Research

What I spend my time thinking about

Black Holes

These are the most compact objects in the Universe, with masses spanning from a few to a few tens of billions of times that of the Sun. They can be formed in different ways, from being the endpoint of stellar evolution, to being the result of the direct collapse of proto-galactic disks. Image credit: Ute Kraus, Physics education group Kraus, Universitat Hildesheim

Neutron Stars & Pulsars

These are the second-most dense object in the Universe, beaten only by black holes. They resist gravitational collapse through neutron-degeneracy pressure, having masses similar to the Sun but packed into a space equivalent to a city. Rapidly rotating neutron stars act like dynamos, generating powerful radiation that is beamed along their magnetic field axis. Image credit: Tonia Klein, NANOGrav NSF Physics Frontier Center

Data Science

Teasing out patterns or signals from data, inferring the odds with which certain hypotheses are favoured by data, and quantifying our certainty or ignorance of the properties of the physical Universe are all the realm of statistical inference. The techniques we need are rich, diverse and fun. Full disclosure, I am a Bayesian. Image: Bayes' Theorem at offices of HP Autonomy

NANOGrav

The North American Nanohertz Observatory for Gravitational waves is a collaborative enterprise of many universities and radio facilities, involving ~100 researchers, and performing precision timing of more than 70 MSPs. We are currently generously funded as an NSF Physics Frontier Center. US facilities like the Arecibo Radio Telescope and the Green Bank Telescope are important for our ongoing successes.

International Pulsar Timing Array

The IPTA is the collaborative union of NANOGrav, the European Pulsar Timing Array (EPTA), the Parkes Pulsar Timing Array (PPTA), and emerging regional PTAs in South Africa, India, and China. Together, we have observed over 60 MSPs pulsars for more than 15 years.

LIGO

The Laser Interferometer Gravitational-wave Observatory made the first detection of GWs on September 14th, 2015. There are detectors in Livingston, Louisiana and Hanford, Washington, and have since been joined by the Virgo detector in Italy. LIGO detects black holes that are a few tens the mass of the Sun, and systems with neutron stars. The emerging catalog of detections is fostering a new era of compact-object demography. Image credit: LIGO Caltech

LISA

The Laser Interferometer Space Antenna is a joint ESA-NASA mission project for launch in ~2034. Three satellites will orbit the Sun at separations of ~2.5 million kilometers from one another. The mission will access the millihertz band of GW frequencies, where many Milky Way white-dwarf binary systems and massive black hole mergers await. These black holes are less massive than what PTAs can see, but LISA will be able to perform exquisite fundamental tests of gravity.

Pulsar Timing Arrays

The main thrust of my research is searching for nanohertz gravitational waves (GWs) from distant sources using a web of Milky Way pulsars (a Pulsar Timing Array), and unveiling the properties of the supermassive black holes, cosmic strings, or early Universe processes that produce these GWs.

Pulsars are rapidly rotating, highly magnetized neutron stars that emit beams of radiation along their magnetic field axis. The misalignment of their magnetic field axis with their rotational axis means that they act like lighthouses, swinging beams of radiation into our line-of-sight to be registered as pulses.

Millisecond pulsars (MSPs) spin hundreds of times a second, and have such stable and predictable pulse arrival times that we can use them as natural clocks in the sky. I work with colleagues in NANOGrav (North American Nanohertz Observatory for Gravitational Waves), and the International Pulsar Timing Array (IPTA) to search within deviations of pulse arrival times from highly-accurate models. It is in these deviations (or "residuals") where we will find gravitational waves. I have developed many of the signal models, statistical techniques, and coding infrastructure that underlies the global effort for PTAs to be the next great GW observatory in the 2020s.

GWs in the PTA band (~1-100 nHz) are highly likely to originate from a cosmological population of inspiraling supermassive binary black holes (SMBBHs), with masses ~10⁸ - 10¹⁰M_☉. Most massive galaxies are thought to harbor massive black holes at their center, so these binares form as a natural by-product of merger-induced galaxy growth. In fact, we expect the first signal to be detected will be the aggregate signal from all binaries added together. This GW background affects all pulsars in a correlated way. My work has shown that we are only ~3-7 years away from detecting it, followed by individual bright systems. I have also developed powerful strategies for mining information about the underlying properties and environments of SMBBHs from PTA detections of GW signals.

Image credit: David Champion

Dynamics & Environments of Supermassive Binary Black Holes

Coming soon...

Cosmology With Gravitational Waves

Coming soon...

Image credit: Fermilab