Research

I am an astrophysicist whose primary research is on pulsars. These rapidly rotating neutron stars emit beams of radio waves, much as lighthouses emit beams of light. As a pulsar rotates, its beam sweeps past the Earth, hitting it with a pulse of radio waves, once per rotation of the pulsar. I use large radio telescopes such as the one in Green Bank, West Virginia and, until its collapse in 2020, the noe in Arecibo, Puerto Rico to discover new pulsars and to exploit known pulsars for a variety of astrophysical studies.

The most precise pulsar observations use millisecond pulsars, neutron stars which rotate hundreds of times a second (and hence have rotation periods of milliseconds). By observing (“timing”) the pulses from any given pulsar, the behavior of a pulsar can be monitored over days, months, and years. Subtle losses of energy can be detected in the pulsar’s rotation, which gives information about the characteristics of the pulsar itself. The motion of the pulsar through the sky can be detected with milliarcsecond precision.

A small fraction of pulsars are in binary orbits with other stars. As its orbit takes a binary pulsar moves closer to and farther from the Earth, its pulses arrive earlier or later. In the best cases, pulses can be measured with accuracy of around 100 nanoseconds, allowing orbits to be mapped out with exquisite prevision. Subtle phenomena can be detected in pulsar orbits which arise because of general relativistic kinematics of their orbits. Detectable relativistic effects include precession of an orbit over time; changes in time dilation and gravitational redshift as the distance between the stars changes; extra time delay in the travel time of pulses when the line-of-sight between the pulsar and the Earth approaches the companion star; and decay of orbits due to emission of gravitational radiation and the consequent loss of energy from the system. The latter provides the only experimental evidence for the existence of gravitational waves. I am seeking to increase the precision of experiments in this area, both by discovering new pulsars and by continually improving the precision of observations of known pulsars.

I am part of the NANOGrav collaboration, which is working to characterize nanoHertz (decade-long time-scale) gravitational waves via their effect on pulsar timing over time scales of several years. We recently published strong evidence for the detection of such gravitation waves; see the summary, articles, and references on my home page.
This effort is being supported by a Physics Frontier Center Grant to Lafayette College and other collaborating institutions from the National Science Foundation.

Research Links:

Pulsar resources (this list badly needs updating):

Radio Telescopes:

Physics, Astronomy, and Computational Resources