Research interests: observational 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. David and his collaborators use large radio telescopes such as those in Arecibo, Puerto Rico, and Green Bank, West Virginia, to discover new pulsars and to exploit known pulsars for a variety of astrophysical studies. These telescopes can be remotely controlled from the Lafayette campus.
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.
Nice is part of the NANOGrav Physics Frontiers Center, which is working to directly detect gravitational waves in the nanoHertz (10-year-time-scale) regime via its effect on pulsar timing. This effort is being supported by a Physics Frontiers Center grant from the National Science Foundation and has involved more than twenty Lafayette students over the past ten years.
A small fraction of pulsars are in binary orbits with other stars. As its orbit makes a binary pulsar move 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 precision. 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. Nice and colleagues seek 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.