NANOGrav Physics Frontiers Center Postdoctoral Fellow at Oregon State University
My research spans two complementary regions of the gravitational wave spectrum: the nanohertz band where pulsar timing arrays like NANOGrav operate, and the millihertz band that the upcoming space-based observatory LISA will probe. Click on a detector below to jump to the related research section.
Pulsar timing arrays (PTAs) detect gravitational waves at nanohertz frequencies (10-10–10-7 Hz) by monitoring tiny variations in the arrival times of pulses from millisecond pulsars. The NANOGrav PTA consisted of 68 MSPs at the time of the 15 yr data release (G. Agazie et al. 2023), all of which were chosen for their pulse shape stability, rotational stability, and/or brightness. (Jacobson-Bell et al. 2026)
With strong evidence now in hand for a stochastic gravitational wave background, most likely sourced by inspiraling supermassive black hole binaries, one of the next big questions is how that background is distributed across the sky. A non-uniform population of binaries should imprint anisotropy on the background, and detecting it would be a strong hint of the underlying astrophysics.
I have recently been developing an analytical framework for the sensitivity of a PTA to anisotropic backgrounds, pulsar-by-pulsar, direction-by-direction. The formalism is described in full in Oliver et al. (2026b), and is released as an extension to the hasasia sensitivity package.
The frequency-dependent (chromatic) delays imprinted on pulse arrival times by the interstellar and interplanetary medium can be up to two orders of magnitude louder than the gravitational wave background itself. I work on building customized chromatic noise models for each pulsar in NANOGrav’s datasets, testing the significance of competing noise prescriptions on a per-pulsar basis.
After applying our chromatic models, we observe significant impacts on the inference of achromatic noise in 19 out of 67 pulsars, finding in several cases that a previously significant achromatic noise process can be partially or entirely described as chromatic. (Larsen, Baier, Oliver, et al. 2026)
We find increased evidence for the Hellings-Downs (HD) correlation signature of the stochastic gravitational wave background (GWB), with a Bayes factor of 1571 ± 14 for HD-correlations over a common uncorrelated red-noise process using a power-law model with 14 Fourier modes. We find this ∼8× increase in Bayes factor from Agazie et al. (2023a) is a result of improved noise mitigation. (Agarwal et al. 2026)
LISA, the planned space-based gravitational wave detector, will be sensitive between roughly 10-5 and 10-1 Hz.
A prime LISA source is the extreme mass ratio inspiral (EMRI): a stellar-mass compact object (~10 M☉) spiraling into a massive black hole (105–108 M☉) at the center of a galaxy.
EMRI orbits typically start out highly eccentric and only emit detectable radiation in brief bursts as the compact object swings past periapsis. Each one of these bursts is a gravitational wave “peep”, firmly in the LISA band even when the long-period orbit itself is not. A single peep is unlikely to be individually resolvable, but a cosmological population of them out to redshift z = 3 contributes to LISA’s signal confusion noise. My dissertation and follow-up work focuses on modeling that background and quantifying how much it can obscure otherwise-detectable LISA sources.
A stellar-mass compact object can radiate in the millihertz band for years to decades before it plunges into its massive black hole, so a substantial fraction of the EMRIs that LISA can detect will still be inspiraling when the mission ends. We find that post-mission EMRIs are roughly one in four of the LISA-detectable population at ρ > 20 over the four-year mission, and roughly one in six over the ten-year extended mission. (Oliver et al. 2026c)
