamental physics, motivating a wide array of theoretical models and experimental searches. Axions and axion-like particles (ALPs) are theoretically well-motivated, as they naturally arise from theories with broken global symmetries. These particles may serve as viable DM candidates themselves or act as mediators between DM and the Standard Model. In this talk, I will present an overview of axion and axion-mediated DM models, including both classic QCD axions and broader classes of ALPs. Emphasis will be placed on novel mechanisms shaping the cosmic DM abundance, and on innovative strategies for detecting these elusive particles. I will highlight how forthcoming laboratory efforts, such as fixed-target and direct detection experiments, alongside astrophysical observations, are poised to explore uncharted regions of parameter space and deepen our understanding of the connections between axion physics and dark matter.